Removal of Sulphur Compounds from Diesel Using ArF Laser and Oxygen

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Removal of sulfur compounds from diesel using ArF laser and oxygen

M. A. Gondal ^a , M. N. Siddiqui ^b & 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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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. SIDDIQUI²and K. AL-HOOSHANI²

1Laser Research Laboratory, Physics Department and Center of Research Excellence in Nanotechnology King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia

2Chemistry 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 undesir- able 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 Nanotechnol- ogy, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia; E-mail: [email protected]

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 photosen- sitizer 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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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 de- signed 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 de- pendence, duration of laser exposure and the laser energy for maximum removal of DMDBT in a model compound like hexane. The above-mentioned parameters were opti- mized. 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

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

3O2 could ensue by reaction between the quenched benzothiophene and¹O2; 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 re- ported 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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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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Fig. 4.Depletion of 4,6-dimethyldibenzothiophene 5 ppm solution in n-hexane upon irradiation of ArF laser at 193 nm. Differ- ent curves correspond to different irradiation times.

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 mea- sured 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. 5. Trend of depletion of 5 ppm solution 4,6-dimethyldibenzothiophene in n-hexane upon irradiation of ArF laser at 193 nm.

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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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.

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. Ab- sorbance 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-

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.

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. Al- though 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 SO2groups and also further cleavage (extrusion of S-containing fragments) as depicted schemat- ically here (Scheme 1). These reactions should be accompa- nied with oxidation of unsaturated hydrocarbons present in hydrocarbon fuel.

The effect of the oxidizing reagents such as H2O2

or others could partly assist in the laser-induced (tran- sient) formation of OH radicals that can induce a chain

Scheme 1.A schematic of laser oxidative cleavage.

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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 in- tervals 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 concentra- tions (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

through Project No. DRP-4-25 and facility support provided by the King Fahd University of Petroleum & Min- erals, Dhahran, Saudi Arabia for this work are gratefully acknowledged.

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