Evaluation and Characterization Parameters of Crude Oils and Petroleum Fractions

Characterization of crude oil has always been an area of interest in the refining field; However, the need to define the properties of crude oil has gained importance in the production and distribution processes, and this is done simply by measuring the properties of crude oil, as the properties of these properties change according to the geological nature in which the oil is found in the traps and thus the physical and chemical properties must be studied and defined for this type Oil or so, from these physical and chemical properties such as API° (American Petroleum Institute), specific weight, pour point, viscosity, total sulfur content, vapor pressure, distillation, initial boiling point (IBP), final boiling point (FBP), were evaluated. The hydrocarbon residue and contents of crude oils were collected from the various oil fields in the Masala, Sarir, and Al Fountain fields using standard ASTM procedures. The results of crude oils in the three oil fields were compared with each other and with other international crude oils. The standards and specifications of their petroleum products were also examined. This study was conducted on the crude oil of the obelisk, bed, and fountain in 2017 that are mixed together to feed the oil refinery in Tobruk. The standards and specifications of the mixture and petroleum products of the refinery, including light Naphtha, heavy Naphtha, Kerosene, and Diesel, were measured and compared with other types of crude oil. According to the evaluation criteria, the examined crude oils can be classified as light sweet crude oil due to the high API value and low sulfur content in it, the percentage of Diesel oil is low and the (K) factor was low. It was also found that light crude oil has a high percentage of light fracture and that the pour point of light crude oil is higher than that of heavy crude oil. The salt content was also shown to be low in the mixture compared to other types. An increase in the boiling point of the distillate was observed with an increase in the percentage of the fraction volume. Moreover, Diesel has a higher boiling point than kerosene which has a higher boiling point than naphtha for all of the combined fractions. It was also found that the water content had few effects on the crude oil.


Introduction
Petroleum (or crude oil) is a complex, naturally occurring liquid mixture containing mostly hydrocarbons, but containing also some compounds of oxygen, nitrogen and sulfur.It is often referred to as the "black gold".
After World War II, the huge oil reserves in the Middle East became available, at a very low cost, and they rapidly revolutionized the way we live.Indeed, the twentieth century with all the dramatic changes that it has brought to society is probably best characterized as the century of oil.For the foreseeable future, oil will remain a critical fuel for the all industrialized nations.
Most of the world's petroleum is to be found in the Middle East, as shown in    (Mendez et al., 2006)

Crude Oil Characterization
Crude oil characterization is one of the most important tasks in refinery units.This becomes more important when the cut has species with more than 10 carbon atoms.In this case, because of more hydrocarbons which have similar properties, it is difficult to characterize them.To solve this problem some researchers have tried to improve the analyses and databanks.
Oil refining is one of the most complex chemical industries, which involves many different and complicated processes with various possible connections.Figure (3) illustration of a standard refinery system (Conaway, 1999).Table (1) depicts the main characteristics of crude oil and their classification on the basis of some parameters.
Table 1.Main characteristics of crude oil and their classification on the basis of some parameters (Habson, 1984) 1 API Gravity= (141.5/Specificgravity) -131.5.Thus water (SG = 1.0) is 10 °API; petroleum lighter than water °API > 10. 2 For comparison, the viscosity of water at 20°C is 1 cP. 3 Volume factor is the ratio of volume in surface storage tanks to volume in the producing formation. 4GOR= cubic feet of gas per barrel of stock -tank oil.The ratios shown conflate R and Rs. 5 Sweet crudes have S < 0.5%; sour crudes have S > 0.5%.Sweeter crudes have greater economic value. 6"Bitumen" is used in its narrower sense here; sometimes "bitumen" is used to denote all petroleum. 7Langenkamp (1984,1985) puts the boundary between heavy and light oils at 20 °API; Miles (1989) puts it at 25 °API.
Numerous important feed and product characterization properties in refinery engineering include: 2.1.API gravity API gravity of petroleum fractions is a measure of density of the stream.Usually measured at 60 °F the API gravity is expressed as; where specific gravity is measured at 60 °F.According to the above expression, 10 o API gravity indicates a specific gravity of 1 (equivalent to water specific gravity).In other words, higher values of °API gravity indicate lower specific gravity and therefore lighter crude oils or refinery products and vice-versa (Kite et al., 1993).As far as crude oil is concerned, lighter API gravity value is desired as more amount of gas fraction, naphtha and gas oils can be produced from the lighter crude oil than with the heavier crude oil (Yeung, 2006).Therefore, crude oil with high values of API gravity are expensive to procure due to their quality.API gravity or density or relative density: can be determined using hydrometer method (ASTM D-287) (Kramer, 2004;and Lambert, 2007).

Watson Characterization Factor
The Watson characterization factor is usually expressed as; where TB is the average boiling point in o R taken from five temperatures corresponding to 10, 30, 50, 70, and 90 vol.% vaporized.Typically, Watson characterization factor varies between 10.5 and 13 for various crude streams (Speight, 1999).A highly paraffinic crude typically possesses a K factor of 13.On the other hand, a highly naphthenic crude possesses a K factor of 10.5.Therefore, Watson characterization factor can be used to judge upon the quality of the crude oil in terms of the dominance of the paraffinic or naphthenic components.It is determined by the standard distillation method (ASTM D-86, ASTM D-1160) (Speight, 2001).

Sulfur Content
Since crude oil is obtained from petroleum reservoirs, sulphur is present in the crude oil.Usually, crude oil has both organic and inorganic sulphur in which the inorganic sulphur dominates the composition.Typically, crude oils with high sulphur content are termed as sour crude.On the other hand, crude oils with low sulphur content are termed as sweet crude.Typically, crude oil sulphur content consists of 0.5-5 wt.% of sulphur.Crudes with sulphur content lower than 0.5 wt.% are termed as sweet crudes.It is estimated that about 80% of world crude oil reserves are sour.Doctor test measures the amount of sulfur (ASTM D-129) (Speight, 2001).

Viscosity Index
It is a measure of the flow properties of the refinery stream.Typically, in the refining industry, viscosity is measured in terms of centistokes (termed as cSt) or saybolt seconds or redwood seconds.Usually, the viscosity measurements are carried out at 100 °F and 210 °F.Viscosity is a very important property for the heavy products obtained from the crude oil.The viscosity acts as an important characterization property in the blending units associated to heavy products such as bunker fuel (Riazi and Al-Sahhaf, 2006).Typically, viscosity of these products is specified to be within a specified range and this is achieved by adjusting the viscosities of the streams entering the blending unit.It is estimated using (ASTM D-445) method (Speight and Ozum, 2002).

Flash and Fire Point
Flash and fire point are important properties that are relevant to the safety and transmission of refinery products.Flash point is the temperature above which the product flashes forming a mixture capable of inducing ignition with air.Fire point is the temperature well above the flash point where the product could catch fire.These two important properties are always taken care in the day to day operation of a refinery.The method used to determine the flash point and fire point are (ASTM D-56, ASTM D-93, and ASTM D-3828) (ASTM, 2000).

Pour Point
When a petroleum product is cooled, first a cloudy appearance of the product occurs at a certain temperature.This temperature is termed as the cloud point.Upon further cooling, the product will ceases to flow at a temperature.This temperature is termed as the pour point.Both pour and cloud points are important properties of the product streams as far as heavier products are concerned.For heavier products, they are specified in a desired range and this is achieved by blending appropriate amounts of lighter intermediate products.The test used to determine the pour point is (ASTM D-97, IP 15) (ASTM, 2000).

Octane Number
Though irrelevant to the crude oil stream, the octane number is an important property for many intermediate streams that undergo blending later on to produce automotive gasoline, Diesel … etc. (Stratier et al., 2010).Typically, gasoline tends to knock the engines.The knocking tendency of the gasoline is defined in terms of the maximum compression ratio of the engine at which the knock occurs.Therefore, high quality gasoline will tend to knock at higher compression ratios and vice versa.However, for comparative purpose, still one needs to have a pure component whose compression ratio is known for knocking.Iso-octane is eventually considered as the barometer for octane number comparison.While iso-octane was given an octane number of 100, n-heptane is given a scale of 0. Therefore, the octane number of a fuel is equivalent to a mixture of an iso-octane and n-heptane that provides the same compression ratio in a fuel engine.Thus an octane number of 80 indicates that the fuel is equivalent to the performance characteristics in a fuel engine fed with 80 vol.% of isooctane and 20% of n-heptane.

TBP/ASTM Distillation Curves
The most important characterization properties of the crude/intermediate/product streams are the TBP/ASTM distillation curves.Both these distillation curves are measured at 1 atm.pressure.In both these cases, the boiling points of various volume fractions are being measured.However, the basic difference between TBP curve and ASTM distillation curve is that while TBP curve is measured using batch distillation apparatus consisting of no less than 100 trays and very high reflux ratio, the ASTM distillation is measured in a single stage apparatus without any reflux.Therefore, the ASTM does not indicate a good separation of various components and indicates the operation of the laboratory setup far away from the equilibrium (McCann, 1998).

Material and Methods
The physiochemical properties of crude oil and petroleum products were determined through the data of tests and chemical analysis that have been carried out on the Messla and Sarir crude oils samples and petroleum products of Tobruk refinery using different analytical techniques in central laboratory of the refinery.

Results and Discussion
Refinery processing is designed to process a number of different crude oils including Messla, Sarir mix and Nafora crude oils.The more important properties of these crude oils are listed in Table (2).
The characterization properties of these crude oils are summarized in Table (2) and compared with other crude oils in the world.The data in Table (2) show that the API gravity of light crude is above 30°, such as Messla and Sarir mix as well as Nafora, MAD, BR, AL and Kuw crudes, while that of heavy crude is less than 30° such as AM and AH crudes.These API gravity values agree with publish data.Furthermore, as expressed the pour point of light crude oil is higher than that of heavy crude oil.The data in Table (3) is expressed graphically in Figures.
(5-7), that give a comparing between the characterization properties of Messla-Sarir mix, Nafora and the other crude oils.The figures reveal that Messla-Sarir mix has an API gravity similar with that of Brent blend crude oil and less than Murban crude oil as well as higher than Arab medium, Arab heavy and Kuwait crude oils.The Kinematics viscosity shows variable values for the different crude oils where the highest value of Arab heavy crude oil and the lowest value of Murban crude oil.On the other hand, Nafora crude oil is similar with that of Arab light.The other parameters such as specific gravity, cetane index, smoke point, acid number and aniline point show no wide variation between them.The sulfur content in Messla-Sarir and Nafora crude oils is lowest value (0.16 & 0.27) while the highest value was recorded in Arab medium crude oil (2.85).The pour point of Messla-Sarir mix and Nafora show the lowest value (-13 and -20℉) and the highest value of Arab heavy crude oil.The data in Table ( 4) of petroleum products of refinery represented Kerosene, Naphtha and Diesel that have different characterization properties, this may attributed to the differences in their chemical characteristics.The API gravity of Kerosene is higher than Diesel (42.5 and 35.95 respectively), this is the vice versa of flash point (35.0 and 90.0 respectively).The sulfur content in Kerosene (0.005) is very low if compared with Diesel (0.07).On the other hand, they have a similar content of acid (0.008).Figures (8 & 9) illustrate the different characterization properties of petroleum products.The data in Table ( 5) are expressed graphically in Figure ( 10) which gives a plot of the true boiling points (TBP) of the distillate fractions versus the percentage cumulative volume of the crude oil.In the petroleum refinery the physical and chemical operations are integrated, where atmospheric distillation process involved.Thus the products are expected to show a wider range in their characteristics.However, our present results show that the product distribution of atmospheric fractional distillation of Messla-Sarir mix is a function of the nature of the crude oils.
The distillation profiles (Figure 12) of the investigated petroleum products also show the same trend.The boiling point of the distillate fractions increases as the volume percent of the fraction increases.Moreover, as expected Diesel has a higher boiling point than kerosene which has a higher boiling point than naphtha for all the fractions collected.

Evaluation Parameters of Crude Oil
There are different parameters can have been applied to evaluate the quality of the crude oils.The most important of them including °API, the factor governing the quality of crude oils.The results from the investigated Sarir crude oils reveal an average value of °API is 38.9.According to this value crude oil can be classified as light crude oil.
The content of sulfur plays an important role for the quality of the crude oil.Generally, it ranges from 0.1% to 5% by weight percent.Crude oil is classified as sweet when sulfur content less than 0.5% and sour if the percent exceeds this value.The concentration of sulfur in the studied crude oils of Messla-Sarir and Nafora are 0.16 and 0.27 respectively, thus it can be classified as sweet crude oil (Table 2).
The pour point of petroleum is an index of the lowest temperature at which the crude oil will flow under specified conditions.The pour point of the examined crude oils shows that both paraffinic and aromatic more or less are similar content, where the average of the pour point is -13°F and -20 °F.
The Characterization Factor was originally devised to show the thermal cracking characteristics of heavy oil.Thus, highly paraffinic oils have K= 12.5-13.0and cyclic (naphthenic) oils have K= 10.5-12.5.The average value of characterization factor for the investigated crude oil is 10.90 and 11.2, these means that the crude oils is naphthenic.
The crude oil of naphthenic base exhibit a wide variation of viscosity index (-60-+40), while the paraffinic base has high viscosity index (79-151), and not affected so much by temperature degree.The intermediate crude oils show viscosity index ranging from 20-98.The viscosity index of Messla-Sarir and Nafora crude oils show low values (24 & 25.2) and this may be attributed to naphthenic base.

Conclusion
The following conclusions may be drawn up as a result of the carried out investigation: The characterization properties of Messla-Sarir mix crude oils are compared with other crude oils in the world.The results reveal that the °API gravity of light crude is above 30°, such as Messla-Sarir mix, Nafora, MAD, BR, AL and Kuw crudes, while that of heavy crude is less than 30° such as AM and AH crudes.The pour point of light crude oil is higher than that of heavy crude oil.
Messla-Sarir mix has a gravity °API similar with that of Brent blend crude oil and less than Murban crude oil as well as higher than Arab medium, Arab heavy and Kuwait crude oils.The Kinematics viscosity shows variable values for the different crude oils.
Figure (1) also illustrates the fact that the world reserves and resources of crude oil are orders of magnitude smaller than those of coal.

Figure ( 4
Figure (4)  presents the general trend of World crude oil quality through the period between 1980 and 2013.It is clear that API gravity decreases while the sulphur and metal contents increase.

Figure 11 .
Figure 11.Volume percent of crude oil fractions versus true boiling point

Figure 12 .
Figure 12.Distillation profiles of petroleum products On the other hand, the petroleum products of the mix crude oils have been compared with the other petroleum products as shown in Figure (13), which shows more or less similarity with that of Brent blend and Arab light.

Table 5 .
Products of distillation of crude oil