Inte rnatio nal Jo urnal o f Sc ie ntific & Eng inee ring Re se arc h, Vo lume 3, Issue 2, February -2012 1
ISS N 2229-5518
Trace Elemental Analysis of Nigerian Petroleum
Products Using AAS Method
Nafi’u Tijjani, P.O Ike, B.B Usman, D. I. Malami and Alaere Matholo
Abs tract - Some physico-chemical analysis of samples of Gasoline, Kerosene, Diesel Oil, Low Pour Point Oil and Residual f uel have been studied. The samples w ere also analyzed f or trace elements. The results conf irmed that Heavy distillates f ractions(Residual f uel and LPFO) contains trace metals higher than the distillates f raction(Automobile Gas Oil or Diesel) f ollow ed by intermediates (Dual Purpose Kerosene) and lights ends f ractions(Premiu m Motor Sprit / Gasoline). The samples w ere analyzed by gravimetric and micro-kjeldahl methods f or sulphur and nitrogen content respectively. The experiment data indicated that, the total sulphur and nitrogen contents increased from light ends, intermediates, distillates to heavy distillates fractions and also the concentrations of the trace metals in the petroleum products f ollow s the same trend i.e. from less viscous petro leum products (Gasoline, Kerosene and Diesel Fuel) to high viscous petroleum products sample (residual f uel and LPFO) . The results w ere discussed in terms of eff ects of this impurities on the ref inery process.
Ke yword: Ashing Method, Pour Point, Specif ic Gravity, Sulphur Content, Trace metals, Gravimetric and micro-kjeldahl methods.
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processes line in the refinery. In a recent
study,Anthony,2005 reported in his comprehensive
Crude oil and its finished products (Petroleum Products)
contains mainly hydrocarbons especially alkanes, naphthenic and aromatics hydrocarbons[1]. It contains also some nitrogen, oxygen and sulphur containing compounds along with trace amounts of elements especially nickel, vanadium , cadmium, iron etc.[2,3] The presence of trace metals and non-metals in the crude oil and petroleum products is destructive, especially in a refining process. This may adversely affect both the processing equipment and storage facilities of the petroleum refinery. The presence of sulphur compound in the finished petroleum products influence the colour, odour, s tability and storage facilities of the finished petroleum products4.Various analytical methods have been reported for the determination of trace metals in petroleum products. Traces of iron, nickel, and vanadium in petroleum products were analyzed using spectrophotometry method. The sample was ashed and taking up with the potassium bisulphate. The measurement were based on the development of coloured solutions by reagents specific for each elements [5]. Henry and George, 1975 employed AAS to determined heavy metals in petroleum products [6]. They used two methods which are based on the decomposition and cold water vapour atomic adsorption. Another method involved acid decomposition of the samples in a closed system while the other method used oxy-hydrogen combustion to decompose the sample.
In another report, Winston and Harry,1975 used AAS to determine trace quantities of cadmium in petroleum and petroleum products in which the sample was digested with sulphuric acid and then ashed[7]. In another report, Oderinde,1984 determined the vanadium and titanium contents of nine Nigerian crude and petroleum products using a spectrometric method[8] He reported that in some of the samples the Vanadium and Titanium content were high enough to cause corrosion in turbines and refining
analysis of various metallic elements in Nigerian petroleum products using Atomic Absorption Spectroscopy technique, he discuss the influence of thes e trace metals contaminants in refinery processes[9].
Farroha et, al,1984 used electrochemical method to determine trace levels of sulphur in petroleum by constant current coulometry. The Tandem mass spectrometer combined with chemical reaction was used to concentrate sulphur containing poly-nuclear aromatic compound by wood et al,[9].Oderinde, 1984 thoroughly investigated the types of sulphur compounds present in Ugheli Quality Control Centre(UQCC) of crude oil distillates fractions. The total nitrogen was in petroleum fractions was determined using the modified Kjedahl method [13,14] The total nitrogen contents of the twenty eight Nigerian well head crudes and their fractions were quantitatively measured by a by coloumetric technique by Nwadinigwe and Maduka[14].
The Petroleum Products sample used in this research paper were Gasoline, Kerosene, Diesel Oil, LPFO and Residual Fuel obtained from Kaduna Refining and Petrochemical company limited, a subsidiary of NNPC, Nigeria.
All the chemicals and reagents used in this study were of analytical grades, the glass wares used were cleansed, rinsed with distilled water and air dried. Physicochemical parameters of petroleum products understudy;
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a) The density, Specific Gravity and API gravity were determined using ASTM method [17,18]. The specific gravity determination involves weighing (W1) a known volume of a sample in a density bottle at certain temperature and weighing W2 an equal volume of water in the same density bottle at the same temperature. The specific gravity is the ratio of the two weights W1/W2. The API gravity of the sample was then calculated by using the equation[18].
API = 141.5/S.G – 131.5 --------------------.(1)
Where S.G means Specific Gravity of the Sample.
b) Viscosity Determination follows the IP method[21]
.This involves measuring the time of flow of the sample between two marked points on the Ostwald viscometer and calculating the viscosity by using the standard equation.
c) The Aniline point determination involves heating sample in a test tube until the two phases become miscible. The mixture was then cool at a controlled rate and temperature was monitored. The minimum temperature at which the two phases separate was taken as the Aniline point [19]
d) The pour point determination involved heating the sample in a test followed by cooling in a cooling bath at a controlled rate. The pour point is the lowest temperature at which there is a loss of fluidity in the sample[20].
e) The water content determination follows the I.P
method by using the Dean and Stark apparatus [19]
.
The procedure involved Ashing 10g of the petroleum products sample already placed on a porcelain crucible at
500 0C in a muffle furnace for 1.5 hours. When the Ashing
was completed , the residue was gently wormed with 10 cm3 of 0.02 M Nitric acid to facilitate the complete extraction of the trace metals from the sample, a glass rod was used to stir and crushed all the acid soluble elements in the ashes. The slurry was then filtered into a 100 ml volumetric flask, and the residue was filtered into 100 ml volumetric flask, and the residue was further washed with
10ml of 0.02 M Nitric acid solution. This was then diluted
with distilled water to the mark. The solution was then analyzed using atomic absorption spectrophotometer[20,22]. The metal content of petroleum products were determined using 305B atomic absorption spectrophotometer attached to graphite atomizer and a HP
960C Computer Printer, the instrument determined the concentration of the trace metals in mg/l in the samples.
The procedure involved weighing 1.5g of the petroleum products sample into 500 ml round bottomed flask and adding 250ml of distilled water to it. Then 5g of solid KOH and 15g of solid KMnO4 were weighed and added to the mixture in the flask together with anti-bombing chips. The mixture was refluxed for 6 hours and allowed to cool then
150ml of the concentrated HCL was added slowly through the condenser. The flask was heated until its contents turned colourless, cooled and the condenser washed into the flask with a little distilled water. The content of the flask with a little distilled water. The content of the flask was collected into a 500ml beaker washed with distilled water and filtered into a beaker. The filtrate was boiled with 10ml of Concentrated HCL and 100ml of hot barium chloride with stirring. This resulted in deposition of a fine white precipitate of barium sulphate after heating the beaker on the water bath for 5hr. the precipitate was filtered and washed with hot distilled water to remove chloride ions. The filter paper containing the precipitate was deposited on a weighed small porcelain crucible and heated strongly on a hot plate until a white powder was left. A drop of concentrated HCL and a drop of concentrated H2SO4 were added and the crucible was reheated until a white powder was left
Mass of BaSO4 precipitated, mB (g) = (Mass of BaSO4 in the sample – mass of BaSO4 in the blank) Gravimetric factor of BaSO4 = 0.137
Mass of sulphur in BaSO4 precipitated (mS) = 0.137g × mass of mB
Mass of sampled analyzed = 1.5g
% sulphur = ms x 100 = 0.137 × mB x 100
1.5g 1.5g
powder was left. The crucible was cooled and weighed and the weight of barium sulphate determined by difference.
The procedure involved weighing 1g of petroleum product sample into a round bottomed flask. Then 5g of Na 2SO4 was added followed by the addition of 0.23g of CuSO4 and
12.5ml of concentrated H2SO4, the mixture was heated to a temperature below the boiling point of the acid until the digestion was completed and digest turned green and clear. The flask and the content were then allowed to cool and the solution transferred into a 250ml standard flask. A 5ml
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portion of each digest was pipetted into a micro-Kjeldahl unit and an excess of 40% aqueous NaOH was added to make the solution strongly alkaline and force ammonia out of the solution. The ammonia was distilled into 5ml of the prepared boric acid indicator in a titration flask containing about 45ml of distillate which contains ammonium borate. The borate anion was titrated with 0.01M HCL to
Blank : 0.06 0.18 0.86 0.12 0.09 0.04 0.26
PMS: 0.64 1.58 1.28 0.72 0.58 0.46 1.48
DPK: 0.26 0.48 1.04 0.28 0.18 0.11 1.28
AGO: 1.06 1.50 1.46 0.84 0.45 0.54 1.74
LP FO: 3.65 4.70 6.6 4.70 2.9 3.65 5.40
R/F: 3.25 4.20 4.8 3.9 2.4 3.10 4.75
4.Discussion
the analysis of physicochemical properties of petroleum
products are contained in Table 1. the data showed that the values of density, specific gravity, viscosity, kinematic
regenerate the bluish color of the boric acid at the end point.
viscosity of petroleum products assayed increase from light fractions to heavy fractions of petroleum distillates. The API gravity of light fractions is above 30 0 while that of heavy fractions is below 300. Furthermore, as expected, the pour point and flash point of light fractions is higher than that of heavy fractions. The data also show that Gasoline has a 0.01% water by volume, while Kerosene, Diesel and LPFO have 0.03, 0.05 and 0.08% water by volume
respectively. The higher pour point and flash point of LPFO and Diesel fuel over Gasoline and Kerosene is probably due to the higher water content in Diesel and LPFO.
The metal content of Nigerian petroleum products are shown in Table 2. The data showed that the metal contents increase on proceeding from light to heavy petroleum distillates (Gasoline > Kerosene > Diesel > LPFO > Residual fuel). It is observed from the results that the metal contents vary with the viscosity of the petroleum products. The nature of this metals and the abundance in petroleum products may probably provides information on the origin, migration and maturation of raw material of these petroleum products as well as indicating the regional geochemical prospecting base as well as the processing and storage channels in the refinery. As a rule, the metal content increase with viscosity of these petroleum products (i.e. the heavier the petroleum product the higher its metal contents) consequently, the lighter distillates contain less metal contents.
The metal contents of the Nigerian petroleum products in the literature show a wide variation. For exampl e Anthony,2005[23], imported iron content in the range of
10.94- 25.0 ppm while our results show the range of 0.25 –
4.75 mg/l. The wide variation is possibly due to the combination of several factors including the type of petroleum refining equipments (because some of them are made up of different percentage of metallic alloys), sample handling procedures and different analytical techniques adopted. The metal contents of Nigerian petroleum products are in general lower than those of petroleum products from other regions of the world. The report of Adolfo, et, al., 2007[24], for example show that iron contents of 12.39ppm , 725 mg/kg and 113.2 ppm for petroleum products from Central American Region (USA), Mexico and Venezuela respectively. However , even as this low levels, the metal contents of Nigerian petroleum products, and especially of vanadium, cadmium, cupper, and iron, is high enough to cause corrosion of turbines distillation towers, etc according to Oderinde,(1984). The data in Table 1 shows that the weight percentage of total sulphur in various
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petroleum products, the data fall within the range for sulphur expected in Nigerian crude oils and petroleum products, i.e. 0.05 – 0.30 w/%[4, 10, 25]. It can be seen from Table 1 that Gasoline, (ligh t ends fraction), Kerosene (Intermediate fraction), Diesel fuel (Distillates), LPFO and Residual fuel (heavy distillates) contain 0.03, 0.04, 0.16 and
0.41 W/% sulphur respectively.
The percentage nitrogen in the sample is calculated by
using the equation:
% Nitrogen in the sample = (0.01 X 0.014)g N2 x 250 x
100 x t 
5 W
Where t= titre value of 0.01M HCL used in the titration . w= weight (g) of the sample taken for the analysis
The experimental data in Table 2 show the weight percentage of total nitrogen in various spetroleum products. These value falls within the range of nitrogen normally reported for Nigerian crude oils and petroleum products. Furthermore as expected, the weight percentage of nitrogen of heavy LPFO and residual fuel is higher than that of gasoline, kerosene and diesel oil. This difference
may depend on the source, process, conditions and the final treatment of the petroleum products in the refining industry.
The nitrogenous compounds in petroleum products are
important impurities because they have deleterious effects on the quality of finished products and the environments. They poison cracking and reform catalysts during petroleum processing, cause instability of finished products, promote sediment and gum formation and impact color and odour. The combustion of liquids fuels containing nitrogenous compounds produces oxides of nitrogen which are the potential health hazards. Nwadinigwe and Maduka carried out a detailed analysis of total, basic and non basic nitrogen contents of 28 Nigerian well head crude’s and their fractions. Our data for petroleum products agree reasonably well with Nwadinigwe and Maduka’s data for petroleum distillate fractions showing the necessity for denitrogenation.
This work showed that Nigerian petroleum products contains trace amount of Cu, Zn, Mn, Si, V, Mo, Fe . The presence or absence of some elements in Nigerian petroleum products may be link to the chemical and geological origin of its raw materials and also industrial process and the transportation method adopted. Hence the concentrations of trace and heavy elements in Nigerian petroleum products may reveal information on the environmental friendliness and quality of petroleum products and probably give an idea of the sources of these trace heavy metals in the petroleum industry. It was
obvious from this study, the Nigerian petroleum products analyzed have low metals contents especially in petrol, kerosene and gas oil sample. However, despite the low concentrations, they could still lead to serious health hazard considering their cumulative effects in the environment.
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