Crude Oil Properties

Crude Classification

Paraffinic base

Mixed or intermediate

Naphthenic base

The majority of crude oil is alkanes, cycloalkanes (naphthenes), aromatics,

polycyclic aromatics, S-containing compounds,

etc.

Gasoline: branched alkanes

Diesel: linear alkanes

Heavier crude contains more polycyclic aromatics

Lead to carboneceous deposits called coke

Some crudes contain a lot of sulfur, which leads to processing considerations

Mallisons method

Classification based on residue left after distillation

>5% paraffins..paraffinic base

2-5 % paraffins.mixed base

US bureau of mines classification

Two key fractions chosen

Key fraction no.1....

Boiling range 250-2750C

Key fraction no.2

Boiling range 389-4220C

.At normal pressure

Crude Assay

The crude assay is a compilation of laboratory and pilot plant data that defines

the properties of the specific crude oil.

Contents of assay

True boiling point curve

Specific gravity curve

Sulphur content

Viscosity

Pour point

EFV curve

True Boiling Point curve (TBP)

It is a plot of the boiling points of almost pure

components, contained

in the crude or fraction

of the crude oil.

ASTM Distillation

Liquid is boiled and vapours are condensed

Vapour temperatures are noted and plotted against the distillate recovered

These are employed as routine tests to measure quality of refinery products

Equilibrium Flash Vaporisation

(EFV)

The apparatus restricts the vapour and the liquid together until the required degree of

vaporization is achieved.

The percentage vaporized is plotted against temperature for several runs to

give the EFV curve.

API gravity (American Petroleum Institute)

The density of petroleum oils is expressed in terms of API gravity rather than specific gravity

An increase in API gravity corresponds to a decrease in specific gravity.

Specific gravity and API gravity refer to the weight per unit volume at 60F as compared to

water at 60F.

Flash Point

The flash point of an oil is the temperature at which the vapour above the oil will

momentarily flash or explode.

Type of apparatus

Pensky Marten(PM) apparatus for middle distillates and fuels

Abel apparatus for kerosene

Flash point

Empirical correlation

Flash point 0F = 0.77(ASTM 5% 0F-1500F)

Aniline Point

The minimum temperature for complete miscibility of equal volumes of aniline and

the test sample.

The test is considered an indication of the paraffinicity of the sample.

The aniline point is also used as a classification of the ignition quality of

diesel fuels.

Reid Vapour Pressure

The vapor pressure at 100F of a product determined in a volume of air four times the liquid volume.

Reid vapor pressure is an indication of the ease of starting and vapor-lock tendency of a motor gasoline as well as explosion and evaporation hazards

It is usually expressed in kPa or psig.

The true vapour pressure is about 5-9% greater than this.

Octane Number

It is a measure of gasolines resistance to knock or detonate in a cylinder of a

petroleum engine.

The higher the resistance the higher is the efficiency of the fuel to produce work.

Octane Number (contd.)

The octane number of gasoline is that percentage of iso-octane and normal heptane which exactly matches the knocking behavior of the gasoline.

Two types of octane numbers (to determine the sensitivity of gasoline to the severity of operating conditions)

Research Octane number (RON)

Motor Octane number (MON)

Research Octane Number (RON)

The percentage by volume of iso-octane in a blend of iso-octane and n-heptane that knocks

with the same intensity as the fuel being tested.

A standardized test engine operating under standardized conditions (600 rpm) is used.

Results are comparable to those obtained in an automobile engine operated at low speed or

under city driving conditions

Motor Octane Number (MON)

A measure of resistance to self-ignition (knocking) of a gasoline under laboratory conditions which correlates with road performance during highway driving conditions.

The percentage by volume of iso-octane in a mixture of iso-octane and n-heptane that knocks with the same intensity as the fuel being tested.

A standardized test engine operating under standardized conditions (900 rpm) is used.

This test approximates cruising conditions of an automobile

Cetane Number

The percentage of pure cetane in a blend of cetane and alpha-methyl-naphthalene

which matches the ignition quality of a

diesel fuel sample.

This quality, specified for middle distillate fuels, is synonymous with the octane

number of gasolines.

Viscosity

It is a measure of liquids resistance to internal flow and is an indication of its

lubricating properties.

Common viscosity scales

Saybolt Universal

Saybolt Furol

Poise

Kinematic (Stokes or centistokes)

Cloud Point

The temperature at which solidifiable compounds present in the sample begin to

crystallize or separate from the solution

under a method of prescribed chilling.

Cloud point is a typical specification of middle distillate fuels

Pour Point

The lowest temperature at which a petroleum oil will flow or pour when it is

chilled without disturbance at a controlled

rate.

Pour point is a critical specification of middle distillate products used in cold

climates.

Sulphur Content

Crude is classified based on sulphur content.

Sour crude: A crude which contains sulfur in amounts greater than 0.5 to 1.0 wt%, or which contains 0.05 ft3 or more of hydrogen sulfide (H2S) per 100 gal

Sweet crude: As evident from the above definitions, a sweet crude contains little or no dissolved hydrogen sulfide and relatively small amounts of mercaptans and other sulfur compounds.

Salt Content

The salt content is expressed as sodium chloride equivalent in pounds per

thousand barrels (PTB) of crude oil.

Typical values range from 1 to 20 PTB.

1 PTB is roughly equivalent to 3 ppm.

Carbon Residue

Carbon residue is a measure of the coke-forming tendencies of oil.

It is determined by destructive distillation in the absence of air of the sample to a coke residue.

The coke residue is expressed as weight percent of the original sample.

There are two standard ASTM tests,

Conradson carbon residue (CCR)

Ramsbottom carbon residue (RCR).

Conradson Carbon Residue

Characterization Factors

An index of feed quality, also useful for correlating data on physical properties.

An indication of carbon-to-hydrogen ratio.

Characterisation Factors (contd.)

The Watson (UOP) characterization factor (Kw) is defined as the cube root of the mean average boiling point in degree Rankine divided by the specific gravity.

TB is the mean avg boiling point in 0R

T0B is the mean avg boiling point in K

G is the specific gravity at 600F

827.0

3/103

1

BBw

T

G

TK

Watson Characterisation Factor

15highly paraffinic

Characterisation Factors (contd.)

Correlation index (CI) The U.S. Bureau of Mines factor for evaluating individual

fractions from crude oil.

The CI scale is based upon straight-chain hydrocarbons having a CI value of 0 and

benzene having a value of 100.

8.45647387552

GT

CIB

Correlation index

Lower the CI value greater is the composition of paraffins

Higher the CI value greater is the composition of naphthenes and aromatics

Nitrogen Content

Nitrogen causes severe poisoning of catalysts used in processing and corrosion

problems such as hydrogen blistering.

Crudes containing nitrogen content above 0.25% by weight require special

processing.

Metal content

Metal contents affect

the activities of catalysts

Corrosion of turbine blades

Refractory furnace linings and stack

Metal content can be reduced by solvent extraction with propane or similar solvents.