CRUDE OIL
The crude unit is the first processing unit in
the refinery. Distillation is used to separate the crude oils into fractions
according to boiling point.
1. 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.
2. 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:
Types of crude
|
Key fraction (oAPI)
|
|
No 1
|
No 2
|
|
Paraffin
|
40
|
30
|
Paraffin, intermediate
|
40
|
20-30
|
Intermediate, paraffin
|
33-40
|
30
|
Intermediate
|
33-40
|
20-30
|
Types of crude
|
Key fraction (oAPI)
|
|
No 1
|
No 2
|
|
Intermediate, naphthene
|
33-40
|
20
|
Naphthene, intermediate
|
33
|
20-30
|
Naphthene
|
<33
|
<20
|
3. Properties of
Crude oil
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. This is expressed in terms of the weight percent
carbon residue by either the Ramsbottom (RCR) or Conradson (CCR).
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 oF
3.8 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 frequent
3.10 MON
It
is Motor method which is the guide to engine performance on the highway or
under heavy load condition
3.11 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. [2]
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.
2.2 Process Involved
 |
| Crude oil cuts |
Crude
is heated in the furnace and charged to distillation column where it is
separated into butanes 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 C5 – 190 oF
or C5-90 oC
which is the cut of naphtha and this naphtha is light also known as light
straight run naphtha (LSR). Second cut is 190-380 oF
which is heavy straight run naphtha (HSR). Next cut is 380-520 oF
which is kerosene. Next cut is 520-650 oF
which is Light gas oil (LGO). Next cut is 650-800oF
which is Heavy gas oil (HGO). Next cut is 800-1000 oF
which is Vacuum gas oil (VGO) and next cut is 1000+ oF
which is Vacuum reduced crude (VRC).
Evaluation of API at different temperature cut
Temperature Range
|
API
|
482 to 527 oF (250 to 275 oC)
|
34.2
|
527 to 572 oF (275 to 300 oC)
|
30
|
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.
 |
| TBP CURVE |
Above Figure
shown the TBP and mid-point curve. Yield of crude decreases as crude is
heated at higher temperature. 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 Appointed
PROPERTY
|
ASTM METHOD
|
APPARATUS
|
Density
|
D4052, D70
|
Digital density meter KYOTO-KEM, Pycnometer
|
Pour Point
|
D5949, D5853
|
Cold flow property analyzer PHASE TECHNOLOGY
|
Water Content
|
D4006, D4928
|
Dean and Stark
distillation apparatus, KF Coulometer METROHM
|
Salt Content
|
D3230
|
Salt-in-Crude analyzer KOEHLER
|
Asphaltene Content
|
D3279
|
Automatic asphaltene analyzer COSMO TRADE & SERVICE
|
Wax Content
|
Manual method
|
|
Reid Vapor Pressure
|
D323
|
Automated Reid vapor pressure tester WALTER HERZOG GmbH
|
Carbon Residue
|
D4530
|
Micro carbon residue tester
|
Kinematic Viscosity
|
D445, D2170
|
Viscometers CANNON and viscometer
holders KOEHLER
|
Flash Point
|
D93
|
Pensky-Martens closed cup flash
point tester TANAKA
|
Aniline Point
|
D611
|
Automatic aniline point tester
TANAKA
|
