Crude oil is separated into components by distillation based on boiling points. It is pumped to storage tanks and preheated before entering distillation units. Single-stage units produce a few products, while two-stage and three-stage units use additional towers and pressure to produce more products. Crude is separated first in a high-pressure tower, then subsequent towers at lower pressures, to efficiently distill its many hydrocarbon components.

2. Processing Of Crude Oil
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Crude oil is separated in to compounds or group of
compounds by distillation process.
Crude oil is composed of complex mixture of
hydrocarbons differing in boiling points.
Components of crude oil distillation are as
components
Gases
Boiling range
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uses
Cooking, heating of furnace
Gasoline(petrol)
Up to 150 C
Motor oil
Kerosene
150-250 C
Illuminating, fuel
Light gas oil
250-300 C
High speed diesel oil
Heavy gas oil
300-350 C
Diesel oil or cutter stock
Fuel oil/residum Above 350 C
Fuel for furnaces

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Crude oil is pumped to crude oil tanks(where it is
allowed to settled and drained of water)
Before entering the primary column of distillation unit
by centrifugal pump, the crude oil is passed through
preheated exchanger &furnace.
This is done by maintaining high pressure at discharge
of these pumps.
All oil streams from the columns are at high temp and
this heat is transferred to incoming oil in heat
exchanger.
Air for combustion is provided by a blower.
A tall stack sucks the hot gases and discharge them in to
the atmosphere.
Crude oil is partly vaporized while entering the primary
column having no of plates.

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Gases and gasoline rise to the top are condensed in
condenser & then to accumulator.
Heavier components such as gas oil & residue fall to the
bottom of the column.
part of condensed liquid pumped back to the column as
reflux for proper liquid flow & proper fractionation.
Greater the reflux with in the certain limit, better is the
quality of the overhead product.
Sea water is utilized to condense the overhead gasoline
vapour. Thus Al-brass tubes are utilized to improve the
corrosion resistance.
In case of fresh water, direct contact condensers are
utilized.
Primary tower is operated under pressure to retain &
dissolve substances like butane to improve the antiknock value of gasoline.

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Shell still distillation
Single stage distillation
Two stage distillation unit
Three stage distillation unit

8. Shell Still Distillation.
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Oldest method of crude distillation accomplished in a
battery of shell stills.
Preheated crude is charged is fed to horizontal externally
fired cylindrical stills.
Lightest product (light neptha) is taken off at first still.
Remaining liquid passes on to the following stills where
process is repeated.
Successively higher boiling point products being obtained
in each still.
Each still is provided with bubble cap column to improve
separation of products with overlapping boiling points.

10. Single Stage Distillation Unit
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Also called crude topping or atmospheric distillation of
crude.
Preheated crude feed is entered to a direct fired furnace
type heater (pipe still).
Materials separate in the distillation column according to
their boiling points.
Lowest boiling fraction leaving at the top of the column.
Desired products may be with drawn as side streams at
appropriate points on the column.
Side streams are further fractionated in small columns called
strippers.
More Volatile components are cut by using steam so that
initial boiling point of products can be adjusted to desire
value.

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Capacity of single stage distillation unit is limited which
is improved by using additional pressure units.
Higher boiling point products can not be separated
(cracking of the products). To overcome this problem
vacuum units are added to work at low temp.

12. Two Stage Distillation Unit
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This scheme include
a primary tower operating at 3 atm pressure.
a secondary tower operating at atmospheric pressure together
with stabilizer.
this scheme is utilized when crude has to be separated in to
six to ten narrow cuts (products with close and overlapping
boiling points).
Two/three side streams (gasoline & naphtha) are with drawn
from primary tower.
Overhead (gas) product become feed to stabilizer.
Primary tower bottoms become the feed of the secondary
tower.

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From secondary tower, side streams like kerosene,
diesel, gas oil are wit drawn at appropriate points after
passing through strippers to remove the light
components.

14. Three stage Distillation Unit
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This system comprises of pressure, atmosphere and
vacuum tower.
This scheme is utilized as some crude products have high
boiling temp or heat sensitive and can’t be distilled at
atmospheric pressure.
So they are distilled under vacuum.
Vacuum distillation is used for production of lubricating
oil, asphalts and feed for catalytic cracking.
Residue from atmospheric tower is fed at about 425 C to
vacuum tower under pressure of 30-80mm Hg.
Overhead gas oil fraction is removed and side streams are
with drawn through strippers.

15. Contd.
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Diameter of vacuum distillation unit is larger as compared to
atmospheric unit due to larger crude vapour volume at lower
pressure(vacuum).
Steam jet ejector or barometric condensers are used for
maintaining desired vacuum in the vacuum tower.

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