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Removal of sulfur compounds from diesel using ArF
laser and oxygen
a
b
M. A. Gondal , M. N. Siddiqui & K. Al-Hooshani
b
a
Laser Research Laboratory , Physics Department and Center of Research Excellence in
Nanotechnology King Fahd University of Petroleum and Minerals , Dhahran , Saudi Arabia
b
Chemistry Department and Center of Research Excellence in Nanotechnology , King Fahd
University of Petroleum and Minerals , Dhahran , Saudi Arabia
Published online: 15 Aug 2013.
To cite this article: M. A. Gondal , M. N. Siddiqui & K. Al-Hooshani (2013) Removal of sulfur compounds from diesel using
ArF laser and oxygen, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental
Engineering, 48:13, 1663-1669, DOI: 10.1080/10934529.2013.815488
To link to this article: http://dx.doi.org/10.1080/10934529.2013.815488
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Journal of Environmental Science and Health, Part A (2013) 48, 1663–1669
C Taylor & Francis Group, LLC
Copyright ISSN: 1093-4529 (Print); 1532-4117 (Online)
DOI: 10.1080/10934529.2013.815488
Removal of sulfur compounds from diesel using ArF
laser and oxygen
M.A. GONDAL1, M.N. SIDDIQUI2 and K. AL-HOOSHANI2
1
Downloaded by [Selcuk Universitesi] at 22:48 09 February 2015
Laser Research Laboratory, Physics Department and Center of Research Excellence in Nanotechnology King Fahd University
of Petroleum and Minerals, Dhahran, Saudi Arabia
2
Chemistry Department and Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals,
Dhahran, Saudi Arabia
A laser-based technique for deep desulfurization of diesel and other hydrocarbon fuels by removal of dimethyldibenzothiophene
(DMDBT), a persistent sulfur contaminant in fuel oils has been developed. We report a selective laser excitation of DMDBT in
diesel and model compounds such as n-hexane in a reaction chamber under oxygen environment where oxidative reactions can take
place. ArF laser emitting at 193 nm was employed for excitation of oxygen and DMDBT, while for process optimization, the laser
energy was varied from 50 to 200 mJ/cm2. The laser-irradiated DMDBT solution under continuous oxygen flow was analyzed by UV
absorption spectrometer to determine the photochemical oxidative degradation of DMDBT. In just 5 min of laser irradiation time,
almost 95% DMDBT was depleted in a diesel containing 200 ppm of DMDBT. This article provides a new method for the removal
of sulfur compounds from diesel by laser based photochemical process.
Keywords: Sulfur removal, clean fuels, DMDBT, photo-oxidation, laser applications.
Introduction
The supply of clean desulfurized hydrocarbon fuel is important due to environmental concerns and compliance with
the regulations set by agencies safeguarding the environment. Under these compulsions, the demand for clean hydrocarbon fuel continues to rise. In this context one major
serious problem in the world is air pollution caused by gases
(SOx), emitted by automobiles exhausts operated by diesel
fuels. The emission of SOx into air is also responsible for
generating acid rain when it combines with water molecules
in the atmosphere. Acid rains may cause severe problems for
all living organisms, especially by polluting water aquifers,
destroying trees and forests and causing many allergies and
respiratory deceases in humans. The other serious problem
with sulfur-containing compounds is that these are undesirable in the refining processes because they tend to deactivate
various catalysts used in downstream processing and in the
upgrading of hydrocarbons. To avoid all above-mentioned
problems caused by sulfur-containing fuels, much research
work has focused on the deep desulfurization of light oil.
Address correspondence to M.A. Gondal, Laser Research Group,
Physics Department and Center of Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran
31261, Saudi Arabia; E-mail: magondal@kfupm.edu.sa
Received April 8, 2013.
Presently, a catalytic hydrodesulfurization (HDS)
method has been applied on an industrial scale, but it
requires both high hydrogen pressure and high temperature.[1-4] Hence to produce deep desulfurized fuels, the
demand for hydrogen has been increased. In addition,
the HDS method has difficulty in the desulfurization of
dibenzothiophene (DBT) and its derivatives among the
sulfur-containing compounds in light oil.[5-8] A desulfurization process for DBTs by photochemical reaction and
liquid-liquid extraction using an organic/water two-phase
system has been proposed by Hirai et al., which has been
extended for benzothiophenes (BTs) and alkyl sulfides
DBTs, in a tetradecane solution, were photodecomposed
using a high pressure mercury lamp and were removed
into the water phase as sulfate anions.[9]
The previous studies, however, revealed the problem
that the removal of DBTs from tetradecane is depressed
remarkably by the presence of naphthalene, which is caused
due to triplet energy transfer from the photoexcited DBT to
the ground-state naphthalene.[5-8] This is a serious problem
for the desulfurization of light oil, since light oil contains
two ring aromatic compounds such as naphthalene whose
derivatives have lower triplet energy. The desulfurization
process was improved by introducing a triplet photosensitizer into the light oil and hydrogen peroxide into the
oil/water two-phase liquid-liquid extraction system.[5,6]
Hydrogen peroxide was found to be effective, since it acts
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1664
as a weak oxidizing agent for the photoexcited DBT and to
some extent interrupts the energy transfer from the excited
DBT to the naphthalene. In these previous studies, the
basic idea for desulfurization was the photodecomposition
of sulfur-containing compounds in the light oil phase,
followed by the transfer of the resultant decomposed
compounds into the aqueous phase. Thus, deactivation of
the photoexcited DBTs by naphthalene appeared more or
less inevitable.[6-9]
As mentioned earlier, bringing down the level of sulfur in
transportation fuels requires deep desulphurization. In addition, the preceding conventional techniques (hydrogenation) have some technical limitations and cost-effectiveness
issues due to low quality fuel (i.e., octane loss) to reduce
certain sulfur compounds like benzothiophene, which is a
major inhibitor, to bring down the sulfur level to <50 ppm
limit. Keeping these constraints in mind, substantial research efforts have been invested to develop novel techniques for reduction of benzothiophene in hydrocarbon
fuels. The unique characteristics of laser radiation such as
coherence and monochromaticity have been well harnessed
in this work to minimize the removal of benzothiophene
compounds such as dimethyldibenzothiophene (DMDBT)
in hydrocarbon fuels.
We report in this work an elegant technique using selective lasers’ excitation of DMDBT in model compounds
such as diesel and n-hexane by atomization and oxidative
reaction of molecular oxygen in a reaction chamber. The
advantages of this laser-based technique over the conventional method can be the minimum quality deterioration
and yield reduction while keeping the liquid properties of
hydrocarbon fuel intact. This work is in continuation of
our commitment to develop technologies for clean fuel and
environment using various techniques.[10-17]
Materials and methods
A setup based on a laser excitation technique for the deep
desulfurization of diesel hydrocarbon fuels by removal of
benzothiophene compounds has been developed. For this
purpose a special reactor as depicted in Figure 1 was designed and fabricated locally. The reactor consists of a
Pyrex cell of 60-mL volume, equipped with an optical grade
quartz window for transmission of the UV laser beam. The
cell is equipped with some ports and rubber septums for
sampling. Keeping in mind the importance of the main experimental parameters and their effects on desulfurization
process, the first step was to see the laser wavelength dependence, duration of laser exposure and the laser energy
for maximum removal of DMDBT in a model compound
like hexane. The above-mentioned parameters were optimized. Eventually, the best excitation wavelength of the
laser was selected as 193 nm using ArF laser (Coherent
model Comprex Pro 201, Santa Clara, CA, USA), while
for optimization of laser energy the energy range studied
Gondal et al.
Fig. 1. Schematic diagram of the experimental setup for laser
desulfurization of diesel and n-hexane.
was 50–200 mJ/cm2. The irradiated DMDBT solution was
analyzed by a high-resolution UV absorption spectrometer
(depicted in Figure 2) to quantify the removal of DMDBT
using this photochemical oxidative degradation process.
Results and discussion
A possible mechanism and processes for removal of sulfur
from dibenzothiophenes compounds using laser excitation
are:
(i) Laser excitation produces triplet states of dibenzothiophenes and oxygen;
(ii) An interaction of benzothiophene triplet state with
3
O2 could ensue by reaction between the quenched
benzothiophene and 1O2 ; and
(iii) The 1O2 produced in intimate proximity of the
quenched benzothiophene could selectively react with
benzothiophene to yield endoperoxides.
Such reactions between 1O2 and thiophene and also
between 1O2 and aromatic hydrocarbons have been reported recently.[18-26] Selective excitation of dibenzothiophenes (DMDBT) with monochromatic sources like lasers
(and not of other aromatic hydrocarbons) should ensure
that the selective oxidation of only these compound(s) is
achieved. The endoperoxides are thermally labile and can
be decomposed to products not containing S atoms. This
scheme is illustrated in Figure 3. In our experiment, the
irradiation of DMDBT solution in diesel and n-hexane using Excimer (Coherent Model Compex pro) laser at 193 nm
resulted in the depletion of DMDBT and proved that laser
photolytic decomposition of DMDBT is possible.
Figure 4 depicts the absorption spectra showing the
degradation of 4,6-dimethyldibenzothiophene using ArF
laser under continuous flow of oxygen gas. In this study, the
laser irradiation at 193 nm wavelength having incident pulse
energy = 50 mJ, and repetition rate = 10 Hz for different irradiation times, which resulted in complete degradation of
5 ppm solution of 4,6-dimethyldibenzothiophene (Fig. 4)
in n-hexane. As one can notice from Figure 4, after 60 min
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Removal of sulphur compounds from diesel
1665
Fig. 2. Schematic diagram of high resolution spectrometer applied for the analysis of the DMDBT removal process after UV laser
irradiation.
Fig. 3. A schematic of photo-oxidative desulfurization of DMDBT using laser excitation process.
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Gondal et al.
Fig. 4. Depletion of 4,6-dimethyldibenzothiophene 5 ppm solution in n-hexane upon irradiation of ArF laser at 193 nm. Different curves correspond to different irradiation times.
Fig. 5. Trend of depletion of 5 ppm solution 4,6-dimethyldibenzothiophene in n-hexane upon irradiation of ArF laser at
193 nm.
of irradiation, almost all DMDBT has been degraded. The
trend of depletion of 4,6-dimethyldibenzothiophene (5 ppm
solution) in n-hexane upon irradiation of ArF laser for various laser irradiation times is depicted in Figure 5. The absorbance trend of DMDBT at various times is depicted in
Figure 6. As can be noticed from Figure 6 approximately
50% reduction took place in the first 30 min of laser irradiation at this incident laser energy (50 mJ).
For the depletion of 4,6-dimethyldibenzothiophene in
diesel, the laser energy was increased. In order to get
higher laser energy, the electrical charging voltage for
pumping the excimer laser was increased. Figure 7 depicts
the absorbance of 4,6-dimethyldibenzothiophene solution
(200 ppm) in diesel upon irradiation of ArF laser at 193 nm
for different laser irradiation time durations under continuous oxygen bubbling through the solution. The flow rate of
oxygen was kept constant throughout the experiment at 2 L
min-1. Here in Figure 7, one can notice that the absorption
spectra due to diesel as a solvent is broadened as compared
with n-hexane. For this study, ArF laser energy was 200 mJ.
The absorbance of DMDBT, which is a real indicator to
show the depletion of DMDBT in the diesel environment,
is plotted in Figure 7.
Based on the spectrophotometric absorption data measured at different laser irradiation times, the trend of depletion of 4,6-dimethyldibenzothiophene solution (50 ppm) in
diesel upon irradiation of ArF laser at 193 nm and laser
Fig. 6. Absorbance versus laser exposure time for the depletion of
5 ppm solution 4,6-dimethyldibenzothiophene in c-hexane upon
irradiation with ArF laser at 193 nm at 50 mJ pulsed energy.
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Removal of sulphur compounds from diesel
1667
Fig. 7. Depletion of 50 ppm solution 4,6-dimethyldibenzothiophene in diesel upon irradiation with ArF laser at 193 nm
having. Different curves correspond to different irradiation times.
Here ArF laser energy was = 200 mJ.
Fig. 8. Trend of depletion of 50 ppm solution 4,6-dimethyldibenzothiophene in diesel upon irradiation of ArF laser at
193 nm. Here ArF laser energy was = 200 mJ.
energy of 200 mJ is depicted in Figure 8. Here one can
also notice an important aspect that as the laser energy
is increased from 50 mJ to 200 mJ, keeping incident laser
wavelength constant, the depletion rate increased, and the
depletion we achieved in 60 min at 50 mJ incident laser
energy could be achieved in 5-min laser irradiation time.
This rapid depletion could be attributed to higher incident
photon flux. The higher number of incident photons in
the reaction zone could enhance the reaction rate much
faster by exciting more DMDB and oxygen molecules. Absorbance versus laser exposure time for the depletion of
4,6-dimethyldibenzothiophene solution (200 ppm) in diesel
upon irradiation of ArF laser at 193 nm at 200 mJ pulsed
energy is depicted in Figure 9. As one can notice that almost
95 % DMDBT has been depleted just in a short laser expo-
sure time = 5.5 min, which is considered to be an excellent
achievement.
We have demonstrated that laser desulfurization of diesel
and other hydrocarbon fuels is quite an efficient process. Although the exact nature of depletion process of DMDBT is
not very clear, we expect the complex chemistry including
laser oxidative cleavage that could lead to aromatic compounds with S-O and SO2 groups and also further cleavage
(extrusion of S-containing fragments) as depicted schematically here (Scheme 1). These reactions should be accompanied with oxidation of unsaturated hydrocarbons present
in hydrocarbon fuel.
The effect of the oxidizing reagents such as H2 O2
or others could partly assist in the laser-induced (transient) formation of OH radicals that can induce a chain
Scheme 1. A schematic of laser oxidative cleavage.
1668
Gondal et al.
through Project No. DRP-4-25 and facility support provided by the King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia for this work are gratefully
acknowledged.
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Fig. 9. Absorbance versus laser exposure time for the depletion
of 50 ppm solution 4,6-dimethyldibenzothiophene in diesel upon
irradiation with ArF laser at 193 nm at 200 mJ pulsed energy.
photo-oxidation of DMDBT in the liquid phase.This reaction could occur in a specific way; it would be interesting
to examine whether DMDBT depletion occurs within intervals or longer than the laser irradiation interval (pulse
width of laser) in future studies.
Conclusions
It has been demonstrated that laser desulfurization of
diesel, which contains sulfur compounds such as DMDBT,
is a very fast process that is hard to remove by normal
hydrogenation processes. In just 5 min of laser irradiation
time, almost 95% DMDBT was depleted in a diesel containing 200 ppm of DMDBT. A research scheme based
on laser-induced photo-oxidation of dibenzothiophenes
with molecular oxygen in laser excited state (1O2 ) is proposed. Different laser-irradiation times, sample concentrations (200 and 5 ppm in diesel and n-hexane) and pulse
energies were employed. Using an ArF laser at 193 nm,
the maximum depletion of DMDBT was achieved at 5 min
with 200 mJ incident pulsed laser energy in diesel and in
60 min with 50 mJ pulsed energy in n-hexane.
Acknowledgments
The financial support provided by King Abdulaziz City for
Science and Technology (KACST), Riyadh, Saudi Arabia
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