LPG RECOVERY UNIT BASIS OF DESIGN

LPG RECOVERY UNIT
BASIS OF DESIGN
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4.5.18 Off-Gas Cooling 11
5. BATTERY LIMIT CONDITIONS 11

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1. INTRODUCTION
1.1 Use of this document
This document defines the basis of design for the LPG Recovery Unit, Unit No.
011.
The information presented in this document was used for the preparation of a
Front End Engineering Design (FEED) package.
1.2 Unit processing objectives
The objective of the LPG Recovery Unit (LPGRU) is to fractionate light streams
(naphtha and lighter) collected from the Crude Distillation Unit (CDU) and other
Process Units to produce:
- Treated off-gas
- Treated LPG, as separate C

3
and C
4
LPG streams
- Stabilised full range naphtha
2. UNIT INDEPENDENCE
The LPGRU is not capable of stand-alone operation, since the CDU Crude
Column Overhead Condenser is utilised as the anti-surge recycle cooler for the LP
Compressor. Also, any condensed material from two of the LPGRU knock-out
drums is returned to the CDU Crude Column Overhead Receiver. Furthermore,
the two Amine Absorbers on the LPGRU require the operation of the Amine
Regeneration Unit (ARU) since the Absorbers receive lean amine from the ARU
and return rich amine to the same Unit.
The LPGRU is not heat integrated with the CDU, refer to section 4.5.2.
3. DESCRIPTION OF UNIT
Off-gas feeds are collected, compressed and combined with the unstabilised
naphtha feeds. The total stream is then cooled before routing to the Feed
Separator. Vapour from the Feed Separator is routed to the bottom of the Off-Gas
Amine Absorber for removal of hydrogen sulphide by absorption in a circulating
methyldiethanolamine (MDEA) stream. Sweetened off-gas from the top of the
Absorber is combined with the Deethaniser overheads net off-gas and then routed
to the Fuel Gas Collection System.
Liquid from the Feed Separator is pumped to the Debutaniser where the full range
naphtha is stabilised by removal of C
4
’s and lighter components. Naphtha from the
bottom of the Debutaniser is routed to the Naphtha Hydrotreater (NHT) Unit, or
storage, after preheating Debutaniser feed and final rundown cooling. The net
overhead liquid product from the Debutaniser is an unstabilised and unsweetened
LPG RECOVERY UNIT

BASIS OF DESIGN
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mixed LPG stream (i.e. includes ethane & lighter, hydrogen sulphide & mercaptan
sulphur components). This mixed LPG stream is routed firstly to an on-plot LPG
Amine Absorber for removal of hydrogen sulphide by absorption in a circulating
MDEA stream and then to a separate licensed saturated LPG Treatment Unit for
removal of mercaptan sulphur components, after water wash.
The net overhead vapour from the Debutaniser may be routed to either:
- the on-plot Off-Gas Amine Absorber for removal of hydrogen sulphide and
then to the Fuel Gas Collection system (“Without Recycle” Operation), or
- the Feed Separator, upstream of the Feed Trim Cooler, for recontact with the
Unit feeds (“With Recycle” Operation).
“Without Recycle” operation produces more off-gas and less LPG product
compared to the “With Recycle” operation.
The sweetened and unstabilised LPG from the licensed LPG Treatment Unit is
combined with the unstabilised LPG from the CCR Platforming Unit for water
washing within the licensed LPG Treatment Unit.
The combined LPG stream is then returned to the Deethaniser on the LPGRU
where it is stabilised and a net off-gas stream is taken from the overhead drum for
routing to the Fuel Gas System. There is no net Deethaniser overhead liquid
product. The stabilised LPG stream from the bottom of the Deethaniser is routed

to the Depropaniser for separation into C
3
and C
4
LPG streams, which are sent to
LPG Storage after final rundown cooling.
4. DESIGN BASIS
4.1 Design objectives
The LPGRU is designed to:
- Collect Off-gas, LPG and Naphtha streams from the CDU and other Process
Units
- Produce an Off-gas stream for use in the Refinery Fuel Gas System after
hydrogen sulphide removal
- Produce a mixed LPG product stream, after removal of hydrogen sulphide and
water washing, for routing to the licensed Saturated LPG Treatment Unit which
removes mercaptan sulphur components
- Receive the sweet mixed LPG stream from the above LPG Treatment Unit for
stabilisation in a Deethaniser and subsequent separation in a Depropaniser to
produce C
3
and C
4
LPG streams, which are routed to LPG storage.
- Produce a stabilised full range naphtha stream, which is routed warm to the
Naphtha Hydrotreater Unit or cold to storage
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- Produce a sour water stream which is routed to the Sour Water Stripper Unit
- Produce a rich amine stream which is routed to the Amine Regeneration Unit
4.2 Design feed stocks
The compositions of the feed streams to the LPGRU are derived from the CDU
operation with 100% Kuwait Export crude. The composition of each feed stream is
detailed in the LPG Recovery Unit Process Flow Diagrams, Heat & Material
Balances, Dwgs. 3550-8110-20-011-0009 to 0020.
Data for the 100% Kuwait Export Crude are included in the Crude Assay,
Document No. 3550-8110-PD-0003. A maximum of 0.5% by volume water is
assumed to be present in the crude.
4.3 Design capacity
4.3.1 Annual throughput
The flow rates of the feed streams to the LPGRU result from the CDU processing
an average of 9.54 Million Tonnes per Annum (MTA) of the design crude defined
in section 4.2. The flow rate of each feed stream is detailed in the LPGRU Process
Flow Diagrams, Heat & Material Balances, Dwgs. 3550-8110-20-011-0009 to
0020.
The total flow of off-gas and unstabilised naphtha to the Unit is 2.33 MTA.
4.3.2 Operating factor
The LPGRU capacity is based on an average of 8320 hours operation per year
giving an operating factor of 0.95. The target operating factor is 0.97.
4.3.3 Nameplate capacity
The total flows to the LPGRU are:

- 45.8 Tonnes/h of Off-Gas
- 234.0 Tonnes/h of Naphtha

In addition the flow of unstabilised LPG from the CCR Platforming Unit is:
- 8.9 Tonnes/h of LPG
4.3.4 Turndown
The Unit is capable of operating at a maximum turndown ratio of 50% of design
feed throughput while meeting the product specifications.

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The Unit may be required to operate at a turndown ratio of greater than 50% on
occasion (e.g. on Refinery start-up when not all Units are operating). In this
circumstance it may be necessary to operate with column reflux rates, and amine
circulation rates, sufficiently high to ensure satisfactory operation of the distillation
and absorption columns. Similarly, mechanical equipment may have to be
operated with recycle flows.
4.4 Product specifications
The LPG Recovery Unit is designed to meet the following product specifications
when processing the design feed referred to in section 4.2, at the design capacity

referred to in section 4.3:
Off-Gas (Off-Gas Amine
Absorber outlet)

H
2
S 100 ppmv max (ASTM D-4084)
Mixed LPG (LPG Amine
Absorber outlet)

H
2
S 50 ppmw max (ASTM D-2420)
Mixed LPG (Deethaniser
bottoms)



C
2
- 3.0 mol% max (ASTM D-2163)
C
5
+ 1.5 mol% max (ASTM D-2163)
The target specifications for the Debutaniser, Deethaniser and Depropaniser are
as follows:
Debutaniser C
5
+ in Debutaniser net overhead LPG liquid product
specified at approx. 1.0 mol% (to meet 1.5 mol%

(max) in C
4
LPG product)

C
4
- in Debutaniser bottoms product specified at 0.8
vol%

Deethaniser C
2
- in Deethaniser bottoms specified at approx. 0.7
mol% (to meet 3.0 mol% (max) in C
3
LPG product)

Depropaniser C
4
+ in Depropaniser overhead (C
3
LPG product)
specified at 2.0 mol% (max)

C
3
- in Depropaniser bottoms (C
4
LPG product)
specified at 2.5 mol% (max)

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4.5 Specific design requirements
4.5.1 Design margins
All elements of the LPGRU have appropriate sizing margins to ensure satisfactory
control and operation as follows, which are generally in accordance with the
Process Engineering Design Guidelines, Doc. No. 3550-8110-PD-0007:

Item Margin Notes

Reflux and Pumparound pumps

20% 1
Feed and other pumps

10% 1

Trays – Flood Factors (FF)

- 2
Compressors

20%

1

Condensers & Reboilers

15% 3
Other exchangers

10% 3

Notes:
1. Margin reflects the difference between ‘normal’ and ‘rated’ flow rates
2. FF of 80% (max) for column diameters greater than 1.5 m,
FF of 75% (max) for column diameters between 0.9 and 1.5 m,
FF of 70% (max) for column diameters less than 0.9 m
3. Margin on both throughput and duty simultaneously

4.5.2 Heat integration
The LPGRU is not heat integrated with the CDU. The Debutaniser reboiler duty is
provided by HP steam and the Deethaniser and Depropaniser reboiler duties are
provided by LP steam.

4.5.3 Pumparound circuits
Deleted

4.5.4 Amine absorbers
The off-gas from the Feed Separator and the mixed LPG from the Debutaniser
overheads are treated in two amine (MDEA) absorbers to remove hydrogen
sulphide. The hydrogen sulphide content of the treated streams is as specified in
Section 4.4. The rich amine streams, containing the absorbed hydrogen sulphide,
are returned to the Amine Regeneration Unit.

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The lean amine from the Amine Regeneration Unit is routed to the amine
absorbers via a trim water cooler. A lean amine booster pump is required to route
amine to the LPG Amine Absorber.

The MDEA solvent strength, lean amine MDEA loading and rich amine stream
compositions are as specified in the Amine Regeneration Unit Basis of Design,
Document No. 3550-8110-PD-083-0001.
4.5.5 Product drying
A coalescer is provided to remove free water from the unstabilised LPG feed from
the licensed saturated LPG Treatment Unit, before routing to the Deethaniser.

4.5.6 Blowdown systems
For operation and maintenance purposes, provision is made for hydrocarbon
blowdown to a closed blowdown system, which comprises a gravity drain
collection header to a drum in a below-ground sump. Drained hydrocarbons are
pumped back to slops for re-processing.

Relief valve discharges are routed to a closed system which includes an on-plot
flare knock-out drum and associated pump. Condensed hydrocarbons are pumped
back to slops for re-processing.
4.5.7 Amine drains
A separate closed amine gravity drain collection header to a drum in a below -
ground sump is provided. Drained amine is pumped back to the Amine
Regeneration Unit (ARU). The amine sump drum is sized to accommodate
batches of solvent drained from equipment which cannot otherwise be routed
directly back to the ARU. The sump drum also collects solvent drained from
equipment, instruments and piping for on-line maintenance purposes.
4.5.8 Common amine wash tank
A common amine wash tank is provided on the Amine Regeneration Unit (Unit
084) to store the wash water which is used to flush the amine systems at the start
of a shutdown. The collection of the wash water flush minimises the loss of solvent
and prevents the discharge of solvent to the Effluent Treatment Plant. The amine
wash tank is to have sufficient capacity for two consecutive amine system water
washes.
The wash water may be recycled for use as make-up water to the appropriate
solution tank during normal operation.
4.5.9 Process chemicals
Facilities are specified for the following process chemicals:
Corrosion inhibitor

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Facilities are provided for corrosion inhibitor injection into the Debutaniser
overhead line in order to protect the overhead system against corrosion by wet
hydrogen sulphide.
Anti-Foam

A connection is provided for anti-foam injection into the lean amine at the Amine
Absorbers (no other anti-foam injection facilities are provided).
4.5.10 Unit metallurgy
The metallurgy for the Unit is based on the 100% Kuwait Export Crude case only.

Equipment design life follows the requirements provided in the Basic Engineering
Design Data, Document No. 3550-8820-SP-0001.
4.5.11 Column internals
All columns, except the LPG Amine Absorber, are specified with conventional
valve trays. All draw-off trays are the non-fractionating type.
The LPG Amine Absorber is specified with random packing in four packed beds.
4.5.12 Exchanger cleaning
Provision for on-line cleaning of exchangers prone to fouling are incorporated into
the design in order to maintain energy efficiency over a four year run length, where

required.
Provision for heat exchanger repair is made where leakage may result in product
contamination.
4.5.13 Effluent streams
All solid, liquid and gaseous effluent streams are identified in terms of
composition, flow rate and hazardous character for both normal and abnormal
modes of operation.
4.5.14 Waste minimisation
Waste minimisation techniques are incorporated, in order to minimise emissions to
air, water and land from both normal and abnormal modes of operation.

4.5.15 Off-Gas Compressors
Three Off-Gas compressors are provided, as follows:
Low-Low Pressure (LLP) Compressor
The small LLP Compressor collects gases from the SWS Train 1 Flash Drum,
RFCC Rich Amine Flash Drum, ARU Rich Amine Flash Drum and miscellaneous
sources. The latter includes off-gases from LPG pump casing vents, compressor
purges etc. All olefinic material is excluded, except from the RFCC Rich Amine
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Flash Drum. The suction pressure is approximately 10 kPa(g) and 1 x 100%
motor-driven two stage reciprocating compressor is provided.
The compressed off-gas is routed to the Fuel Gas Collection System after
hydrogen sulphide removal in the Off-Gas Amine Absorber.

Low Pressure (LP) Compressor
The large LP Compressor collects gases from the CDU, GO HDS, HCDS PSA and
Kerosene HDS Units. The suction pressure is approximately 20 kPa(g) and 1 x
100% turbine-driven two-stage centrifugal compressor is provided.

High Pressure (HP) Compressor
The HP Compressor collects gases from the NHT, RHDS and InAlk Units. The
suction pressure is approximately 300 kPa(g) and 1 x 100% motor-driven single
stage centrifugal compressor is provided.

The compressed off-gas streams from the LP & HP Compressors are combined
and recontacted with the unstabilised feed naphtha streams upstream of the Feed
Separator.

4.5.16 Debutaniser Net Overhead Off-Gas
Two alternative routings are possible for the net off-gas stream from the
Debutaniser Overhead Drum, as follows:
Without Recycle
Debutaniser net overhead off-gas is combined with the off-gas from the Feed
Separator and routed to the Fuel Gas Collection System after hydrogen sulphide
removal in the Off-Gas Amine Absorber. This operating mode controls the sizing
of the Off-Gas system, including the Off-Gas Amine Absorber, and produces more
off-gas and less LPG compared to the “With Recycle” operation.
With Recycle
Debutaniser net overhead off-gas is recycled to upstream of the Feed Trim Cooler

for recontacting with the Unit feeds. This operating mode controls the sizing of the
Debutaniser, Deethaniser, Depropaniser and LPG Amine Absorber Systems and
produces less off-gas and more LPG compared to the “Without Recycle”
operation.
The flow rates of net off-gas and LPG products for the two operating modes are
detailed in the LPG Recovery Unit Process Flow Diagrams, Heat & Material
Balances, Dwgs. 3550-8110-20-011-0009 to 0020.

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4.5.17 LPG Water Wash
The net LPG product from the Debutaniser overheads is routed to the LPG Amine
Absorber for hydrogen sulphide removal. In order to meet the LPG amine content
(dissolved & entrained) required by the Saturated LPG Treatment Unit (012)
Licensor, the LPG is routed to a two-stage water wash system. Each stage
includes a LPG amine wash water mixer and downstream LPG amine wash water
drum with coalescer. The design of the mixer and coalescer in each stage
effectively determine the efficiency of the water wash process and an integrated
design shall be provided by the Contractor/Vendor(s).

4.5.18 Off-Gas Cooling
The Off-Gas Amine Absorber operates at a relatively low pressure, which makes
the absorption of hydrogen sulphide into the MDEA solvent relatively more difficult
(i.e. higher pressure favours absorption). In order to improve the absorption
process, an Off-Gas Trim Cooler is provided to lower the absorber operating
temperature (i.e. lower temperature favours absorption).


5. BATTERY LIMIT CONDITIONS
The design of the LPGRU shall allow the reception of feeds from, and the delivery
of products to the battery limits, at the following conditions, measured at grade.
Maximum temperature means that a lesser temperature shall be acceptable.
Maximum pressure refers to the maximum backpressure that may be imposed at
the LPGRU battery limits by the downstream system. Contractor shall ensure that
the LPGRU can deliver products at up to this pressure at all times, and also that
the LPGRU can control satisfactorily throughout the entire operating range, should
a lesser backpressure be imposed.

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Stream Origin / Destination Temp
(°C)
Pressure
(kPag)
Feeds From

Off-gas CDU 55 27
Off-gas GasOil HDS Unit 49 27
Tail Gas HCDS Unit (PSA) 30 27
Off-Gas Kero HDS Unit 51 27

Off-gas Naphtha HDT Unit 37 330
Off-gas Residue HDS Unit 48 330

Off-Gas InAlk Unit 44 330

Off-gas SWS (Train 1, Flash Drum) 55 10
Off-gas RFCC (Rich Amine Flash Drum) 47 10
Flash Gas ARU 68 10
Off-Gas Miscellaneous Sources 46 10

Mixed LPG (1, 2) LPG Treatment Unit 41 1600

Naphtha (3) CDU 55 1200
Naphtha Naphtha HDT Unit 40 1200
Naphtha Residue HDS Unit 39 1200

Lean Amine Amine Regen Unit 55 1330

Products To

Off-gas Fuel Gas Collection System 43 507
Mixed LPG LPG Treatment Unit 42 1389
C
3
LPG Storage 42 1900
C
4
LPG Storage 42 1000
Stabilised Naphtha Naphtha HDT Unit 75 450
Stabilised Naphtha Storage 42 600

Rich Amine Amine Regen Unit 56 350
Sour Water Sour Water Stripper 45 300

Notes:
1. After mercaptans removal
2. Includes LPG from CCR Platforming Unit
3. Includes unstabilised naphtha from Kerosene HDS (Unit 013) & GO HDS (Unit
014)