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DETAILED PROCESS DESCRIPTION
GAS CONCENTRATION UNIT:
The Gas concentration unit is designed to recover C3/C4 LPG and to produce a
stabilized Naphtha feed to the Naphtha Hydrotreating unit.
Gases from various process units are compressed in compressor section and routed
through an absorption section to maximize the recovery of LPG. Off gas from the
absorption section will be sent to fuel gas treater.
Unstabilized Naphtha from various process units is routed through the distillation
section to separate unstabilized Naphtha and LPG. The recovered LPG and Stabilized
Naphtha are sent to LPG merox unit and Naphtha Hydrotreating Unit, respectively.
The removal of lighter ends from the liquid by distillation process is called
stabilization. Enough of the light ends are removed to make a stabilized liquid i.e. the
liquid with low enough vapor pressure to permit its storage in ordinary tanks without
vapor loss.
OPERATION OF GAS CONCENTRATION UNIT:
The operation of Gas Concentration Process Unit comprises of the following sections:
1. Compressor Section
a) High Pressure Separator
b) Two Stage Centrifugal Compressor
2. Absorption Section
a) Primary Absorber
b) Sponge Absorber
3. Distillation Section
a) Stripper Section
b) Debutanizer Section
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DETAILED PROCESS DESCRIPTION
1) Co mpressor Section:
The feed source to the compressor section of the Gas Concentration Unit consists of:
Gas from Crude Distillation Unit (CDU)
Gas from fractionator receiver at visbreaking unit.
Gas from flash fractionator receiver at the Diesel MAX unit.
Gas from stripper receiver at the Diesel MAX unit.
Gas from stripper at Naphtha Hydrotreating Unit.
Un-stabilized Naphtha from Crude Distillation Unit (CDU)
Un-stabilized Naphtha from diesel max Product Fractionator
Un-stabilized Naphtha from fractionator at visbreaking unit.
Un-stabilized Naphtha from Flash Fractionator at Diesel MAX unit
All incoming gas streams flow into a two stage centrifugal compressor via the
compressor suction cooler and the compressor suction drum. Mists separated
and accumulated in the compressor suction drum is manually drained out and sent to
the Crude Distillation Unit (CDU).
Gas streams is pressurized to 3.7 to 8.2 kg/cm 2 in the first stage and then it
enters the second stage of compressor via inter-stage cooler and compressor
inter-stage suction drum and compressed to 17.1 kg/cm2(g). This pressurized gas
mixes with the Stripper overhead vapor, the incoming liquid streams, and Primary
Absorber bottoms. The combined stream is cooled in the high pressure cooler and
sent to the high pressure receiver . Liquid separated and accumulated in the
compressor inter-stage suction drum is pumped to the high pressure separator feed stream.
High Pressure Receiver:
High pressure inlet streams are as follows:
Compressed gas from compressor.
Un-stabilized Naphtha from Diesel Max Unit
Un-stabilized Naphtha from fractionator at visbreaking unit.
Unstabilized Naphtha from Crude Distillation Unit.
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DETAILED PROCESS DESCRIPTION
Stripper overhead vapor.
Primary absorber bottom liquid.
These all streams enter the high pressure receiver where water is drained out.
Overhead gases from the high pressure separator are sent to the absorption section and
liquid stream is pumped to the s t r ipper .
2) Absorption Section:
Primary Absorber:
The first column in the absorption process is the primary absorber. Primary absorber
contains bubble cap trays. In the primary absorber stabilized naphtha from debutanizer
bottom is used as lean oil. The C3/C4 rich gas stream from the high pressure receiver enters
the column below the bottom tray. The absorption process is exothermic and to improve C 3
and higher ends recovery, liquid from one or more of the middle trays is pumped through
an intercooler and returned to tray above. The intercooler reflux is returned to the column
at approximately the same temperature as the stabilized naphtha entering the top of the
column. The primary absorber rich oil from the bottom of the column returns to
the high pressure receiver, where most of the light material recovered is pumped
with the heavier material to the stripper for further processing.
Sponge Absorber:
The second column in this section is sponge absorber. This column is smaller in size
and filled with packing that allows intimate contact between liquid and gas streams.
The overhead of the primary absorber is directed to the bottom of sponge absorber,
while circulating diesel from crude unit used as lean oil enters top of the column.
The rich diesel at bottom of the column is returned to crude unit. Gases leaving the
sponge absorber are sent to Amine treating Unit.
The process principle of the sponge absorber is the same as the primary
absorber, in that the overhead gas from the primary absorber is intimately contacted
with the heavier liquid to recover the C3/C4 material that was not absorbed in the
primary absorber. The main differences between the primary and the sponge absorber are
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DETAILED PROCESS DESCRIPTION
the internals of the column and the type of lean oil used. The sponge absorber has a packed
section instead of trays for the intimate contact between the down flowing liquid and
upward flowing gases. The sponge absorber does not have a reflux flow as the primary
absorber has a demister blanket installed near the top of the column. The lean oil used in
the sponge absorber is circulating diesel from the crude distillation unit. After passing
down the column the diesel is collected in the bottom of the column where it is returned to
the crude distillation unit as rich oil on level control at the bottom of the sponge absorber.
The lean gas is sent to the amine treating unit and then to the refinery fuel gases system
after passing through a pressure control valve maintaining a constant pressure on the entire
absorption system.
3) D istillation Section:
Stripping Section:
The liquid hydrocarbon stream from the high pressure receiver consists mostly of C 3
and higher hydrocarbons; however it also has smaller fractions like methane
ethane etc., and is pumped to the stripper to strip off light ends (lighter than C3) plus
the bulk of hydrogen sulfide. Stripper feed enters the column above the top tray. Feed to
the stripper is heated using the debutanizer bottoms. The stripping action is provided by the
reboiler present at the bottom of the column and heat is provided to rebolier by the
debutanizer bottom stream and Medium Pressure Steam. The unstabilized naphtha flows
downward through the column. The vapors stripped from the unstabilized naphtha leave
the top of the stripper and pass through the high pressure cooler into the high pressure
receiver. The stripper bottoms under level control on the bottom of the stripper
are pressured to the debutanizer column.
Debutanizer Section:
The feed to the debutanizer is the stripper bottom stream which contains C 3, C4 and
naphtha. The debutanizer feed enters the column in the top half of the column. Heat is
supplied via reboiler using circulating diesel from crude unit as heating oil. The lighter
components (C3/C4) of the feed vaporize and are carried overhead where it is
condensed and stored in debutanizer receiver and sent to LPG Merox Unit for further
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DETAILED PROCESS DESCRIPTION
processing. Some portion of the overhead product can be returned to the tower as
reflux.
The debutanizer bottoms leave the column and after passing through one of the stripper
reboiler and the stripper feed preheater, the flow is divided into two flows. The first flow
is directed on flow control to top of the primary absorber as lean oil or sent to the
stabilized naphtha storage tank. The second flow is directed on level control from the
bottom of the debutanizer to the Naphtha Hydrotreating Unit.
Feed and P roducts :
Feeds:
The gas feed to the Gas Concentration Process Unit are as follows:
Gas from Crude Distillation Unit (CDU)
Gas from fractionator receiver at visbreaking unit.
Gas from flash fractionator receiver at the Diesel MAX unit.
Gas from stripper receiver at the Diesel MAX unit.
Gas from stripper at Naphtha Hydrotreating Unit. `
Un-stabilized Naphtha from Crude Distillation Unit (CDU)
Un-stabilized Naphtha from diesel max Product Fractionator
Un-stabilized Naphtha from fractionator at visbreaking unit.
Un-stabilized Naphtha from Flash Fractionator at Diesel MAX unit
Products:
A mixed C3/C4 LPG Stream is recovered and sent to LPG Merox unit. Debutanizer
naphtha is sent to Naphtha Hydrotreating Unit for further processing.
Off gas from the absorber is routed to the fuel gas treater at the Amine treating
Unit.
CHARACTERISTICS OF UNSTABILIZED NAPHTHA
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DETAILED PROCESS DESCRIPTION
Property 100% Light Arabian Crude Oil
TBP Cut Point, °C 21-150
Gravity, API (Sp. Gr.) 64.72
Sulphur Wt. %
Nitrogen Wt. %
0.85
0.00
Paraffin, Vol. % 33.3
Olefin, Vol. % 38.1
Napthenes, Vol. % 22.7
Aromatics, Vol. % 5.8
Feed composition incase of 70/30% Upper Zakhum/Murban Crude Mix:
Feed Gas
From From From From From
Stripper Flash fractionator
Fractionator Stripper Net gas comp.
H2O 1.54 3.05 1.32 0.93 3.57
NH3 - - - TRACE -
H2S 20.01 1.91 3.81 1.33 -
H2 41.94 - 2.47 36.81 -
C1 12.9 102.68 17.2 13.26 -
=C2 - - 0.78 - -
C2 39.27 45.88 10.29 7.48 2.56
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DETAILED PROCESS DESCRIPTION
=C3 - - 2.9 - -
C3 28.28 45.55 7.7 2.28 21.82
=NiC4 - - 2.54 - -
=iC4 - - - - -
iC4 3.95 3.33 0.99 0.3 3.93
nC4 6.7 19.92 2.59 20.64 13.01
=C5 - 3.31 0.57 - -
iC 5 1.97 0.35 0.31 5.74 4.11
nC5 1.6 2.85 0.55 2.11 5.84
CP - 0.14 - 0.01 0.16
22DMB - - - 0.02 -
23DMB - 0.1 - 0.02 -
2MP 0.89 0.4 - 0.14 -
3MP - 0.15 - 0.07 -
nC6 0.26 2.49 - 0.11 -
MCP 0.41 0.65 - 0.01 -
CH - 0.15 - TRACE -
BZ 0.07 - - 0.01 -
Naphtha 0.59 2.35 2.55 TRACE 8.07
Kg-mole/h 160.39 235.26 56.58 91.29 62.27
Flow Rate, kg/h
4514 7581 1946 2493 3575
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DETAILED PROCESS DESCRIPTION
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DETAILED PROCESS DESCRIPTION
Feed
Unstabilized Naphtha
From From From From
Crude column
Prod. fractionator
Fractionator Flash fractionator
H2O 1.57 0.05 - 0.2
NH3 - 0.07 - -
H2S - - 0.46 0.23
H2 - - 0.01 -
C1 - - 0.31 2
=C2 - - 0.04 -
C2 2.7 - 0.77 3.89
=C3 - - 0.61 -
C3 34.29 0.08 1.84 12.39
=NiC4 - - 1.64 -
=iC4 - 0.44 - -
iC4 21.8 - 0.56 2
nC4 93.16 1.36 1.94 16.18
=C5 - - 1.24 6.7
iC 5 64.17 1.99 0.56 0.65
nC5 99.53 2.11 1.25 6.75
CP 4.95 - - 0.51
22DMB - - - -
23DMB - - - 0.44
2MP - 3.58 - 2.02
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DETAILED PROCESS DESCRIPTION
3MP - - - 0.81
nC6 - 1.54 - 16.71
MCP - 2.77 - 5.07
CH - - - 1.68
BZ - 0.37 - -
Naphtha 749.52 7.16 27.9 59.2
Kg-mole/h 1071.7 21.52 39.1 137.44
Flow Rate, kg/h
104808 1923 3827 12195
Product composition is as follows:
Product
LPG Stabilized naphtha Off gas
H2O TRACE - 1.75
NH3 - 0.02 -
H2S 0.47 - 23.25
H2 - - 80.92
C1 - - 144.59
=C2 TRACE - 0.76
C2 0.27 - 99.07
=C3 3.1 TRACE 0.3
C3 122.04 TRACE 7.53
=NiC4 4 0.15 0.01
=iC4 0.42 0.01 TRACE
iC4 31.95 0.81 0.08
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DETAILED PROCESS DESCRIPTION
nC4 145.1 15.23 0.69
=C5 0.19 10.98 0.01
iC 5 2.04 69.12 0.15
nC5 0.32 110.65 0.06
CP TRACE 5.44 TRACE
22DMB - 0.02 TRACE
23DMB - 0.54 TRACE
2MP TRACE 6.86 TRACE
3MP - 1 TRACE
nC6 - 20.7 TRACE
MCP - 8.76 TRACE
CH - 1.81 -
BZ - 0.45 -
Naphtha TRACE 839.39 TRACE
Kg-mole/h 309.92 1091.94 359.19
Flow Rate, kg/h
16259 113570 6715
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