CRUDE OIL- Classification of Crude Oil
"Crude oil" is usually black or dark brown (although it may be yellowish, reddish, or even greenish). Paraffin base, Naphthene base, asphalt base or mixed base are the "classification of Crude oil". In the reservoir it is usually found in association with natural gas, which being lighter forms a "gas cap" over the petroleum, and saline water which, being heavier than most forms of Crude oil, generally sinks beneath it. "Crude oil" may also be found in a semi-solid form mixed with sand and water. Distillation is used to separate the "Crude oil" into fractions according to boiling point. The crude unit is the first processing unit in the refinery to separate the "Crude oils".
COMPOSITION OF CRUDE OIL
Crude oils are composed of critical homologous series of hydrocarbon. The hydrocarbons present in the crude petroleum are classified into general types-1.1 Paraffin’s
When carbon atom is connected to single bond and other bond are saturated with hydrogen atom.1.2 Olefins
Olefins do not naturally occur in crude oil but are formed during the processing.1.3 Naphthenes
Naphthenes is also known as Cycloparaffins. Cycloparaffin hydrocarbon in which all of the available bonds of the carbon atoms are saturated with hydrogen are called naphthenes.1.4 Aromatics
The aromatics series of hydrocarbon contain a benzene ring which is unsaturated but very stable and frequently behaves as a saturated compound.CLASSIFICATION OF CRUDE OIL
"Crude oils" are classified as paraffin base, Naphthene base, asphalt base or mixed base. The U.S Bureau of mines has developed a system which classifies the crude according to two key fraction obtained in distillation: No 1 from 482 to 527 oF (250 to 275 oC) at atmospheric pressure and No 2 from 527 to 572 oF (275 to 300 oC) at 40 mmHg pressure.
The gravity of these two fractions is used to classify crude oils into types a shown below:
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The more useful properties are-
3.1 API Gravity
The density of petroleum oils is expressed in the United States in terms of API gravity rather than specific gravity. API is inversely proportional to the specific gravity.
The units of API gravity are oAPI and the relation between API and specific gravity is shown in equation.
In above equation, specific gravity and API gravity refer to the weight per unit volume at 60 oF as compared to water at 60 oF.
3.2 Sulfur Content, wt%
The sulfur content is expressed as percent sulfur by weight and varies from less than 0.1% to greater than 5%. Sulfur content is one of the properties that effect the crude oil prices.
Crude with greater than 0.5% sulfur is more expensive and refers to sour crude oil whereas crude with less than 0.5% sulfur refers to sweet crude.
3.3 Pour point oF or oC
The pour point of the crude oil is a rough indicator of the relative paraffinicity and aromaticity of the crude. The lower the pour point, the lower the paraffin content and the greater the content of aromatics.
3.4 Carbon Residue, wt%
The carbon residue is roughly related to the asphalt content of the crude. And to the quantity of the lubricating oil fraction. Lower the carbon residue, the more valuable the crude.
3.5 Salt content, lb/1000bbl
Crude oil passes through the desalter before going in the Atmospheric distillation if the salt content in the crude is greater than 10lb/ 1000bbl.
Corrosion problem may be encountered, if the salt is not removed. The unit in which salt content measure is PTB.
3.6 Characterization Factors
The Watson characterization factor ranges from less than 10 for highly aromatic materials and 15 for highly paraffinic compounds. Kw vary from 10.5 for a highly naphthenic crude to 12.9 for a paraffinic base crude.
Formula used to calculate the Watson characteristic is given below-
The correlation index is useful in evaluating individual fraction from crude oil. The CI scale is based upon straight-chain paraffins having a CI value of 0 and benzene having a CI value of 100.
Lower the value of CI, the greater the concentration of paraffin hydrocarbon in the fraction, and the higher the CI value, the greater the concentration of naphthenes and aromatics.
3.7 RVP
Reid vapor pressure is approximately the vapor pressure of gasoline at 100 oF3.8 Octane number
Octane number is defined as percentage volume of Iso-octane (2,2,4-trimethyl pentane) in a mixture of iso-octane and n-heptane that gives the same knocking charactristic as the fuel under consideration.3.9 RON
It is research method which represents the performance during city driving when acceleration is relatively frequent3.10 MON
It is Motor method which is the guide to engine performance on the highway or under heavy load condition3.11 Sensitivity
The difference between the research and motor octane number. Low sensitivity is better.
3.12 PON
Posted octane number is arithmetic average of the research and motor octane number.
3.13 Wax Content
The waxes present in most crude oils include n-alkanes, iso-alkanes, alkyl cyclic compounds and alkyl aromatics. In most crude n-alkanes are the predominant species.
There is no standard definition for wax content but it is generally accepted that n-alkanes from C18 to C40 represent waxy material.
Waxy crude oils are highly non-Newtonian materials known to cause handling and pipelining difficulties and whose flow properties are time and history dependent. https://chemengineering1.blogspot.com/2018/10/blog-post.html
3.14 Aniline point
The minimum temperature at which equal volumes of anhydrous aniline and oil mix together. High aniline point indicates that the fuel is highly paraffinic and hence has a high diesel index and very good ignition quality.
3.15 Asphaltenes content
Asphaltenes are composed mainly of polyaromatic carbon ring units with oxygen, nitrogen, and sulfur heteroatoms, combined with trace amounts of heavy metals, particularly chelated vanadium and nickel, and aliphatic side chains of various lengths.
Asphaltenes are defined operationally as the n-heptane (C7H16)-insoluble, toluene soluble component of a carbonaceous material such as crude oil, bitumen, or coal.
3.16 Kinematic Viscosity
Viscosity is a measure of a fluid’s resistance to flow. The term “kinematic” means that the measurement is made while fluid is flowing under the force of gravity. The kinematic viscosity of a fluid is measured in centiStokes.
PROCESS INVOLVED
Crude is heated in the furnace and charged to distillation column where it is separated into butane's and light wet gas is come out from the top and side stream product is come out from the distillation column at different temperature cut.
First cut is naphtha and this naphtha is light also known as light straight run naphtha (LSR). Second cut is heavy straight run naphtha (HSR). Next cut is 380-520 oF which is kerosene. Similarly 520-650 oF , 650-800oF , 800-1000 oF and 1000+ oF cuts are for Light gas oil (LGO), Heavy gas oil (HGO), Vacuum gas oil (VGO) and Vacuum reduced crude (VRC) respectively.
Evaluation of API at different temperature cut
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At 482 to 527 oF (250 to 275 oC) at atmospheric pressure the API is 34.2 oAPI and at 527 to 572 oF (275 to 300 oC) at 40 mmHg pressure, the API is 30 oAPI therefore from Table 1 Gravity of two fraction to classify crude oil” it’s seem that crude oil is Intermediate, paraffin.
Above Figure shown the TBP and mid-point curve. LSR (C5-190 oF) cut have higher yield than Vacuum residue crude (VRC -1050+ oF). The API gravity increases with
the yield. API is inversely proportional to the specific gravity. Higher the API, more will be lighter components. Lighter cuts have higher yields.
APPENDIX -Tests Methods and Apparatus
Table Properties, ASTM methods and apparatus information
PROPERTY
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ASTM METHOD
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APPARATUS
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Density
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D4052, D70
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Digital density meter KYOTO-KEM, Pycnometer
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Pour Point
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D5949, D5853
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Cold flow property analyzer PHASE TECHNOLOGY
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Water Content
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D4006, D4928
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Dean and Stark distillation apparatus, KF Coulometer METROHM
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Salt Content
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D3230
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Salt-in-Crude analyzer KOEHLER
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Asphaltene Content
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D3279
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Automatic asphaltene analyzer COSMO TRADE & SERVICE
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Wax Content
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Manual method
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Reid Vapor Pressure
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D323
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Automated Reid vapor pressure tester WALTER HERZOG GmbH
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Carbon Residue
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D4530
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Micro carbon residue tester
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Kinematic Viscosity
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D445, D2170
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Viscometers CANNON and viscometer holders KOEHLER
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Flash Point
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D93
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Pensky-Martens closed cup flash point tester TANAKA
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Aniline Point
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D611
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Automatic aniline point tester TANAKA
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