Characterization of Petroleum Crude Oils by Fourier Transform Infrared (FT-IR) and Gas Chromatography-Mass Spectrometerys

Samples Collection and Physico-Chemical Characterization

Three indigenous crude oils were used in current investigation and coded as:

1. Light crude denoted as LC, Pariwali, oil field
2. Medium crude denoted as MC, Naspha oil field
3. Heavy crude denoted as HC, Turkawali oil field.

The as received oils were characterized by ASTM methods [22]. The properties determined were:

Physico-Chemical Properties:

1. Density; ASTM D 1480
2. API gravity; ASTM D 4052
3. Conradson carbon residue; ASTM D 189 – 05
4. kinematic viscosity; ASTM 445
5. pour point; ASTM D 97 – 05
6. flash point; ASTM D 93
7. aniline point; ASTM-D 611-04
8. Whatson characterization constant. Elemental Analysis

FTIR and GC-MS Characterization

A double beam FTIR spectrophotometer was used in this study. In a typical analysis, potassium bromide (KBr) pellets were prepared by using analytical grade KBr. A small drop of the test oil was applied to the prepared KBr disc. The GC-MS analysis was carried with a gas chromatograph coupled with MS analyzer (GC-MSQP2010 Shimadzu).

Physico-Chemical Characterization

It is well documented elsewhere that the physical and chemical characteristics of a crude oil vary considerably with its composition [2]. The physico-chemical characterization of the crude oils understudy was carried out. The values for different physicochemical parameters obtained for the oil samples were; densities (0.76, 0.85,and 0.94 g/cm3); API gravities (54, 35, 19); viscosities (10, 8, and 14 mm2/s); CCR (0.98, 0.92, and 0.98 % wt); pour point (-12, -14, and -16oC ), flash point (96, 93, and 96oC) and Watson characterization constant (K) values( 12, 11 and 10),for LC, MC and HC, respectively. The elemental analysis were; C (85.44, 85.91 and 86.10 %), H(11.15, 11.30,and 11.55; O(1.85, 1.21, and 0.96), N (0.11, 0.14, and 0.15) and S (1.45, 1.44, and 1.24) for LC, MC and HC, respectively. It can be seen that the nitrogen and sulphur contents are mostly concentrated in the naphthenic and heavier fractions. The results indicate that the LC, MC, and HC oils have different physcio-chemical properties agreed to the values of published data and are found to be light, medium and heavy crudes, respectively [4, 23, 24, 25].

Structural analysis of petroleum based products is performed to know about fraction recognition, combustion properties, and efficiency of fuel and quality inspections. It also influence the quality of the marketable products derived from crude products [26]. It has been reported elsewhere that crude oils which have similar physical properties but possess widely varying chemical compositions [27]. The absorption bands of aliphatic C−H bonds, with additional bands originating from groups containing aromatics, oxygen, sulfur, and nitrogen, usually dominate the spectra of crude oils [28]. The FTIR of the oils understudy was carried out. The results are discussed as under: LC: The FTIR analysis of LC was performed. The data is given in Table 1. The clear larger size absorption peaks were observed at 2954 and 2822 cm–1 corresponded to the CH and CH2 stretching vibrations. The other well developed bands centered at 1446 and 1419 cm–1 corresponded to the CH, CH3 bending vibrations (deformations) were also observed. A medium intensity peak appeared at 727 cm–1 showed CH2 bending. Another medium intensity band at 1078 cm–1 observed evident the CH bending vibrations [29]. The appearance of these peaks reveals the paraffins characterized by CH, CH2 and CH3vibrations.The results exhibited bands appearing at 852 and 1647 cm–1 due to CH bending and C=C stretching, respectively [30]. A medium peak exhibited at 1725 at cm–1 is indicative of carbonyl compounds i.e. ketone. Similarly, a small band observed at 3550 cm–1 due to O-H bond stretching vibrations confirmed the alcoholic contents of the sample under study. It is clear from the results that the intensities of the bands appeared for paraffins were found to be high which reveals its high concentration in the sample under test. On the other hand band, the peaks observed for aromatics/olefins and oxygenated compounds have poor intensities which indicate their presence in low concentrations. It may be concluded from the results that the LC possess long chain paraffins as the major constituents and the aromatics/olefins and oxygenated compounds as the minor components.

Table 1: FTIR analysis of LC: Major Absorption Peaks Observed. MC: The FTIR analysis of MC was carried out and data have been presented in Table 2. Some distinct bands appeared at 2922 and 2852 cm–1 characterizing the CH and CH2 stretching vibrations. Peaks observed at 1456 and 1377 cm–1 corresponded to the CH, CH3 bending vibrations (deformations) were evident. A moderate intensity peak at 727 cm–1 indicated CH2 bending while a peak at 1078 cm–1 showed CH bending vibrations. The observation of these bands implies the existence of paraffins which evidence CH, CH2 and CH3vibrations. The aromatics/olefins as constituent of the oil under test was ensured by the appearance of some medium intensity peaks centered at 790 and 1580 cm–1 due to CH bending and C=C stretching, respectively. A small peak exhibited at 1715 cm–1 corresponded to ketone as carbonyl compound. For alcoholic contents, a small intensity peak can be observed at 3525 cm–1 which is due to O-H bond stretching vibrations.

Table 2: FTIR analysis of MC: Major Absorption Peaks Observed. HC: The results of FTIR analysis of HC are compiled in Table 3. Some strong intensity bands were observed at 2922 and 2852 cm–1which indicate the CH and CH2 stretching vibrations. Few peaks noticed at 1456 and 1373 cm–1 corresponded to the CH, CH3 bending vibrations (deformations). A medium intensity band at 740 cm–1 revealed CH2 bending and another medium intensity peak at 1078 cm–1 show CH bending vibrations. The appearance of these peaks indicates the presence of paraffins which represent CH, CH2 and CH3 vibrations. The aromatics/olefins contents of the oil under test was verified by the appearance of some medium intensity peaks centered at 795 and 1584 cm–1which can be attributed to CH bending and C=C stretching, respectively. At 1715 cm–1, a small peak evidenced the presence of ketone. A band at 3525 cm–1 due O-H bond stretching vibrations can be seen which evident the presence of oxygenates. The results revealed some larger intensity bands characterized the paraffin content in high concentration of the oils. Conversely, the smaller intensity peaks for aromatics/olefins and oxygenate content accounted for their low concentration. Thus, it may be inferred that the crude understudy contained long chain paraffins as the major components and the aromatics/olefins and oxygenate as the least concentrated components.

The chemical and physical properties of petroleum are directly related to its chemical composition, which determines its market value [31]. The GC-MS distinguishes and characterizes crude oil sample according to their compound classes [32]. The chemical compositions in terms of paraffins, naphtnenes, aromatics, oxygenates and olefins (PNAOO) of the three Very few olefins recognized to be present included 2,3,4- trimethyl-2-pentene, 11-chloro-1-undecene. The aromatics compounds verified in the crude oil mostly included alkyl derivatives of benzene. Some oxygenated hydrocarbons were also present which included alcohol and ketonic derivatives of paraffinic and aromatic hydrocarbons. The GC-MS data of MC reveal similar results as obtained for LC. Compared with the main NIST library, most of the paraffins verified were found to be polyalkyl derivative of aliphatic hydrocarbons with carbon atoms from 6 to as high as 30 per molecule. Abundantly occurring paraffins were alkylated, hexane, heptane, octane, nonane and decane. The naphthenic hydrocarbon identified commonly mono, di or tri alkylcyclohexane. Among the olefins, the specific compounds identified were found to be 2, 3, 4-trimethyl-2-pentene, 11-chloro-1- undecene. The aromatics identified were solely the alkyl benzene derivatives. A number of largely occurring of oxygenates, including hydroxy and ketonic compounds were also identified. From the GC-MS data of HC, the major paraffins verified were found to be alkylated aliphatic hydrocarbons containing 7 to as high as 30 carbon atoms per molecules. Mostly these compounds included alkyl derivatives of hexane, heptane, octane, nonane, decane, eicosane, henicosane, docosane, tricosane, tetracosane, pentacosane, hexa cosine, octacosane and triacontane. The naphthenes confirmed were found to be di and tri alkyl cyclohexane. Among the olefins the specific compounds identified were found to be 2,3,4-trimethyl-2- pentene, 11-chloro-1-undecene. The aromatics identified were; sec-butyl benzene, 1-methyl, 2-propyl benzene, 1-4- diethyl benzene, butane, 2-phenyl, 3-hydroxy, 4-cyano, (1, 1-dimethyl propyl benzene), (3, 3-dimethyl butyl) benzene. The oxygenates triasterianone, 3, 3-dimethyl 2- hexane-one, 2-butyl octanol. In case of original HC, the concentrations of paraffins, naphthenes, aromatic, oxygenates and olefins were found. The relative abundance of the hydrocarbons types were calculated. The results of LC are provided in Table 4. The proportions of the paraffins, naphthenes, aromatic, oxygenates and olefins were found to be 85.67, 6.13, 6.16, 1.43 and 0.73 %, respectively. The carbon range hydrocarbons were found to be C5-C10, C11-C13, C14-C18 andC5-C10 in concentration of 25.26, 58.81, 6.21 and 9.72 %, respectively (Table 5). The relative abundance of hydrocarbons types i.e. paraffins, olefins, naphthenes and aromatics were calculated in case of MC. The results are provided in Table 4. The proportions of paraffins, naphthenes, aromatic, oxygenates and olefins were found to be 63.45, 26.20, 7.00, 2.70 and 0.90 %, respectively. The different carbon range hydrocarbons determined were C5-C10, C11-C13, C14-C18 and C19-Cn in concentrations of 74.40, 16.92, 4.63 and 4.07 %, respectively. The distribution of the different hydrocarbons range compounds calculated from GC-MS data determined in case of HC is given in Table 4. The concentrations of paraffins, naphthenes, aromatic, oxygenates and olefins were found to be 64.52, 14.67, 7.59, 2.70 and 12.08 and 1.14 %, respectively. The different carbon range hydrocarbons were i.e. C5-C10 (61.52 %), C11-C13 (18.69), C14-C18 (8.12 %) and C19-Cn (11.67) as per results compiled in the Table 5.

It is inferred from the results that all of the three crude oils studied have quite similar chemical compositions in terms of various individual hydrocarbons. However, they differ in paraffinic, olefinic, naphthenic, and aromatic group types with varying carbon range distributions. The relative abundance of saturated hydrocarbons suggests that the oils are predominantly aliphatic. LC is dominated by C11–C13, MC by C5-C10 while HC also by C5-C10.