Gas Concentration Unit Workpaper

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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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


Unstabilized Naphtha from Crude Distillation Unit.

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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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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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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 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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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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=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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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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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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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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Gas Concentration Unit Workpaper