The presence of Naphthenic Acids compounds contributes to the acidity of crude oils and is one of the major sources of corrosion in oil pipelines and distillation units in oil refineries. Consequently, crude oils with high naphthenic acid concentrations are considered to be of poor quality and marketed at a lower price. Therefore removal of those acids before refining operations is of paramount importance. In this research, Naphthenic acids were isolated from Nigerian crude oil using ammoniated ethylene glycol as the extraction solvent, the extracted acid was then determined using MS/MS spectroscopic technique. Some standards were used to ascertain whether naphthenic acids were actually from the sample or not. It was found that Naphthenic acids were isolated from the sample as separate entities without derivatization to other compounds.
Naphthenic acids (NA), as used in the petroleum industry, refer collectively to all of the carboxylic acids present in crude oil. Naphthenic acids are classified as monobasic carboxylic acids of the general formula RCOOH, where R represents the naphthene moiety consisting of cyclopentane and cyclohexane derivatives, (Lai et al., 1996) Naphthenic acids are composed predominantly of alkyl-substituted cycloaliphatic carboxylic acids, with smaller amounts of acyclic aliphatic acids. The cycloaliphatic acids include single and fused multiple cyclopentane and cyclohexane rings. The carboxyl group is usually attached to a side chain rather than directly to the ring. Aromatic, olefinic, hydroxy and dibasic acids are present as minor components. Naphthenic acids recovered from refinery streams occur naturally in the crude oil and are not formed during the refining
process. Heavy crudes have the highest acid content, and paraffinic crudes usually have low acid content, (Brient et al., 1995). The presence of NA compounds contributes to the acidity of crude oils and is one of the major sources of corrosion in oil pipelines and distillation units in oil refineries. Consequently, crude oils with high naphthenic acid concentrations are considered to be of poor quality and marketed at a lower price (Speight, 1999). It is highly desirable to determine the ring-type distribution and the carbon number distribution of each ring type because the corrosivity of naphthenic acids is dependent on their size and structure (Messer et al., 2004). Naphthenic acids are quite soluble in water, and water affected by processes in the petroleum industries generally contains naphthenic
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acid which is considered to be in the range of toxicity to human consumption, and fractions are known to be toxic to mussels and fish (Herman et al., 1994), and perhaps even more importantly to mammals. Hence water contaminated with NA from petroleum production needs to be treated before it is use by the remote communities, (Holowenko et al., 2002). Apart from causing corrosion problems in the refining process, naphthenic acids are also important in many applications. For instance as oil-soluble metal soaps for driers and other catalysts, wood preservatives, tire cord adhesion promoters, in amine derivatives for corrosion inhibitors, as copper naphthenate in wood preservation (Grove, 1987).
Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions using an electrospray in which a high voltage is applied to a liquid to create an aerosol, (Ho et al., 2003). It is especially useful in producing ions from macromolecules because it overcomes the propensity of these molecules to fragment when ionized. ESI is different than other atmospheric pressure ionization processes (e.g. MALDI) since it may produce multiple charged ions, effectively extending the mass range of the analyser to accommodate the kDa-MDa orders of magnitude observed in proteins and their associated polypeptide fragments, (James, 2009)
ESI coupled with MS/MS techniqueis extremely valuable in the determination of the molecular weight distribution as well as confirmation of the identity of naphthenic acid samples. Carboxylic acids generate neutral losses of 44, 28 and 18 Dalton that can be respectively attributed to M-CO2, M-CO, and M-H2O (Rudzinski et al., 2002). The aim of this study is to isolate/extract naphthenic acids from the Oki-Oziengbe oil field crude oil using ethylene glycol as the extraction solvent. MATERIALS AND METHODS
Materials
All reagents used such as Ammonia, ethylene glycol, acetonitrile petroleum ether are all of analytical grade and obtained from Agilent. The standards used: Benzoic acid, decane sulfonic acid and butyl benzoate were obtained from Sigma-Aldrich.
Sample Collection
Petroleum crude oil was obtained from Nigerian National Petroleum Cooperation (Warri refinery) and was supplied from Oki-Oziengbe South field located in the Niger Delta sedimentary province. The field lies approximately 20 km south of Benin City and 80 km north of Warri in Delta State.
Preparation of Ammoniated ethylene glycol
Anhydrous ammonia was bubbled at a constant flow rate through 10 mL of ethylene glycol in a three-necked, round bottom flask maintained at 0°C in an ice bath for about 2 hours for the solution to be saturated. Extraction of Naphthenic Acids
A measured volume of ammoniated ethylene glycol (10 mL) was added to of petroleum crude and then stirred for 1 hour at 0 °C. The mixture was transferred into a single neck round bottomed flask capped with a rubber stopper, heated at 60-65 °C for an hour. The solution was transferred into a separating funnel and, upon cooling in about 20-25 minutes, two layers were formed: an oil phase (top layer) and an ethylene glycol phase (bottom layer). (Note: If the ammonium naphthenate salts in the ethylene glycol phase are left too long in the funnel, they may go back into the oil phase). The top layer was then discarded, and the ethylene glycol layer, which contains ammoniated naphthenate salts, was then transferred into a round bottom flask and heated at 130-135 oC using an oil bath for 1.5 – 2 hours or until no ammonia gas evolves (The ethylene glycol layer turns to a yellow colour on heating. The heat removes the ammonia, which has a low boiling point, from the ammoniated naphthenate salt, and the naphthenic acids that have high boiling points remained undecomposed). The ethylene glycol layer was transferred into a separating funnel while hot and left to cool to room temperature. Ten milliliters (10 mL) of petroleum ether was added to the funnel and all the naphthenic acids are extracted from ethylene glycol. The suspension was left to settle for 15-20 minutes. The bottom layer (ethylene glycol) was discarded. The petroleum ether layer containing the naphthenic acids was washed with 3-5 mL of water to remove the residual ethylene glycol from the petroleum ether. The naphthenic acids were obtained by evaporation of the solvent at 40oC for 5-8 minutes (Wang et al., 2006).
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Mass Spectrometry (MS)
MS and MS/MS experiments were performed using a Thermo Finnigan Mass Spectrometer controlled by Tuneplus software (Excalibur), with the ESI (Electrospray Ionization) source in negative ion mode. The concentration of the sample solutions was 1.5 mg of naphthenic acid extract in 2 mL of acetonitrile as a solvent (i.e. 0.75 mg/mL). ESI (-) MS Experiments
The sample flow rate was 4 μL – 6 μL/min; the mass scan range was between 180 to 450 m/z; mass spectra were based on 3 microscans with a 50 msec ion injection time; automatic gain control was enabled. The sheath gas was maintained at 60 psi, the temperature of the API stack at 220 °C, the capillary voltage at 7 V, and the spray voltage at 3.15 kV. ESI (-) MS/MS Experiments
For all ions, the collision activated dissociation (CAD) energy was set in such a way that the MS/MS scan of the precursor and fragment ions could both be detected. CAD energy was between 20-40 %. The optimum peak isolation width was 1 Da, (whereas a wider width of more than 1 Da resulted in fragmentation of ions of species with m/z near that of the precursor ion). Activation Q was 0.25 and activation time was 30 msec. The MS and MS/MS experiments were performed on a series of standards: benzoic acid, butyl benzoate, decanesulfonic acid and lastly on the naphthenic acids extract from crude oil, so as to understand the characteristic fragmentation patterns of the extracted acids with respect to these compounds. RESULTS AND DISCUSSION
The result of MS and MS/MS analyses of standard acids
and naphthenic acids are shown in Figure 1 to 9.
Benzoic acid: The MS/MS experiment on a carboxylic acid (benzoic acid) was run to observe the fragmentation pattern. There is a mass loss of 44 Da due to carbon dioxide loss shown by benzoate, leaving a fragment ion peak at 77 Da (benzene ring), (Rudzinski, et al., 2002) (Figure 1). Ester compound: Heating an acid at high temperature (120-180 °C) in presence of a base/ water can favour the formation of an ester, (Lateefah et al., 2006). Our
extraction procedure favours the formation of esters from acids in the crude oil. MS/MS experiments were performed on butylbenzoate, with m/z 179 in acetonitrile in negative ion mode and showed a peak at m/z 79.9 (Figure 2).
Figure 1: Mass spectra of benzoic acid (122 Da) in acetonitrile using
ESI -ve ion mode, mass range 50-125 m/z.
Figure 2: Mass spectra of butylbenzoate (178 m/z) in acetonitrile
using ESI -ve ion mode with mass range 50-182 m/z.
Decanesulfonic acid: (Figure 3) MS/MS experiment was performed on a standard sulphur compound (1-decanesulfonic acid) in order to observe the fragmentation pattern of the precursor ion. In negative ion mode, the precursor of 1-decanesulfonic acid of molecular weight 222 (sodium salt peak with molecular mass of 244), showed a peak at m/z 221 in methanol, and the MS/MS experiment showed a peak at m/z 79.9, which corresponds to SO3.
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Figure 3: MS of 1-decanesulfonic acid in methanol using ESI -ve ion
mode in the mass range 60-225 m/z
MS and MS/MS of Naphthenic Acid Extract
A number of peaks in the range of 200 to 400 m/z were found for the ESI negative ion mode mass spectrum of naphthenic acid extract. The distribution appears very Gaussian with a maximum at around 300 and a spacing of 14 Da (due to –CH2) between most of the large peaks. MS and MS/MS spectrums are shown in Figures 4 and 5.
Figure 4: MS of Naphthenic acid extract in acetonitrile USING ESI -
ve ion mode, mass range 180-425 m/z in.
Figure 5: MS of Naphthenic acid extract in acetonitrile, ESI -ve ion
mode, mass range m/z 256-350.
We also performed MS/MS experiments on the naphthenic acid extract focusing on all of the more intense common peaks. The MS/MS of naphthenic acid extract peaks that were chosen for MS/MS experiments had a relative abundance of 40 and higher in the MS spectrum. The results showed mass losses of M-1, M-18, M-28, and M-44, which corresponds to a loss of hydrogen, water, carbon monoxide, and carbon dioxide, which are all indications for the presence of carboxylic acid functional group.
Figure 6: MS/MS of m/z 245.1 for naphthenic acid extract in acetonitrile using ESI -ve ion mode, mass range 100-248 m/z
Figure 7: MS/MS of m/z 287.1 for Naphthenic acid extract in
acetonitrile ESI -ve ion mode, mass range 100-290 m/z.
Figure 8: MS/MS of m/z 301.1 for Naphthenic acid extract in acetonitrile using ESI -ve ion mode, mass range 100-305 m/z.
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Figures 6, 7, 8, and 9, illustrate representative MS/MS spectra for peaks at 245, 287, 301, and 313 (found in the MS spectrum), which confirm that these are due to naphthenic acids as found by Rudzinski et al. (2002). There was a mass loss of M-34 and M-62 from the parent molecule which have not been identified. The M-1 hydrogen loss can be seen in each spectrum. The M-18 water loss was prominent and was found in all of the spectra. The mass loss of M-28 corresponding to carbon monoxide was small in some of the MS/MS spectra. All of the spectra exhibited a mass loss of M-44, which indicated the presence of –CO2 group.
Figure 9: MS/MS of Naphthenic acid (m/z 313.2) extract in
acetonitrile ESI -ve ion mode, mass range 100-317.
CONCLUSION Mass Spectrometry of the sample extract shows fragmentation peaks at 18, 28, and 44 which corresponds to water loss, carbon monoxide, and carbon dioxide loses respectively and these confirms the presence of carboxylic acids functional group which is an indication of the naphthenic acids in the extract as found by Rudzinski et al.,(2002).
RECOMMENDATIONS Further research should be done on the nature,
properties and reactions of the isolated acids with the view of producing usable naphthenic acid compounds as well as efficient ways of removing those acids from the refinery waste waters to avoid contamination since those acids are toxic to humans and the aquatic life.
Refineries should employ the isolation methods of naphthenic acids removal instead of caustic washing in which it’s not easy to recover the acids since those acids have a lot of commercial applications.
Polar and short chain naphthenic acids and other polar species might be more soluble in ethylene glycol than petroleum ether, so there was a possibility that the more polar species could be lost during the extraction process, therefore future research should be directed toward the recovery of those short chain and more polar naphthenic acids.
In order to solve the problem of unresolved signal with high biodegraded oils, lost of the heavier part of the sample in the column, etc. Government and petroleum industries should provide efficient instruments for the efficient isolation and characterization of naphthenic acid such as Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR MS) which can overcome some of these inconveniences and provides very detailed information about those acids.
CONFLICT OF INTEREST None declared.
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Article’s citation:
Rahman UA and Gayari MS (2016). Extraction and determination of naphthenic acid from Nigerian crude oil from Oki-Oziengbe south field by Tandem (ms/ms) mass spectroscopy. Ew J Petrochem Res Innov 1(1): 1-6.
