Crude Oil Properties & Classification Methods

Crude Oil Properties & Classification Methods
?Crude Oil Properties:
Crude petroleum is very complex and, except for the low-boiling components,no attempt is made by the refiner to analyze for the pure components containedin the crude oil. Relatively simple analytical tests are run on the crude and the
results of these are used with empirical correlations to evaluate the crude oils asfeedstocks for the particular refinery.
Each crude is compared with the other feedstocksavailable and, based upon the operating cost and product realization, is
assigned a value. The more useful properties are discussed.
API Gravity
The density of petroleum oils is expressed in the United States in terms of APIgravity rather than specific gravity; it is related to specific gravity in such a fashionthat an increase in API gravity corresponds to a decrease in specific gravity.
The units of API gravity are °API and can be calculated from specific gravityby the following:



In this equation, specific gravity and API gravity refer to the weight perunit volume at 60°F as compared to water at 60°F.
API gravitiesare not linear and, therefore, cannot be averaged. For example, a gallon of30°API gravity hydrocarbons when mixed with a gallon of 40°API hydrocarbonswill not yield two gallons of 35°API hydrocarbons, but will give two gallons of
hydrocarbons with an API gravity different from 35°API. Specific gravities canbe averaged.
?Using the following formula, API gravity can also be used to calculate how many barrels of crude oil can be produced per metric ton.
Barrels of crude oil per metric ton= 1/(141.5/(131.5+API) x 0.159)
= 1/ (sp.gr x 0.159)
Example: How many barrels per metric ton of West Texas Intermediate crude oil (with an API of 39.6)?
A metric ton of West Texas Intermediate, with an API of 39.6, will produce 7.6 barrels (at 42 gallons each).
Carbon Residue, wt%
Carbon residue is determined by distillation to a coke residue in the absence of air. The carbon residue is roughly related to the asphalt content of the crude and to the quantity of the lubricating oil fraction that can be recovered.
In most cases the lower the carbon residue, the more valuable the crude. This is expressed in terms of the weight percent carbon residue by either the Ramsbottom (RCR) or Conradson (CCR) ASTM test procedures (D-524 and D-189).

Salt Content, lb/1000 bbl
If the salt content of the crude, when expressed as NaCl, is greater than 10 lb/1000 bbl, it is generally necessary to desalt the crude before processing. If the salt is not removed, severe corrosion problems may be encountered.
If residua are processed catalytically, desalting is desirable at even lower salt contents of the crude.
Although it is not possible to have an accurate conversion unit between lb/1000 bbl and ppm by weight because of the different densities of crude oils,1 lb/1000 bbl is approximately 3 ppm.

Nitrogen Content, wt%
Ahigh nitrogen content is undesirable in crude oils because organic nitrogen compounds cause severe poisoning of catalysts used in processing and cause corrosion problems such as hydrogen blistering.
Crudes containing nitrogen above 0.25% by weight require special processing to remove the nitrogen.

Metals Content, ppm
The metals content of crude oils can vary from a few parts per million to more than 1000 ppm and, in spite of their relatively low concentrations, are of considerable importance.
Minute quantities of some of these metals (nickel, vanadium,
And copper) can severely affect the activities of catalysts and result in a lower value product distribution.
Vanadium concentrations above 2 ppm in fuel oils can lead to severe corrosion to turbine blades and deterioration of refractory furnace linings.
Distillation concentrates the metallic constituents of crude in the residues,but some of the organometallic compounds are actually volatilized at refinery distillation temperatures and appear in the higher-boiling distillates.
The metallic content may be reduced by solvent extraction with propane or similar solvents as the organometallic compounds are precipitated with the asphaltenes and resins.

Sulfur Content, wt%
The sulfur content is expressed as percent sulfur byweight and varies from less than 0.1% to greater than 5%.
Crudes with greater than 0.5% sulfur generally require more extensive processing than those withlower sulfur content.

Pour Point, °F (°C)
The pour point of the crude oil, in °F or °C, is a rough indicator of the relativeparaffinicity and aromaticity of the crude.
The lower the pour point, the lowerthe paraffin content and the greater the content of aromatics.

Characterization Factors
There are several correlations between yield and the aromaticity and paraffinicityof crude oils, but the two most widely used are the UOP or Watson ‘‘characterizationfactor’’ (KW) and the U.S. Bureau of Mines ‘‘correlation index’’ (CI).


where
TB :Average boiling point, °R , [?F+460].
SG : Specific gravity at 60°F.
The Watson characterization factor ranges from less than 10 for highlyaromatic materials to almost 15 for highly paraffinic compounds.
Crude oils showa narrower range of KW and vary from 10.5 for a highly naphthenic crude to 12.9for a paraffinic base crude.
The correlation index is useful in evaluating individual fractions from crudeoils. The CI scale is based upon straight-chain paraffins having a CI value of 0and benzene having a CI value of 100. The CI values are not quantitative, butthe lower the CI value, the greater the concentrations of paraffin hydrocarbonsin the fraction; and the higher the CI value, the greater the concentrations ofnaphthenes and aromatics.

Distillation Range
The boiling range of the crude gives an indication of the quantities of the various products present. The most useful type of distillation is known as a true boiling point (TBP) distillation and generally refers to a distillation performed in equipmentthat accomplishes a reasonable degree of fractionation.
There is no specifictest procedure called a TBP distillation, but the U.S. Bureau of Mines Hempeland ASTM D-285 distillations are the tests most commonly used.
Neither ofthese specify either the number of theoretical plates or the reflux ratio used and,as a result, there is a trend toward using the results of a 15:5 distillation (D-2892) rather than the TBP. The 15:5 distillation is carried out using 15 theoreticalplates at a reflux ratio of 5:1.
The crude distillation range has to be correlated with ASTM distillationsbecause product specifications are generally based on the simple ASTMdistillation tests D-86 and D-1160.The TBP cut point for various fractions can be approximated by use of Figure 3.1.


Cetane Number
The cetane number measures the ability for auto ignition and is essentiallythe opposite of the octane number. The cetane number is the percentage ofpure cetane (n-hexadecane) in a blend of cetane and alpha methyl naphthalenewhich matches the ignition quality of a diesel fuel sample. This quality is
specified for middle distillate fuels. One of the standard tests is ASTM D976.


Smoke Point
The smoke point is a test measures the burning qualities of kerosene and jetfuel. It is defined as the maximum height in mm, of a smokeless flame offuel. One of the standard tests is ASTM D1322.
Reid Vapour Pressure
The reid vapour pressure (RVP) of a product is the vapour pressure determined in a volume of air four times the liquid volume at 37.8oC (100oF). This property measures the vapour-lock tendency of a motor gasoline in which excessive vapours are produced in the fuel line causing interruption of the supply of liquid fuel to the engine. It also indicates the explosion and evaporation hazards of the fuel. One of the standard tests is ASTM D323.










?Classification Methods of Crude Oil:














Sweet vs. Sour Crude Oil
Sweet
The terms sweet and sour are a reference to the sulfur content of crude oil. Early prospectors would taste oil to determine its quality, with low sulfur oil actually tasting sweet. Crude is currently considered sweet if it contains less than 0.5% sulfur.
Sweet crude is easier to refine and safer to extract and transport than sour crude. Because sulfur is corrosive, light crude also causes less damage to refineries and thus results in lower maintenance costs over time. Due to all these factors, sweet crude commands up to a $15 dollar premium per barrel over sour.
Major locations where sweet crude is found include the Appalachian Basin in Eastern North America, Western Texas, the Bakken Formation of North Dakota and Saskatchewan, the North Sea of Europe, North Africa, Australia, and the Far East including Indonesia.
Sour: Sour crude oil will have greater than 0.5% sulfur and some of this will be in the form of hydrogen sulfide. Sour crude also contains more carbon dioxide. Most sulfur in crude is actually bonded to carbon atoms, nevertheless, high quantities of hydrogen sulfide in sour crude can pose serious health problems or even be fatal.
Hydrogen sulfide is famous for its “rotten egg” smell, which is only noticed at low concentrations. At moderate concentrations, hydrogen sulfide can cause respiratory and nerve damage. At high concentrations, it is instantly fatal. Exposure to high levels of hydrogen sulfide is thought to be in part responsible for Gulf War Syndrome, which is characterized by chronic fatigue, headaches, dizziness, memory problems, serious breathing problems, and even birth defects. Hydrogen sulfide is so much of a risk that sour crude has to be stabilized via removal of hydrogen sulfide before it can be transported by oil tankers.
Sour crude is more common in the Gulf of Mexico, Mexico, South America, and Canada. Crude produced by OPEC Member Nations also tends to be relatively sour, with an average sulfur content of 1.77%.













Example:
Calculate the Watson characterization factor and correlation index for n-pentane (TB = 97 ?F and sp.gr. = 0.63).
Solution: