Testing of desulfurization effect in the Petroleum Industry

1Manny Mathuthu, Roseline Y. Olobatoke and Nelisa N. Gaxela

Centre for Applied Radiation Science and Technology (CARST),

North-West University (Mafikeng), Cnr Albert Luthuli and University Drive,

P. Bag X2046, Mmabatho, 2735, South Africa.

Manny.Mathuthu@nwu.ac.za ; yemisirose205@yahoo.com ; nelisa.gaxela@gmail.com

Tel: +27 18 389 2777

Abstract: Research has shown that such sour petroleum can be "sweetened" by gamma irradiation to desulfurize the crude oil. In this research we will report experimental results of desulfurizing petroleum oil locally procured. The feedstock for this work was LRB95 Grade petrol from the pump, consisting mainly of the n-alkanes (Cyclohexane-C6H12; Octane-C8H18; Octadecane-C18H38; Cyclopentane, n-pentane-C5H12. Ethylene (Propylene) was also identified. The reference standards used were made from a cocktail of n-Octane standard, (n-C8H18, GC ≥99% purity), from Merck KGaA. The total sulfur content in the un-irradiated product was found to be 42,700 ppm which means it is “sour” petroleum and therefore has poor quality as a pump petrol for the motor engines. The objective of this research is to improve the quality of product delivered to the motor market and also reduce the environmental pollution due to SO

2 emissions from engines. The gamma irradiated

(desulfurized petroleum) was analyzed using the Bruker IR Spectrometer. The preliminary results show that the petroleum sulfides and thiophenes etc., are oxidized by gamma radiation to sulfoxides and sulfones as a result of molecular transformation. The results indicate that the gamma oxidation of sulfur in petrol fuel might increase the calorific value of the fuel. IR Spectrums are presented for the identified sulfone and S-S / S=C bonds after gamma irradiation. Keywords: desulfurization, gamma irradiation, sulfur pollution, sulfones, sweetening.

1Corresponding author.

1.1 Introduction

The high capital and processing costs by thermal cracking of hydrocarbons has led to the emergence of new technologies like hydrocarbon enhancement electron-beam technology (HEET) (Mirkin et al., 2003), which operates at lower temperatures, and higher outputs than thermo-catalytic (TC) processing, and many catalytic steps are omitted by the production of radiation-induced active particles such as radicals, ions etc. (Zaykina and Zaykin, 2002a). A high degree of sulfur oxidation has been achieved due to the application of ozone-containing air. Sulfur hydrocarbons are oxidized to sulfoxides and sulfones leaving the carbon skeleton intact (Zaykina et al., 2004b), but under certain conditions, the skeleton is transformed.

Unlike in thermal cracking, radiation processing controls the molecule destruction rate by dose rate not temperature variations (Zaykina and Zaykin, 2002a). Research shows that lubricants and fuel oil can be upgraded by radiation processing to higher saturated hydrocarbon content (> C5H12). As shown by IR–Spectroscopy, absorption decreases compared with original feedstock fuel due to increased acidity of the product. Also radiation processing can be efficiently used for conversion of mercaptans and other light sulfuric species to the heavy liquid sulfur compounds like sulfones, sulfuric oxides and sulfonic acids that can be easily extracted by conventional methods. However partial desulfurization of light oil fractions by RTC which concentrated all the sulfur in these high boiling temperature products has been reported (Mirkin et al., 2003; Zaykina and Zaykin, 2002a). The Gamma Source can be used to replace the HEET in laboratory experiments where one deals with smaller volumes of feed stock (Zaykina et al., 2002c). This paper describes the desulfurization of LRB95 Grade petrol fuel using a Co-60 gamma source with elevated doses of up to 50 kGys at temperatures between 20oC and 22oC. The total sulfur content before gamma irradiation was found to be in the sour region. This suggest that the refineries could not completely remove sulfur from the original crude oil, probably due to high costs of processing by conventional methods. However this amount of sulfur in petroleum refineries and engines results in severe corrosion of metal parts (Hussein and Halim, 2013). Removal or reduction of sulphur provides an increase in the antiknock value of the produced gasolines (Potapenko et al., 2014).

1.2 Materials and Methods Processing high-sulfuric oil by ionized (ozone) air in combination with gamma –irradiation, increases the gasoline fraction (e.g. iso-alkenes) by up-to 2 times (Zaykin and Zaykina, 2004b). This work investigated the effect of gamma

irradiation of LRB95 Grade petrol with different doses varying from 10 to 50 kGys in order to determine whether there was any oxidation and/or transformation of the heavy liquid sulfur compounds like sulfides, thiophenes and sulfuric oxides to the sulfoxides and sulfones – which represents desulfurization and/or feedstock oxidation. This was done by measuring the total SO3 and n-octane content of the irradiated fuel.

1.2.1 Gamma Irradiation of samples

The Petroleum oil samples were LRB95 Grade petrol from the pump. A set of three vials was each irradiated with Gamma doses of 10, 20, 30, 40, & 50 kGys with a dose rate shown in Table 1. The processing of this motor petrol fuel

(normally called gasoline) was done at room temperature of 22oC with increasing

dose.

Table 1: Processing parameters for LRB95 Grade petrol irradiation

Target Dose

(kGys)

Measured dose

(kGy)

Irradiation Time

(min)

Dose rate

(kGys /hr.)

Uncertainty

(%)

10 10.16 61 9.99 5.30

20 19.52 114 10.27 2.40

30 30.41 186 9.81 3.70

40 40.63 248 9.83 2.10

50 50.46 313 9.67 1.40

Mean 9.91 2.98

Due to transport logistics, both the un-irradiated and the gamma irradiated samples were analyzed many days after irradiation. The Bruker ALPHA IR Spectrometer to determine the effect of gamma dose on the concentration of heavy hydrocarbons while the Energy Dispersive EDX model Rany-720 from Shimadzu was used to determine the concentration of the sulfoxide and total sulfur in the final products from the un-irradiated and irradiated sample jars, respectively. The OPUS Quantitative analysis was used to determine the quantity of n-Octane in both un-irradiated and gamma irradiated samples–see Table 3, & 4 and Fig 3 - 5.

A set of three jars (vials) of fuel sample were irradiated per dose.

All doses were measured by GEX Type B3 dosimeters (Batch No BD) in a Genesys 20 spectrophotometer (3SGM-049001). As samples were small, only

one dosimeter was attached to each dose group of three, therefore the variation was assumed to be constant per group. The temperature of the irradiation

chamber varied between 20⁰ C and 22⁰ C and all samples entered the chamber at ambient temperature The irradiation time was increased such that the dose rate was relatively constant per sample. An un-irradiated set of jars was used as a control and the above parameters were also determined from this set. Table 1 shows the processing parameters for this experiment.

1.2.2 Reagents and Standard solutions

Table 2: Characteristics of the LRB95 Grade petrol feed stock at 0 kGy. (Potapenko et al., 2014)

Hydrocarbon Content

(n-alkanes)

Boiling Temperature

(oC)

Density (g/L)

C:H Content

Cyclohexane 80.7 C6H12

Octane (mg/mg/mL)

124-126

0.684

C8H18

280.833 ± 0.769

Octadecane

316.12 0.777 C18H38 nd

Phenyldodecane

nd nd C18H30 nd

Total Sulfur content (% wt) nd nd S 4.270

Petroleum standard n-Octane (mg/mL)

(Sánchez et al., 2013)

125.7 0.684 C8H18 280.84

n-Pentane nd nd C5H12 nd

Ethylene (propylene) nd nd

The characteristics of the original feedstock used in this work are shown in Table 2 above.

This paper reports results of an analysis performed on gamma irradiated petroleum using the Bruker ALPHA IR Spectrometer and the Energy Dispersive EDX model Rany-720 from Shimadzu.

Five samples of n-Octane were used as the Calibration standards. The calibration results are shown in Table 3 below. The correlation coefficient for the calibration was 0.9964 and sigma was 0.769. X represents the integration area. Two additional and different samples (n-Octane (3mg/mL) and n-Octane 25 (mg/mL) were then used to TEST the calibration. The results (Fit) are in good agreement with original (True) concentrations as also shown.

Calibration standards were made from a cocktail of n-Octane standard, (n-C8H18, GC ≥99% purity), dissolved in carbon tetrachloride (CCl4, GC ≥ 99.8% purity), both purchased from Merck KGaA.

Table 3: Calibration results from OPUS Quantitative analysis

Sample ID True (mg/mL) Fit (mg/mL) Difference (mg/mL)

X (Int. Result)

n-Octane (CAL)

5.00 4.41 0.59 133.4

“ 8.00 8.89 -0.89 133.1

“ 10.00 9.55 0.45 133.0

“ 15.00 15.53 -0.53 132.5

“ 25.00 24.63 0.37 131.8

n-Octane (TEST)

3.00 3.00 0.00 133.6

“ 25.00 24.66 0.34 131.8

n-Octane (this work)

280.833 ± 0.769

(0 kGy dose) -- --

n-Octane (this work)

617.303 ± 0.769

(10 kGy dose) -- --

. 1.2.3 ALPHA IR Spectrometer Analysis NIR and MIR spectroscopies have been used alone or in combination for the prediction of benzene content and research octane (Khanmohammadi et al., 2012). Another application of MIR analysis has been made to evaluate crude oils quality by using spectroscopic indices related to aliphatic hydrocarbon and aromatic composition in order to calculate the ratio ALI/ARO (Abbas et al., 2012). In this work, an ALPHA FT-IR spectrometer (Bruker Optics) equipped with a RT-DLATGS detector, an MIR source and a KBr beam splitter was employed. An Attenuated Total Reflectance (ATR) cell (Platinum Diamond 1, Ref# D00E962D) was used to analyse the samples. Air was taken as a reference (background acquisition) before the collection of each sample spectrum. Data acquisition,

made at ambient room temperature ( 24 oC) with an absorbance scale, was from 4000 to 400 cm-1 with 4 cm-1 nominal resolution and 32 scans. The samples were labelled by their Dose (kGys) and each sample had three replicates. After thawing the samples to room temperature, they were then deposited on the ATR cell without any prior preparation or dilution.

Spectra of the samples was acquired using the OPUS Software that came with the Spectrometer. Both the un-irradiated and the gamma irradiated samples were characterised using the ALPHA FT- IR Spectrometer. Identification of samples in Table 2, was by using the installed OPUS/SEARCH Library facility of the Program. 1.3 Results and Discussions The results from the un-irradiated jars are shown in Table 2.

The Table 2 shows that the un-irradiated petrol samples contain stretching vibrations of alkanes (2850 - 3000 cm-1); bending vibrations of alkanes (1470 – 1350 cm-1) (Xiaolan et al., 2006) and bending vibrations of alkenes (780 - 675 cm-1) as also confirmed by the IR spectra in Fig 1 below. In Table 3, the octane content at 10 kGy dose was evaluated and found to be about twice that at 0 kGy, which indicates isomerization by gamma RTC (Zaykin and Zaykina, 2004b).

Table 4: Results of Gamma-irradiated gasoline using ED-XRF

nd = not detectable

Dose (kGy) NiO (%)

CuO (%)

Fe2O3

(%) Cr2O3 (%) SO3

(%)

0 2.329 0.044

0.832 0.032

29.502 0.088

11.202 0.062

10.662 0.204

10 7.075 0.140

1.921 0.104

62.573 0.199

17.486 0.100

10.608 0.200

20 nd 0.65 0.05

6.118 0.103

2.907 0.130

77.952 2.600.

30 nd nd 13.154

0.302 nd

47.051 0.625

40 nd nd 4.690 0.157 nd

61.994 1.520

50 6.340 0.098

1.921 0.073

62.959 0.166

18.572 0.082

43.340 0.821

Fig 1: Typical FT-IR spectra for the petroleum sample at some doses used in this work

The results in Table 4 show the presence of Sulfur (as SO3) in the petroleum

from the pump (dose of 0 kGy) indicating that conventional methods could not

totally eliminate it. The 10.662 % wt. was calculated to be equal to 106,620 ppm.

This indicates that the evaluated petroleum feedstock was sour (Hussein and

Halim, 2013). The total sulfur content in the un-irradiated feedstock was

calculated to be 42,700 ppm (= 4.270 % wt). This table shows that the

concentration of sulfoxide in the petroleum is increased from 10.662 % wt to

77.952 % wt after RTC with gamma rays at 20 kGy. An increase of sulfone in the

petroleum represents desulfurization as the C-S bonds are broken and sulfur is

oxidized by the radiation. This effect peaks at around 20 kGy after which

saturation dominates the transformation reactions – see also Fig 2. Gamma

irradiation does not evaporate sulfur from the petroleum, but it oxidizes it to a

sulfoxide or sulfone which are more polar compounds (Ali et al., 2009) with

increased acidity. However these oxidized products of sulfur can then be easily

removed by conventional means like hydrodesulphurization or solvent extraction

steps (e.g. using methanol etc.) (Duarte et al., 2011). These results indicate that

RTC by gamma rays has the potential of reducing sulfur content by seven times

before conventional processing (Potapenko et al., 2014).

0 kGy Dose, 10 kGy Dose & 50 kGy Dose

When the gamma irradiation cracking caused an increase in the content of

cyclohexane (a hydrogen donor) in the feedstock, the content of thiophene

compounds also decreased, and the selectivity for hydrogen sulfide increased.

Also the olefin (for ≥ C8) contents decreased favouring higher Octane number

(better quality petrol) (Zhang et al., 2008) at a dose of about 10 kGy.

The RCT results suggest that even pump petrol, as is this case, which was not

processed to “sweet” engine oil can be reprocessed to meet international

standards. Finally it has been observed that reaction rates (isomerization) at low

temperature favor the formation of iso-alkenes (Zaykina et al., 2001), (Andrade,

2013). Table 4 also shows that as the dose is increased in petroleum, the metals

that might not be wanted are oxidized and would thus be easily removed by

conventional methods. The catalytic metals like Ru, Fe and Ni can still be

recycled. This improves the quality of the petroleum.

Fig 2a: Sulfur (as SO3) removal with applied gamma dose

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50

Sulf

oxi

de

rem

ova

l (%

wt)

Gamma dose (kGy)

Fig 2b: Variation of n-Octane content in petroleum with gamma dose

At doss ≥20 KGy saturation occurs due to the developing reactions of the addition of light hydrocarbon radicals CnH(2n+1) to the double bonds of the hydrocarbon olefins (Zaykina et al., 2002c). The effect of saturation is shown in the graph of Fig 2a & 2b above.

0

100

200

300

400

500

600

700

0 10 20 30 40 50

N-O

ctan

e C

on

ten

t (m

g/m

L)

Gamma dose (kGy)

Fig 3: Typical S-S spectra at wavenumber 538 cm-1 for different doses

The disulfide (S-S) spectra is located in the frequency band of 500 - 540 cm-1

(Fig 3) and the C S in the band of 1050 -1200 cm-1 (Fig 4) (Reusch, 2013) (Duarte et al., 2011).

The detection of S C in the frequency band of 1030-1060 (sulfoxide in Fig 4) and 1325± 25 & 1140± 20 (sulfone in Fig 5) shows desulfurization (Ali et al., 2009). Oxidation of the bridging sulfur to sulfone greatly increases the acidity of the resultant sulfone and reduces its absorbance (Hiroaki et al., 2002). This phenomena is confirmed in the graphs of Fig 3-5 where the highest absorbance (weakest oxidation) is at doses of 0 kGy and lowest absorbance (strongest oxidation) at doses of 10 kGy.

0 kGy Dose, 10 kGy Dose & 50 kGy Dose

Fig 4: S=C spectra in the frequency band of 1050 -1200 cm-1 for different doses

The sulfur containing C ═ S species are converted to H-C species by gamma

oxidation (Sandeep et al., June 2012)

Fig 5: Sulfone spectra in the frequency band of 1080 -1350 cm-1 for different

doses (Ali et al., 2009; Duarte et al., 2011)

The sulfone peaks have been observed in the wavenumbers of 1160 cm-1 and

1280 cm-1 (Ali et al., 2009). In this work, in Fig 5, only the sulfone peak at 1160

cm-1 is observed clearly. At 1280 cm-1 it is very week for the doses shown. It is

possible that this peak could have been clear also if the doses had been varied at

smaller increments (e.g. steps of 1 kGy) from 0 to 15 kGy.

1.4 Conclusions

Results of this study show that the quality of sour petroleum can be improved by

gamma irradiation doses up to 10 kGys, while doses greater than 20 kGy cause

saturation of the target compounds with no further increase in sulfonation. In this

preliminary work the un-irradiated petroleum had a sulfur concentration of 4.270

% wt. which was converted to sulfone by a factor of seven after gamma

irradiation. This RTC technique oxidizes sulfur containing species to a sulfoxides

or sulfones which are more polar compounds with increased acidity and reduced

absorbance. The sulfone peaks are observed in the wavenumbers of 1160 cm-1

(strong) and 1280 cm-1 (very weak). These oxidized products of sulfur can then

be easily removed by conventional means like hydrodesulphurization or

methanol solvent extraction steps.

The conclusions of the study will provide information about the processing

requirements for locally made petroleum and reference standard. This can be

extended on a small scale to locally produced cooking oils. The dose rate was

about 10.6 kGy/hr. Also once sulfur is converted to SO3 by gamma irradiation,

less heat is extracted from the fuel to burn the sulfur. This suggests that gamma

irradiation increases the calorific value of fuels.

1.5 Acknowledgements

.

The authors gratefully acknowledge the Faculty Focus Area, MaSIM, for

providing funds to attend the IMRP2013 Conference on this work and we also are

grateful to the Faculty Research Committee of the Faculty of Agriculture Science

and Technology (NWU-Mafikeng) for providing Sponsorship to attend the

European Nuclear Conference (ENC 2014) to present this work. Grant Ref: FRC

Activity Account N150941. Finally the assistance of the Chemistry Technicians,

Mr. Kagiso Mokalane and Dr Johan Jordaan is respectfully acknowledged during

the analysis of the samples by FTIR.

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