OPERATING MANUAL OF
ISOMERISATION UNIT (UNIT NO.73)
VISAKH REFINERY CLEAN FUELS PROJECT
HINDUSTAN PETROLEUM CORPORATION LIMITED VISAKH REFINERY
A Issued for comments NM
Rev No. Date Purpose Prepared by Checked by Approved by
This operating manual for Isomerization Unit of HPCL, VISAKH has been prepared by M/s Engineers India Limited for M/s Hindustan Petroleum Corporation Limited.
This manual contains process description and operating guidelines for the unit and is based on documents supplied by the Process Licensor (Axens). Hence the manual must be reviewed /approved by the licensor before the start-up /operation of the unit.
Operating procedures & conditions given in this manual are indicative. These should be treated as general guide only for routine start-up and operation of the unit. The actual operating parameters and procedures may require minor modifications/changes from those contained in this manual as more experience is gained in operation of the Plant.
For detailed specifications and operating procedures of specific equipment, corresponding Vendor's operating manuals/instructions need to be referred to.
Table of Contents SECTION-1...6 INTRODUCTION...6 1.1 INTRODUCTION...6 SECTION-2...8 DESIGN BASIS...8 2.1 INTRODUCTION...8 2.2 UNIT CAPACITY...8 2.3 ON-STREAM FACTOR...8 2.4 TURNDOWN CAPABILITY...8 2.5 FEED...8 2.6 PRODUCTS...11
2.7 BATTERY LIMIT CONDITIONS...17
2.8 UTILITY CONDITION...17
SECTION-3...21
CHEMISTRY OF THE PROCESS...21
3.1 INTRODUCTION...21 3.2 ISOMER REACTIONS...21 3.3 ISOMAR CATALYST...25 3.4 CATALYST CONTAMINANTS...27 SECTION-4...30 PROCESS DESCRIPTION...30
4.1 ISOMERIZATION UNIT PROCESS SYSTEM:...30
4.2 PROCESS DESCRIPTION...34
4.3 CHLORIDE INJECTION FACILITIES...41
4.4 UTILITY SYSTEM...41
4.5 WASTE EFFLUENT FROM ISOMERIZATION SECTION...45
SECTION – 5...47
PRE-COMMISSIONING PROCEDURE...47
5.1 GENERAL...47
5.2 PRE-COMMISSIONING ACTIVITIES...47
5.3 INSPECTION / CHECKING...47
5.4 PREPARATION FOR PRE-COMMISSIONING...49
5.5 PRE-COMMISSIONING OPERATION...50
SECTION – 6...60
COMMISSIONING PROCEDURE...60
6.1 FIRST START UP...60
6.3 CHRONOLOGY...60
6.4 COMPLETE INERTISING...61
6.5 HYDROCARBON CIRCULATION AND INITIAL DRY DOWN...63
6.6 HYDROGEN SWEEP...64
6.7 OIL CIRCULATION, AND STABILIZER / DEISOHEXANIZER / LPG RECOVERY SECTION START-UP...66
6.8 REACTION CIRCUIT OIL-IN...71
6.9 ACIDIZING AND FINAL DRY-DOWN...75
6.10 HYDROGENATION AND ISOMERISATION CATALYST LOADING UNDER N2...81
6.11 HYDROGENATION REACTION SECTION START-UP...81
6.12 ISOMERIZATION REACTION SECTION START-UP...84
6.13 ISOMERIZATION UNIT LINE-OUT AT DESIGN CAPACITY...87
6.14 LPG RECOVERY SECTION LINE-OUT...88
6.15 UNIT RESTART...89
SECTION –7...90
NORMAL OPERATING PROCEDURES...90
7.1 INTRODUCTION...90
7.2 CONTROL PHILOSOPHY...90
7.3 OPERATING PARAMETERS...93
7.4 OPERATING PARAMETER...95
7.5 SET POINTS OF ALARMS AND TRIPS...122
7.6 EQUIPMENT LIST...127
7.7 LIST OF INSTRUMENTS...136
7.8 PRESSURE SAFETY VALVES...143
7.9 ANALYZERS...145
SECTION - 8...147
SHUTDOWN PROCEDURES...147
8.1 GENERAL...147
8.2 NORMAL SHUTDOWN PROCEDURE...147
8.3 SHORT DURATION SHUTDOWNS...148
8.4 LONG DURATION SHUTDOWNS...149
8.5 SHUTDOWN FOR CATALYST REPLACEMENT...151
8.6 EMERGENCY SHUTDOWN PROCEDURE...152
8.7 AUTOMATIC SHUTDOWNS...159 SECTION-9...163 PROCESS VARIABLE...163 9.1 BENZENE SATURATION...163 9.2 ISOMERIZATION...164 SECTION-10...170 TROUBLE SHOOTING...170
10.1 WATER BREAKTHROUGH FROM THE DRYERS...170
10.2 SULPHUR BREAKTHROUGH: SULPHUR STRIPPING PROCEDURE...172
10.3 LOSS OF C2CL4 INJECTION...173
10.4 CHLORIDE BREAKTHROUGH...174
SECTION -11...175
SPECIAL PROCEDURES/INSTRUCTION/INFORMATION...175
11.1 CATALYSTS LOADING UNDER N2...175
11.2 SPENT CATALYST UNLOADING...183
11.3 SPECIFICATIONS OF CATALYSTS...184
11.4 CHEMICALS DURING NORMAL OPERATION...187
11.5 CHEMICALS DURING TRANSIENT OPEARTION...188
SECTION-12...189
UTILITY CONSUMPTION SUMMARY...189
12.1 INTERMITTENT CONSUMPTIONS FOR ISOMERIZATION SECTION...189
SECTION-13...191
SAMPLING PROCEDURE AND LABORATORY ANALYSIS REQUIREMENT...191
13.1 GENERAL...191
13.2 SAMPLING PROCEDURE...191
13.3 LABORATORY TEST METHOD AND SCHEDULE...194
SECTION -14...198
SAFETY PROCEDURE...198
14.1 INTRODUCTION...198
14.2 SAFETY OF PERSONNEL...198
14.3 WORK PERMIT PROCEDURE...199
14.4 PREPARATION OF EQUIPMENT FOR MAINTENANCE...201
14.5 PREPARATION FOR VESSEL ENTRY...203
14.6 HAZARDOUS CHEMICAL HANDLING...209
14.7 FIRE FIGHTING SYSTEM...218
14.8 USE OF LIFE SAVING DEVICE...219
SECTION -15...221
GENERAL OPERATING INSTRUCTIONS FOR EQUIPMENT...221
15.1 GENERAL...221
15.2 CENTRIFUGAL PUMPS...221
15.3 POSITIVE DISPLACEMENT PUMPS...224
15.4 CENTRIFUGAL COMPRESSOR...226
SECTION-1 INTRODUCTION
1.1 INTRODUCTION
Isomerization is the conversion of low octane straight chain compounds to their higher octane branched isomers. The purpose of this process is to saturate benzene and to isomerise normal paraffins to improve the research and motor octane number of the light
naphtha feed (predominantly C5/C6) before blending into the gasoline pool. The light
naphtha fraction is typically high in normal isomer content resulting in a low octane number (typically < 68). The isomerization process converts an equilibrium proportion of these low octane normal isomers into their higher octane branched isomers.
This process developed and licensed by Axens consists of three fixed bed adiabatic reactors, with benzene saturation carried out in the first reactor, and C5/C6 isomerization
reactions completed in the following two reactors. The isomerization reactions are carried over a fixed chlorinated platinum catalyst bed in a hydrogen environment. Operating conditions are not severe as reflected by moderate operating pressure, low temperature, low hydrogen partial pressure and high catalyst space velocity. These operating conditions promote the isomerization reaction, minimize hydrocracking and minimize equipment capital costs.
The feedstock of the Isomerization unit is light hydrotreated naphtha coming from Naphtha Hydrotreating Unit. Naphtha is mixed with hydrogen. A small amount of chlorinating agent is continuously injected in to the isomerization catalyst. The mixture enters in first stage reactor where isomerization reaction occurs. The effluent is cooled before entering the second stage reactor. Remaining isomerization reaction occurs in third stage reactor. The effluent is then routed to stabilizer to reduce C4 rate in the isomerization reactor effluent. The stabilizer overhead is partially cooled and vapor phase is routed to LPG recovery section while liquid is used as reflux. Stabilizer bottom
is routed to deisohexaniser where low octane C6- n paraffin and methyl pentane is recycled to the reactor circuit in order to obtain a high octane product.
SECTION-2 DESIGN BASIS
2.1 INTRODUCTION
The isomer unit is required to produce an Isomerate product which is suitable for gasoline pool.
Important content of design basis is given below.
2.2 UNIT CAPACITY
Isomar Unit is designed for a capacity of 228641 T/yr.
2.3 ON-STREAM FACTOR
The facility is designed for 8000 operating hours per year.
2.4 TURNDOWN CAPABILITY
The facility is capable of operating at 50 percent of design feed capacity, while maintaining the designated product specification.
2.5 FEED
The feedstock of the Isomerization unit is light hydrotreated naphtha coming from Naphtha Hydrotreating Unit.
Three different feeds were considered for the design of the unit NIT CASE, AM CASE (Arabian Mixed) and BH CASE (Bombay High).
2.5.1. Light Hydrotreated Naphtha
NIT case AM case BH case
Molecular weight 83.5 81.1 81.3 Sp Gravity @ 15°C 0.689 0.6647 0.711 Composition, wt% IBUTANE 0.00 0.00 0.01 NBUTANE 0.00 0.23 0.24 IPENTANE 8.48 10.89 11.41 NPENTANE 9.51 24.07 12.32 22DMB 0.44 0.30 0.52 23DMB 1.41 9.28 6.83 3METHYLPENTANE 7.98 9.93 5.36 NHEXANE 27.09 25.19 11.98 CYCLOPENTANE 1.15 1.39 1.62 METHYLCYCLOPENTANE 19.51 4.26 7.78 BENZENE 4.24 1.63 16.51 CYCLOHEXANE - 2.79 15.39 NHEPTANE 0.72 9.13 9.81 METHYLCYCLOHEXAN 1.67 0.46 0.20 Iso C7 8.41 0.00 0.00 TOLUENE 0.01 0.45 0.03 C8 0.14 0.00 0.00 Total 100 100 100 Impurities:
The Isomerization catalyst is an activated chloride, alumina based catalyst with a platinum promoter. It is highly sensitive to impurities in the feedstock, in particular water or other oxygenated compounds. Feed to this unit is hydrotreated and both feed and make-up hydrogen are passed through dryers to remove any traces of water. Some
impurities are considered to be irreversible poisons such as water and nitrogen compounds. Others are reversible such as sulfur.
It is critically important that the operating performances of both upstream hydrotreater and dryers are such that the impurity levels in the isomerization reactor feed do not exceed the levels shown hereafter. Discussions regarding feed poisons and their effects will be presented in the Operating Instructions Section.
MAXIMUM ALLOWABLE IMPURITY LEVELS IN THE FEED
Total Sulphur 0.5 ppm wt max. (1)
Total Nitrogen compounds 0.1 ppm wt max. (1)
Water + Oxygenates 0.1 ppm wt max. (1)
Metals 5 ppb wt max.
Chloride 1 ppm wt max. (1)
Olefins 1% wt max.
Arsenic 1 ppb wt max.
(1) At the outlet of the feed dryer. 2.5.2. Hydrogen
Hydrogen make-up to this unit is needed for isomerization unit for benzene hydrogenation to cyclohexane and in order to satisfy the hydrogen partial pressure requirement for the naphtha hydrotreatment and for the isomerization reactions.
Hydrogen is supplied from a new CCR unit. A gas compressor is required to satisfy the process requirements of the different users.
MAXIMUM ALLOWABLE CONTAMINANT LEVELS FOR HYDROGEN TO ISOMERIZATION UNIT
Total Sulphur 1.0 ppm wt max.
Total Nitrogen compounds 1.0 ppm wt max.
Water 0.5 ppm wt max.(1)
CO + CO2 10 ppm wt max.(1)
Olefins 10 ppm wt max.
Chloride 5 ppm wt max
(1) At the outlet of the hydrogen dryers.
Components (Mole %) Origin CCR Unit H2 93.0 C1 2.3 C2 2.2 C3 1.7 iC4 0.3 nC4 0.3 C5+ 0.2 Impurities: H2S HCl CO COS CO + CO2 H2O Olefins N2 compounds Hydrogen quality 5 ppm vol max 0.5 ppm vol max 6-10 ppm vol 1 ppm vol max 25 ppm vol max 50 vol ppm 10 ppm wt max 1 wt ppm max
2.6 PRODUCTS
There are four products. The primary product is light Isomerate i.e. deisohexaniser distillate. The secondary product is heavy Isomerate, i.e. deisohexaniser bottom. The third product Fuel gas comes from LPG separator drum. The fourth product is LPG stream produced isomerization stabilizer overhead.
Light Isomerate product
The light isomerate product estimated flowrates and product characteristics are presented hereafter:
SOR NIT CASE AM CASE BH CASE
Isomerate Product flowrate, kg/h 17859 21203 11961
Sp. Gravity @15°C 0.6456 0.6414 0.6391 Estimated RON 88.9 89.4 88.5 Estimated MON 88.2 88.3 86.7 Estimated Yield, wt % (1) 65.7 78 44 Estimated RVP, kg/cm2 abs 13.56 14.9 16.7 Composition, wt % PROPANE 0.01 0.02 0.01 IBUTANE 1.4 1.02 0.71 NBUTANE 0.53 0.43 0.39 IPENTANE 25.56 35.38 43.53 NPENTANE 8.28 10.82 16.47 22DMB 39.02 41.32 34.01 23DMB 7.25 3.6 0.75 2METHYLPENTANE 13.73 5.22 0.71 3METHYLPENTANE 2.51 0.71 0.07 NHEXANE 0.11 0.02 0 CYCLOPENTANE 1.55 1.45 3.35 METHYCYCLOPENTANE 0.05 0.01 0 BENZENE 0 0 0 CYCLOHEXANE 0 0 0 NHEPATANE 0 0 0
SOR NIT CASE AM CASE BH CASE METHYLCYCLOHEXANE 0 0 0 11DIMET-CYCLOHEXANE 0 0 0 Total 100 100 100 100 * ) / ( ) / ( h kg flowrate Freshfeed h kg flowrate rate LightIsome Yield
EOR NIT CASE AM CASE BH CASE
Isomerate Product flowrate, kg/h 18377 22292 12575
Sp. Gravity @15°C 0.6448 0.6401 0.6366 Estimated RON 89.0 89.4 88.5 Estimated MON 88.2 88.3 86.4 Estimated Yield, wt% (1) 64.3 78 44 Estimated RVP, kg/cm2 abs 13.98 15.80 17.3 Composition, wt % PROPANE 0.02 0.03 0.02 IBUTANE 1.66 1.93 1.65 NBUTANE 0.44 0.66 0.69 IPENTANE 26.8 35.76 47.24 NPENTANE 8.76 10.99 17 22DMB 39.05 40.11 29.41 23DMB 6.87 3.45 0.33 2METHYLPENTANE 1.54 4.95 0.31 3METHYLPENTANE 2.16 0.67 0.03 NHEXANE 0.09 0.02 0 CYCLOPENTANE 1.58 1.43 3.32 METHYCYCLOPENTANE 0.03 0 0 BENZENE 0 0 0 CYCLOHEXANE 0 0 0 NHEPATANE 0 0 0 METHYLCYCLOHEXANE 0 0 0 11DIMET-CYCLOHEXAN 0 0 0 Total 100 100 100 (1) *100 ) / ( ) / ( h kg flowrate Freshfeed h kg flowrate rate LightIsome Yield
SOR NIT CASE AM CASE BH CASE
Isomerate Product flowrate, kg/h 7148 3791 13788
Sp. Gravity @15°C 0.7562 0.7243 0.7402 Estimated Yield, wt % (1) 26.3 13.9 50.7 Estimated RVP, kg/cm2 abs 3.31 4.3 4.4 Composition, wt % IBUTANE 0.28 0.34 0.08 NBUTANE 0.1 0.23 0.1 IPENTANE 0.08 0.84 0.94 NPENTANE 0.01 0.07 0.09 22DMB 0 0.02 0.03 23DMB 0.02 0.31 0.62 2METHYLPENTANE 0.1 1.7 2.72 3METHYLPENTANE 0.3 3.75 4.32 NHEXANE 1.63 16.6 8.56 CYCLOPENTANE 0 0 0 METHYCYCLOPENTANE 7.61 8.05 22.18 BENZENE 0 0 0 CYCLOHEXANE 31.63 21.1 33.84 NHEPATANE 18.02 23.52 11.7 METHYLCYCLOHEXANE 23.24 12.27 7.56 11DIMET-CYCLOHEXAN 16.97 11.21 7.27 Total 100 100 100 100 * ) / ( ) / ( h kg flowrate Freshfeed h kg flowrate rate HeavyIsome Yield ]
EOR NIT CASE AM CASE BH CASE
Isomerate Product flowrate, kg/h 7669 3857 14239
Sp. Gravity @15°C 0.7542 0.7252 0.7408 Estimated Yield, wt % (1) 26.8 13.5 49.8 Estimated RVP, kg/cm2 abs 3.58 4.1 4.2 Composition, wt % IBUTANE 0.23 0.35 0.08 NBUTANE 0.15 0.2 0.08 IPENTANE 1.43 0.49 0.67 NPENTANE 0.12 0.04 0.06 22DMB 0.02 0.02 0.04 23DMB 0.02 0.31 0.61 2METHYLPENTANE 0.1 1.77 2.72 3METHYLPENTANE 0.29 3.9 4.37
NHEXANE 1.52 17.43 8.62 CYCLOPENTANE 0 0 0 METHYCYCLOPENTANE 6.26 7.34 21.48 BENZENE 0 0 0 CYCLOHEXANE 27.23 17.88 28.79 NHEPATANE 17.61 21.85 11.33 METHYLCYCLOHEXANE 26.9 15.014 11.63 11DIMET-CYCLOHEXAN 18.11 13.41 9.52 Total 100 100 100 LPG Product
LPG is recovered at LPG stripper 73-C-04 draw-off. Estimated flowrates and compositions are presented hereafter.
SOR NIT Case AM case BH case
LPG normal flowrate kg/h 2427 2320 1930 Composition, wt% H2 0 0.01 0.02 C1 0.03 0.58 0.95 C2 0.76 32.72 27.68 C3 28.53 59.75 61.38 iC4 65.33 6.68 9.72 nC4 5.11 0.24 0.25 C5+ 0.24 0 0 Total 100.00 100.00 100.00 Specific gravity @15°C 0.5448 0.5432 0.5456 Yield estimated, wt % (1) 8.9 8.5 7.1
EOR NIT Case AM case BH Case
LPG normal flowrate kg/h 2794 2566 2283 Composition, wt% H2 0 0 0 C1 0.02 0.01 0.02 C2 0.68 0.59 0.95 C3 27.31 33 27.46 iC4 65.71 60.38 62.69 nC4 6.03 5.77 8.63 iC5 0.25 0.25 0.24 Total 100 100 100 Specific gravity @15°C 0.5459 0.5429 0.5455 Yield estimated, wt % (1) 9.8 9.0 8.0
(1) *100 ) / ( ) / ( h kg flowrate Feed NHDT h kg flowrate LPG Yield
Note: LPG flowrates described here above take into account the LPG stream coming from reforming unit.
Fuel gas
Fuel gas is recovered at caustic scrubber 73-V-11. Estimated flowrates and compositions are presented hereafter:
SOR NIT Case AM Case BH Case
Off gas normal flowrate (kg/h) 285 261 590 Composition, mol % H2 83.49 86.33 69.06 C1 6.89 5.14 11.39 C2 5.22 3.88 9.78 C3 3.37 3.43 7.43 iC4 0.3 0.27 0.78 nC4 0.03 0.07 0.08 C5+ 0.7 0.88 1.48 H2O 0 0 0 Total 100 100 100 Molecular Weight 6.7 6.2 11.2
EOR NIT Case AM Case BH Case
Off gas normal flowrate (kg/h) 312 292 647 Composition, mol % H2 82.9 85.46 68.19 C1 6.97 5.27 11.49 C2 5.31 3.95 9.76 C3 3.65 4.03 8.03 iC4 0.32 0.33 0.98 nC4 0.05 0.06 0.07 C5+ 0.8 0.9 1.48 H2O 0 0 0 Total 100 100 100 Molecular Weight 6.9 6.5 11.6
2.7 BATTERY LIMIT CONDITIONS
Temperature Pressure
°C Kg/cm2 g
Feedstock :
o Light Naphta feed 40 7.0
o H2 make-up 40 22.0 o H2 for start-up 45 20.0 o 10% caustic soda 40 3.0 Products : o LPG 40 16.0 o Light Isomerate 40 7.0 o Heavy Isomerate 40 7.0
o Sweet Fuel gas 40 4.5
o Spent caustic soda 45 6.0
2.8 UTILITY CONDITION
UTILITY CONDITION AT UNIT BATTERY LIMIT (All battery limit pressures are measured at grade)
Sr. No. Parameter Minimum (for
thermal design) Normal Maximum
Mech. Design
1 HIGH PRESSURE STEAM
Pressure, Kg/cm2g 33 35 38 40
Temperature, oC 340 360 380 400
2 MEDIUM PRESSURE STEAM
Pressure, Kg/cm2g 9 10 11 12.5
Sr. No. Parameter Minimum (for
thermal design) Normal Maximum
Mech. Design
3 LOW PRESSURE STEAM
Pressure, Kg/cm2g 2.5 3.0 4.0 5.5 Temperature, oC Satd 150 170 190 4 STEAM CONDENSATE Pressure, Kg/cm2g -- 5.5 -- 10 Temperature, oC -- 100 -- 185
5 COOLING WATER SUPPLY
Pressure, Kg/cm2g -- 5.3 -- 7.6
Temperature, oC -- 33 -- 65
6 COOLING WATER RETURN
Pressure, Kg/cm2g -- 3.5 -- 7.6
Temperature, oC -- 44 -- 65
7 BOILER FEED WATER ( VHP )
Pressure, Kg/cm2g
47 50 -- 71
Temperature, oC 120 120 -- 120
8 BOILER FEED WATER ( MP )
Pressure, Kg/cm2g 17.5 20.5 -- 29 Temperature, oC 120 120 -- 120 9 DEMINERALIZED WATER Pressure, Kg/cm2g -- 3.0 -- 9.0 Temperature, oC -- Ambient -- 65 10 PLANT AIR Pressure, Kg/cm2g 3.0 4.0 5.0 9.0 Temperature, oC -- Ambient -- 65
Sr. No. Parameter Minimum (for
thermal design) Normal Maximum
Mech. Design 11 INSTRUMENT AIR Pressure, Kg/cm2g 4.0 5.0 6.0 9.0 Temperature, oC -- Ambient -- 65 12 NITROGEN Pressure, Kg/cm2g 5.0 6.0 7.0 10.5 Temperature, oC -- Ambient -- 65
13 FUEL OIL SUPPLY
Pressure, Kg/cm2g 7 8 11 17
Temperature, oC 100 130 170 200
14 FUEL OIL RETURN
Pressure, Kg/cm2g 2.5 -- -- --Temperature, oC -- -- -- --15 FUEL GAS Pressure, Kg/cm2g 2.5 3.0 3.5 9.0 Temperature, 30 40-50 60 100 16 FLARE HEADER Super imposed back pressure at B/L(kg/cm2g) -- 0.1 -- --Built up back pr. (kg/cm2g) -- 1.5 -- --Total back pressure at PSV outlet(kg/cm2g) -- 1.7 --
--SECTION-3
CHEMISTRY OF THE PROCESS
3.1 INTRODUCTION
The main purpose of this process is to saturate benzene and to isomerise normal paraffins to improve the research and motor octane number of the light naphtha feed (predominantly C5/C6) before blending into gasoline pool. The light naphtha fraction is
typically high in normal isomer content resulting in a low octane number (typically<68).The isomerization process converts an equilibrium proportion of these low octane normal isomers into their higher octane branched isomers. In addition the LPG recovery section allows separating light products (H2, C1, C2) from C3+ either from
isomerization or reformer stabiliser column.
This process consists of three fixed bed adiabatic reactors, with benzene saturation carried out in the first reactor, and C5/C6 isomerization reactions completed in the
following two reactors. The isomerization reactions are carried over a fixed chlorinated catalyst bed in a hydrogen environment. Operating conditions are not severe as reflected by moderate operating pressure, low temperature, low hydrogen partial pressure and high catalyst space velocity. These operating conditions promote the isomerization reaction, minimize hydrocracking and minimize equipment capital costs.
General process variables will be presented and discussed first and process performance will then be discussed with respect to these variables. Finally, the Isomar kinetic model
will be discussed. This model helps to define the operation of the unit and provides insight into monitoring and adjusting plant operation.
3.2 ISOMER REACTIONS
The isomerization reactions are carried out in two steps: 1st step: Hydrogenation of benzene.
2nd step: Isomerization of normal paraffins 3.2.1 Hydrogenation of benzene:
This reaction is highly exothermic and occurs in a separate reactor, namely the Benzene Saturation Reactor. This allows carrying out the isomerization reactions separately at lower temperatures in the downstream isomerization reactors.
C6H6 + 3H2 C6H12 3.2.2 Isomerization:
Isomerization is the conversion or rearrangement of the structure of a compound to its more branched, higher octane structure. These rearrangements are depicted by the following formula:
CH3 - CH2 - CH2 - CH2 - CH3 CH3 - CH - CH2 - CH3 CH3 n-pentane isopentane (RON = 62) (RON = 93)
CH3 - CH2 - CH2 - CH2 - CH2 - CH3 CH3 - CH - CH2 - CH2 - CH3 n-hexane
(RON = 30) 2 methylpentane(RON = 74 ) CH3 CH3 - C - CH2 - CH3 2,2 dimethylbutane CH3 CH3 (RON = 92) CH3 - CH - CH - CH3 2,3 dimethylbutaneCH3 CH3 (RON = 104) CH3 - CH2 - CH - CH2 - CH3 3 methylpentane (RON = 75 ) CH3
These reactions, as shown, are reversible and the final distribution of the isomers is based on the equilibrium composition which is dictated by the reactor process conditions and kinetics.
In addition to the isomerization reactions, there are other side reactions taking place as well, and some of them are not desirable.
3.2.3 Naphthene Ring Opening :
The three naphthene, which are typically, present in an isomerization feed are cyclopentane (CP), methyl cyclopentane (MCP) and cyclohexane (CH). These naphthenic rings break and hydrogenate to form paraffins. Ring opening reactions
increase with increasing temperature and again are governed by equilibrium compositions at the reactor process conditions. At typical isomerization reactor conditions the conversion of naphthene rings to paraffins will be approximately 20-30 percent. + H2 C5 H12 + H2 C6 H14 C5H10 (cyclopentane) C6H12 (cyclohexane) + H2 C6 H14 C6H12 (methyl cyclopentane) CH3
Naphthenic or cyclic components tend to inhibit the isomerization reactions and are therefore undesirable in large quantities. The cyclic components are absorbed on the catalyst and reduce the active sites available for paraffin isomerization. They also consume hydrogen, produce exothermic heat which is undesirable from the isomerization equilibrium standpoint. However undesirable as they are, they are a natural fraction of C5/C6 cut naphtha and are difficult to eliminate without also
3.2.4 Hydrocracking
Operating at the low severity reactor conditions, very little C5/C6 hydrocracking occurs
in the isomerization reactors. C7 paraffins however hydrocrack readily to produce C3 and
C4 components. Much of the hydrocracking occurs in the first reactor which typically
operates at a higher temperature. Hydrocracking reactions consume hydrogen, and hence it is recommended to restrict the C7+ content of the isomerization feedstock.
C7 H16 + H2
C3 H8 + C4 H103.3 ISOMAR CATALYST
There are several reaction mechanism theories presented based on the type of catalyst used, i.e. whether it is a dual functional catalyst consisting of a metal and a support or if its acidity is enhanced by a halogen. Whatever the type of catalyst used the intermediate step is the same i.e., the formation of a carbonium -ion.
Isomerization on dual-function catalysts in a hydrogen atmosphere is generally described by the following reaction scheme:
CH3 - CH2 - CH2 - CH2 - CH3
CH3 - CH2 - CH2 - CH = CH2 + H2 PtThe metallic function of the catalyst, in this case platinum, catalyses the formation of an olefin intermediate by dehydrogenation of the paraffin. The olefins are then converted into carbonium ions by the addition of a proton during adsorption onto the acidic surface of the catalyst.
CH3 - CH2 - CH2 - CH = CH2
CH3 - CH2 - CH2 - CH - CH3+H
+
Skeletal rearrangement then occurs:
CH3 - CH2 - CH2 - CH - CH3 CH3 - CH2 - C - CH3
C
The rearranged carbonium ion is then desorbed as an iso-olefin which is then hydrogenated by the catalyst metallic function to the iso-paraffin:
CH3 - CH2 - C - CH3 C
CH3 - CH2 - C = CH2 + H C CH3 - CH2 - C = CH2 + H2
CH3 - CH2 - CH - CH3 Pt C CDual-functional hydro-isomerization catalysts which operate at very low temperature have stronger acid sites than those which require higher temperatures. In this case, the theory postulates that the carbonium ion is formed by direct hydride ion abstraction from the paraffin by the acid function of the catalyst.
CH3 - CH2 - CH2 - CH2 - CH3 + H A
CH3 - CH - CH2 - CH2 - CH3 + H2 + A After rearrangement, isopentane is formed and the chain is propagated by the generation of a new acid site.
CH3 - C - CH2 - CH3 + A + H 2
CH3 CH3 - CH - CH2 - CH3 + H A CH3 Another principal theory of initiation and propagation is that based on the Friedel-Crafts theory. Friedel-Crafts isomerization is believed to require the presence of traces of olefins or alkyl halides as carbonium ion initiators with the reaction thereafter proceeding through chain propagation. The initiator ion, which needs to be present only in small amounts, may be formed by the addition of HCl to an olefin which is present as an impurity in the paraffin or is formed by paraffin hydrocracking.
RCH = CH2 + HCl
R CH CH3 + Cl
The initiator then forms a carbonium ion with the paraffin to be isomerized.
R CH CH3 + CH3 - CH2 - CH2 - CH2 - CH3
R CH2 CH3 + CH3 - CH - CH2 - CH2 - CH3
After skeletal rearrangement, isopentane is formed and the chain is propagated by the generation of a new normal carbonium ion.
CH3-C-CH2-CH3 + CH3-CH2-CH2-CH2-CH3 C CH3-CH-CH2-CH3 + CH3-CH-CH2-CH2-CH3 C 3.4 CATALYST CONTAMINANTS
The contaminants and poisons for this catalyst are mainly sulfur, mercury and free water. Table below shows the maximum allowable limit of impurities for feed and hydrogen make-up gas.
The isomerization unit feed is dried and hydrotreated prior to contact with the isomerization catalyst IS-614 A. These operations eliminate or reduce to an acceptable level the contaminants naturally present in the feedstocks. However, it is possible that upstream unit upsets or misoperation could lead to a contaminant breakthrough. The following discusses their impact on the process.
The presence of sulfur in the feed or make-up hydrogen will immediately decrease the activity of the catalyst. Sulfur reacts with the platinum to form platinum sulfide, and this reduces the metallic function of the catalyst affecting the hydrogen transfer mechanism. This decrease in activity is temporary and recovery is normally rapid once the sulfur has been removed. However, an increase in reactor temperature and a higher make-up hydrogen rate will assist in purging the sulfur from the catalyst more rapidly. While sulfur is present in the feed, an increase in temperature may help to partially compensate for the reduced catalyst activity.
b) Water / Oxygenates:
The deactivation which results from water or other oxygenates breakthrough is permanent. The oxygenated compounds react chemically with the active chloride on the catalyst, which is chemically bound into the alumina structure during manufacture. The water, once it reacts with the catalyst is chemically bound as hydroxyl to the alumina and the chloride is removed as HCl. Approximately one kilogram of oxygen in any form will deactivate 100 kg of catalyst. If breakthrough occurs, the catalyst deactivation will occur in a piston-like fashion moving down the first catalyst bed. In addition to the loss of product quality, a clear sign is the lack of reactor T in the top portion of bed and this inactivity slowly moving down the bed. Once the catalyst is deactivated, it must be removed for platinum recovery and replaced with fresh catalyst.
c) Nitrogen Compounds:
This refers to organic nitrogen or ammonia, not to molecular nitrogen N2. Nitrogen
compounds react to form ammonia which in turn reacts with the chloride in the catalyst or the HCl to form ammonium chloride salt. This leads to a permanent deactivation of the catalyst by a coating of the catalyst active sites, loss of chloride and possibly inactivity due to maldistribution from salt deposits.
This is also a permanent catalyst poison caused by the fluoride bonding to catalyst active sites affecting the catalyst acidity. Again one kilogram of fluoride will deactivate
100 kilograms of catalyst.
Maximum Allowable Contaminant Levels in a) Hydrogen Make-up :
Contaminants (by wt)
Total Sulfur 1 ppm wt. Max.
Water 0.5 ppm wt. Max. (1)
CO + CO2 10 ppm wt. Max.
Olefins 10 mol wt max.
Chloride 5 ppm wt. Max.
Total Nitrogen 1 wt ppm max.
(1) At hydrogen dryer outlet Maximum Allowable Contaminant Levels in
b) Light Naphtha Feed:
Contaminants (by wt)
Total Sulfur 0.5 ppm max (1)
Water + oxygenates 0.1 ppm max. (1)
Metals 5 wt ppb max.
Olefins 1% wt max
Chloride 1 ppm wt max
Arsenic 1 ppb wt max
Total Nitrogen 0.1 ppm max (1)
SECTION-4
PROCESS DESCRIPTION
4.1 ISOMERIZATION UNIT PROCESS SYSTEM:
Isomerization of the light hydrotreated naphtha is carried out in a series of two fixed bed reactors. As benzene content in feed is high and benzene hydrogenation reaction is highly exothermic, benzene hydrogenation takes places in a separate fixed bed reactor, installed upstream of the two isomerization reactors. In order to limit the temperature increase across the hydrogenation reactor, part of the effluent is cooled and recycled to dilute the reactor.
Isomerization is the conversion of hydrocarbons to their isomers, which have the same
molecular formula but a different arrangement of molecules. The C5 /C6 Isomerization
Section specifically converts the normal C5 / C6 paraffins to their isomers, i.e. to a higher
octane branched arrangement, over a proprietary platinum catalyst in the presence of hydrogen. The conversion per pass of the normal paraffins to their isomers is determined by the reaction equilibrium at the reactor operating conditions. The low octane methyl-pentanes and the unconverted n-hexane are recycled back to the isomerization reactors to achieve the objective to produce light isomerate stream with an estimated minimum octane no. of 88.5 in order to meet gasoline pool constraints.
A stabilizer column is used for removing light ends from the reactor effluent. Stabilizer bottom is routed to deisohexaniser column via chloride guard bed.
A Deisohexaniser tower is used for recovering stabilized isomerate product and recycling the low octane methyl-pentanes and the unconverted n-hexane to the reactor. The Deisohexaniser distillate goes as light isomerate to MS Pool via storage and bottoms streams is pumped to LPG recovery section, where it is used as lean oil make-up.
The stabilizer reflux drum off gas contains LPG, hydrogen and chloride which is sent to the LPG recovery unit. The chloride is removed by scrubbing with caustic soda in Caustic Scrubber.
The unit consists of the following sections: - Dryer Section - Hydrogenation Section - Isomerisation Section - Stabilizer Section - Deisohexaniser - Scrubber Section - LPG Recovery Section - Dryer Regeneration - Chloride Injection Facility
Chemical reactions in Isomerization Section
There are principally two fundamental reactions
occurring:-- Benzene hydrogenation
- Isomerization
The first reaction is highly exothermic. In order to monitor the isomerization temperature, the benzene hydrogenation takes place in the hydrogenation reactor 73-R-01.
The second reaction is the isomerization reaction itself and it occurs in the two last reactors 73-R-01 & 73-R-02.
Benzene hydrogenation:
Benzene and hydrogen react to form cyclohexane. C6H6 + 3H2 C6H12
This reaction takes place in the superior part of the first reactor 73-R-01. Benzene hydrogenation is an exothermic reaction (16660 kcal/kmole of consumed hydrogen).
Isomerization
Isomerization is the conversion of hydrocarbons to their isomers, which have the same molecular formula but a different arrangement of molecules. C5 /C6 Isomerization
section specifically converts normal C5 / C6 paraffins to their isomers, i.e. to a higher
octane branched arrangement, over a proprietary platinum catalyst in presence of hydrogen. The conversion of normal paraffins to their isomers is determined by the reaction equilibrium at reactor operating conditions.
The term isomer refers to compound which have the same molecular formula but different structural formula or a different arrangement of molecules. In general any compounds that are linear or straight chain are termed “normals”, if they are in a different arrangement, i.e. branched, they are termed “isomers”.
Examples:
n-pentane C5 H12:
H C C C C C H
H H H H H
H H H H H
Pentane has five carbon atoms. When in a straight chain as shown, this is called normal pentane or n-pentane. The Research Octane Number (RON) of n-pentane is 62.
H C C C H H H H H H H C H H C H H
This is isopentane, one of the isomers of n-pentane; it has the same molecular formula but a different arrangement or structure. Isopentane or i-pentane has a Research Octane Number (RON) of approximately 93.
Isomerization is simply the conversion of compounds to their isomer, i.e., to a higher octane branched arrangement.
Similarly with hexane:
n-hexane C6 H14: H C C C C H H H H H H H H C H H H H C H
Hexane has six carbon atoms. When in a straight chain as shown, this is called normal hexane or n-hexane. The Research Octane Number (RON) of n-hexane is 30.
i-hexane C6 H14: H C C C C H H H H H H H H C H H H C H H
This is isohexane, one of the isomers of n-hexane; again it has the same molecular formula but a different arrangement or structure. This is called 2,2 Dimethylbutane (2,2,DMB) and has a Research Octane Number (RON) of approximately 92.
Not all n-pentane and n-hexane will convert to their isomer but a certain percentage or equilibrium amount will exist at given operating conditions. This is then the measure of the performance of an Isomerization unit, i.e. the percentage of the isomer in the reactor effluent.
The conversion of n-pentane to isopentane and n-hexane to 2,2,DMB are only two of the many reactions occurring simultaneously in the Isomerization process. These reactions take place in the 73-R-02 and 73-R-03 reactors.
4.2 PROCESS DESCRIPTION
4.2.1. Dryer section
The hydrotreated naphtha is fed to the feed surge drum 73-V-01. Then it is pumped by pumps 73-P-01 A/B under flow control to the two feed dryers 73-DR-01 A/B in series. The feed dryers protect the isomerization catalyst from irreversible damage with water, which is extremely poisonous to the reactor catalyst.
The make-up hydrogen gas from CCR unit also needs to be dried for the same reason. Hydrogen from the CCR unit goes through make-up H2 K.O. drum 73-V-02. It is
compressed to the desired pressure level by compressors 73-K-01 A/B and cooled in exchanger 73-E-01 A/B (one spare cooler is required due to frequent maintenance on cooling water side). A part of the cooled gas is sent to the Prime G+: HDS section and SHU section (unit 75). Hydrogen to Isomerization section is sent to dryers 73-DR-02 A/B in series, under flow control; excess hydrogen is routed to the flow control valve on
H2 compressor recycle (73-K-01 A/B). Set point of flow controller is reset by flow
controller on the feed to the isomerization reactor. Remaining dried hydrogen is mixed with dried Naphtha under flow control and sent to the benzene hydrogenation section.
Set point of this flow controller is adjusted manually, according to the benzene composition of the feed, as indicated by on line analyzer.
Remark: Isomerization section cannot be fed directly by hydrotreated naphtha from a storage tank.
4.2.2. Hydrogenation Section
The combined two phase feed from the dryer section is preheated in Deisohexanizer recycle/ Reactors feed exchanger 73-E-02 and in first stage reactor feed/effluent exchanger 73-E-03. It is mixed with the diluent recycle from hydrogenation reactor and further heated by HP Steam, if required, in hydrogenation reactor feed heater 73-E-04 to the reaction temperature.
Feed enters the hydrogenation reactor 73-R-01, where benzene is hydrogenated to cyclohexane. The hydrogenation reaction is highly exothermic.
The reactor effluent is routed to hydrogenation reactor flash drum 73-V-04. Operating pressure leads to an almost liquid phase at the hydrogenation reactor effluent. Thermodynamic model heat and material balances define reactor outlet as fully liquid, but some vapor phase could be present.
The pressure of vessel 73-V-04 is controlled by split range controller, with an injection of dry hydrogen to pressurize the drum and a vent line routed to the stabilizer.
A major part of the liquid from 73-V-04 is pumped via recycle pumps hydrogenation reactor recycle pumps 73-P-03A/B under flow control and used as feed diluent. The other part is pumped, under flow control; by isomerization reactor feed pumps 73-P-04 A/B to first stage isomerization reactor 73-R-02.
In case of high benzene content in feed, a part of diluent is cooled in reactor recycle air cooler 73-A-01, while remaining goes through the air cooler by-pass line, in order to allow the control of the isomerization reactor inlet temperature (and consequently of the hydrogenation reactor). An additional pitch fan control is used for higher temperature rise in benzene saturation reactor.
In case of low benzene content in feed, the temperature is controlled via HP steam of the 73-E-04 (air cooler is not required and is fully bypassed).
Diluent recycle is the key factor to control the temperature of the hydrogenation reactor. Indeed, the rate of diluent recycle controls the reactor temperature gradient, while the level of cooling on the diluent recycle controls the reactor inlet temperature.
The rate of diluent recycle must be adjusted by operator in order not to exceed 35°C of exothermicity in the hydrogenation reactor.
4.2.3. Isomerization Section
The isomerization reactors feed from flash drum is fed under flow control and mixed with hydrogen under ratio flow control.
A small amount of chloriding agent is continuously injected into isomerization reactor feed, by pump 73-P-02 A/B, in order to maintain the chloride balance on the isomerization catalyst. This is a make-up for catalyst chloride, which is lost in reactor effluent.
The mixture is routed to a static mixer 73-M-01 (homogenous mixing) before entering the first stage isomerization reactor 73-R-02, where isomerization reactions occur. These reactions are slightly exothermic. The reactor effluent, that leaves the first isomerization reactor, has to be cooled before entering the second stage isomerization reactor 73-R-03. Temperature of the second isomerization reactor inlet is controlled by first stage isomerization reactor feed / effluent exchanger 73-E-03 bypass. In 73-R-03, remaining isomerization reactions occur.
Both 1st and 2nd stage isomerization reactors are mixed phase, down-flow reactors, with a
single catalyst bed. The isomerization reactors are designed to operate in the lead/tail position or in a single reactor configuration.
The reactor circuit inlet pressure is controlled using a back pressure controller located on the reactor effluent stream, routed to the stabilizer 73-C-01. The temperature profile in each of the reactors is monitored with multiple temperature indicators located across the
catalyst bed. The differential temperature between adjacent thermocouples is a measure of the extent of reaction, while also indicating the reactive zone of the catalyst bed. The effluent is then routed to the stabilizer 73-C-01 under pressure control.
4.2.4. Stabilizer Section
The raw isomerate is received in the stabilizer column 73-C-01 under pressure control. The purpose of the stabilizer is to reduce C4- rate in the isomerization reactor effluent.
LPG, H2 and HCl are stripped and sent to LPG recovery section. The Stabilizer
operating pressure is optimized to strip out the C4 components from the reactor effluent,
while minimizing the C5 hydrocarbon vent losses and reducing the C4 content of the
Isomerate product.
The stabilizer overhead is partially cooled in stabilizer air condenser 73-A-02. It is collected in the stabilizer reflux drum 73-V-05 where the vapor phase is routed under pressure control to the scrubber while the liquid is pumped by pump 73-P-05 A/B under flow control with level reset to the stabilizer as reflux.
Stabilizer bottoms are reboiled with high pressure steam in exchanger 73-E-05. Reboiler duty is under temperature control at the sensitive tray of the column. The temperature control is cascaded with the flow control on low pressure condensate.
Stabilizer’s bottom is routed to deisohexaniser column 73-C-02 under flow control cascaded with the level control on the column bottom, via a chloride guard bed 73-V-06. This guard bed prevents any chloride component in the deisohexaniser section during upset.
4.2.5. Deisohexaniser
Deisohexaniser (called DIH) 73-C-02 is fed with stabilizer bottom which preheat the DIH pump around through DIH feed/recycle exchanger 73-E-07. Deisohexaniser recovers stabilized isomerate product and recycles low octane methyl-pentanes and n-hexane to the reactors. This is done via the DIH recycle drum 73-V-07, which is fed on level control. Recycle is pumped, by pump 73-P-07 A/B as a recycle to the reactor
section after being cooled successively in the deisohexaniser recycle/reactor feed exchanger 73-E-02 and recycle trim cooler 73-E-08.
Remaining liquid from 73-P-07 A/B is fully vaporized by heat exchange with DIH feed in 73-E-07, and is recycled to the DIH column, in order to reduce the heat load required for reboiling of the column.
Overhead vapor of the column is totally condensed through deisohexaniser air condenser 73-A-03 and is routed to the deisohexaniser reflux drum 73-V-08. Reflux is pumped by deisohexaniser reflux pump 73-P-06 A/B under flow control. Isomerate product is pumped, under flow control reset by 73-V-08 level control, by pumps 73-P-09 A/B and is further cooled in sea water cooler 73-E-09 and routed to storage.
The Deisohexaniser reflux drum pressure is controlled by split range pressure controller at column overhead.
Deisohexaniser bottom is reboiled by medium pressure steam in exchanger 73-E-10. Steam flow controller to the reboiler is reset by temperature control at the sensitive tray of the column.
The bottom stream is concentrated in C7+ and C6 Naphthenes. It is pumped by
Deisohexaniser bottoms pumps 73-P-08 A/B to the LPG recovery section, where it is used as lean oil make-up. The lean oil is under flow control cascaded by the column level control.
4.2.6. LPG Recovery Section
The purpose of this section is to optimize LPG recovery from the stabilizer off gas, by using of DIH bottom stream as lean oil.
The dry off gas is mixed with the recycle gas from LPG stripper reflux drum 73-V-12. This stream is contacted with cold lean oil from LPG separator drum 73-V-11. The two phase stream is further cooled LPG refrigeration system 73-E-13B exchanger and fed to the LPG separator drum, in which gas phase and liquid phase are separated. The gas phase is send to a packed zone, located in upper section of drum, for counter current contacting with the cold lean oil, in order to maximize LPG recovery.
Off gas from this section is sent, under pressure control, to Fuel Gas.
The rich oil containing the absorbed LPG is pump from the cold separator drum through 73-P-14 A/B, underflow control cascaded with level control of separator drum 73-V-11, to stripper 73-C-04. The stripper feed is preheated through heat exchangers 73-E-14 and 73-E-15, in order to optimize the recovery.
LPG stripper reboiler with HP steam as in 73-E-17 allows recovering the LPG product which is produced as side stream liquid draw-off below pasteurization zone. Draw off is cooled in LPG trim cooler 73-E-18 A/B and pumped by LPG product pumps 73-P-16 A/B to the battery limit. The LPG product is on flow control cascade by column top temperature control.
Stripper overhead is partially condensed in LPG stripper condenser 73-E-19 A/B. The stripper off gas is recycled and mixed with the dry scrubber off gas. The stripper overhead is under pressure control.
The stripper bottoms stream is routed, under flow control cascade with the level control, to feed/bottom heat exchangers of LPG stripper via 73-P-15 A/B. then lean oil is combined with DIH bottom stream. The mixture is cooled down through LPG stripper feed/bottom exchanger 73-E-15 and through water cooler 73-E-16 A/B. Part of this stream is sent to export via storage. The other part is fresh lean oil (to be cooled down before absorption of LPG) and is recycled to packed zone of the LPG separator drum. Reboiler duty is under temperature control at the sensitivity tray of column.
In case LPG recovery is in shut down, facility is provided to by-pass completely this section and route scrubber off gas directly to Fuel gas or to flare.
4.2.7. Scrubber Section
As the gas from stabilizer overhead contains HCl, it must be caustic treated and water washed before released to LPG recovery section. Off gas enters the bottom of the caustic scrubber 73-C-03 through hold-up and is first caustic washed, in a first packed bed. Then off gas, saturated by caustic, is water washed in second packed bed before being routed to scrubber off gas dryer. Caustic weight fraction varies from 10% wt to 2% wt as
it reacts with HCl to produce NaCl. The caustic is re-circulated by 73-P-11 A/B and is
maintained at the 55 oC through the caustic recycle heater 73-E-12, in order to keep the
caustic a few degrees warmer than the feed gas to avoid potential foaming problems due to any hydrocarbon condensation.
Both the scrubber section are packed with raschig rings. The caustic inventory requirement is stored in the tower bottom section and feed gas is bubbled through this caustic inventory. A portion of the circulating caustic is sprayed on to the column walls below the caustic wash packed section to avoid any wet hydrogen chloride corrosion in this part of the scrubber.
When the concentration of circulating caustic has been decreased to about 2% wt, the caustic inventory is drained through pump 73-P-11 A/B discharge and tower bottom is filled up with 10% wt fresh caustic. Spent caustic is sent to the spent caustic system. Fresh 10% wt is made up through pumps 73-P-10 A/B.
The gas leaving the caustic wash section is washed with water in the top packed section, to remove any entrained caustic. Water is collected in the chimney tray below the water wash packed section, and is circulated using the pumps 73-P-13 A/B. water loss to the vent gas leaving the scrubber is made-up periodically by fresh de-mineralized water addition (via pump 73-P-12 A/B and de-mineralized water drum 73-V-10), and once every several days the water inventory is drained and replaced.
4.2.8. Dryers regeneration:
Naphtha feed, H2 make up and scrubber off gas flow through their respective dryers in
series. Molecular sieve becomes saturated after a certain period of time. Then they need regeneration. The on line moisture analyzer is used to monitor the moisture content of the streams leaving each dryer.
The feed dryers (73-DR-01 A/B), the Hydrogen dryers(73-DR-02 A/B) and the scrubber off gas dryers are regenerated using vaporized deisohexaniser distillate product (from 73-P-09 A/B) as regenerant medium to remove down-flow the water trapped by the
molecular sieves. The concerned dryer is isolated from the other one which is still in service.
This regenerant is completely vaporized by high pressure steam in dryers regenerant vaporizer 73-E-20. Liquid level in the vaporizer is monitored closely to avoid liquid carryover to dryers regeneration superheater 73-F-01 (electrical heater). The steam to the vaporizer is under flow control and is cascaded to the level control of the vaporizer. After passing through the dryers, the regenerant stream is cooled down by dryers regenerant air condenser 73-A-04 and further by dryers regeneration trim cooler 73-E-21. This stream is routed via dryers regenerant degasser 73-V-13 to isomerate storage. The Regenerant Degasser is a liquid flooded drum, releasing the effluent regenerant liquid on pressure control for mixing with the isomerate product. Any light components which accumulate in the Regenerant Degasser are purged to flare as required through a liquid level controller, which maintains a low liquid level at the very top of the drum. Free water collected at the bottom of the Regenerant Degasser is periodically drained to oily water sewer.
4.3 CHLORIDE INJECTION FACILITIES
Tetra-chloro-ethylene, C2Cl4 is the recommended chloriding agent which is injected
continuously from the chloride agent injection drum (73-V-03) to the Isomerization reactors by pumps 73-P-02 A/B. This injection corresponds to the make-up for the amount of chloride loss from the chlorinated platinum catalyst. C2Cl4 is fully converted
to HCl at reactor normal operating conditions. Hence chloride lost in the reactor effluent is HCl form, which is finally converted into NaCl in the caustic scrubber.
4.4 UTILITY SYSTEM
The utility system consists of HP steam, MP steam, LP Steam, Condensate, Service Water, Cooling Water, Bearing Cooling Water, Instrument Air, Plant Air, Nitrogen, Fuel Gas, etc. Closed Blow Down (CBD), Flare is also provided within the unit.
4.4.1. HP Steam System
An 8” High pressure steam header has been provided for ISOM unit. HP steam is distributed to the following equipment of the ISOM unit.
- Hydrogenation reactor feed heater 73-E-04
- Stabilizer reboiler 73-E-05
- LPG stripper reboiler 73-E-17
- Dryer regenerant vaporiser 73-E-20
4.4.2. MP Steam System
A 10” header supplies MP steam to the ISOM unit. Use of MP steam in ISOM unit is mainly as follows:
- Deisohexaniser reboiler 73-E-10
- Start-up ejector 73-J-01
4.4.3. LP Steam System
A 4” header supplies LP steam to the ISOM unit. Mainly use of LP steam in ISOM unit is as follows:
- Utility hose station
- Feed Surge drum 73-V-01
- Chloriding agent injection drum 73-V-03
- Compressor 73-K-01 A/B
- DIH Recycle drum 73-V-07
- Deisohexaniser 73-C-02
- Refrigeration package 73-LC-01
- LPG stripper 73-C-04
4.4.4. Cooling Water System
The cooling water requirement for cooling purpose in the ISOM unit is met through sea cooling water system. A 20” cooling water supply header supplies water to ISOM unit. Cooling water from the supply header is taken to the following equipment in ISOM unit.
- Compressor 73-K-01 A/B
- LPG trim coolers 73-E-18 A/B
- Make up H2 trim cooler 73-E-01 A/B
- Lean oil trim coolers 73-E-16 A/B
- To sample points
- To analysers
- Isomerate trim coolers 73-E-09 A/B
- Recycle trim coolers 73-E-08 A/B
- Stabilizer trim cooler 73-E-06 A/B
- Dryers regeneration trim cooler 73-E-21 A/B
The return water is collected from all the equipments through cooling water return header and sent to B/L.
Cooling water regularly PH, Chlorine, turbidity and conductivity to be monitored regularly. In this PH and chlorine are most important parameters. Due to this, chances of exchanger shell or tube damage increases.
4.4.5. Bearing Cooling Water System
Bearing cooling water is required for pump and compressor cooling purpose. A 4” bearing cooling water supply header supplies water to ISOM unit. Bearing Cooling water from the supply header is taken to the pumps and compressors.
4.4.6. Service Water System
The 3” common service water header supplies service water to the ISOM unit. The service water header supplies water to various hose stations and chloriding agent injection pump 73-P-02 A/B, to prevent any spillage, in the units. Service water is required mainly for cleaning and washing.
4.4.7. Instrument Air System
A 2” Instrument Air header supplies IA to ISOM unit. Apart from instrument air tapping instrument air is supplied to the following facilities.
- Hydrogen Compressor 73-K-01 A/B
- Refrigeration package 73-LC-01
Instrument air Dew point (-400 C) is to be monitored regularly. If moisture is more then it
will create problems in control valves (mainly scaling). 4.4.8. Plant Air System
A 3” plant air header supplies plant air to the ISOM unit. Plant air is required for the hose stations, Anhydrous HCL bottle, Refrigeration package etc.
4.4.9. Nitrogen
A 3” header supplies N2 to the ISOM unit. N2 is used for various purposes in equipment,
line etc. for inertisation, blanketing, purging etc. In addition to that following permanent connections are provided for N2 in the ISOM unit.
- Hydrogen make up compressor (73-K-01 A/B)
- Isomerisation feed surge drum (73-V-01)
- Close Blow Down system (73-V-16)
- Stabiliser bottom chloride guard bed(73-V-06)
- Deisohexaniser reflux drum (73-V-08)
- Stabiliser (73-C-01)
- Hydrogenation reactor flush drum (73-V-04)
- Isomerization reactors (73-R-02 / 03)
- Hydrogenation reactor (73-R-01)
- LPG separator drum (73-V-11)
- LPG stripper (73-C-04)
- ISOM feed dryers (73-DR-01 A/B)
- ISOM hydrogen dryers (73-DR-02 A/B)
- Scrubber offgas dryer (73-DR-03 A/B)
- Dryer regenerant vaporizer (73-E-20)
- Anhydrous HCL bottle etc.
Nitrogen purity is to be maintained by controlling oxygen ppm (<0.5). If nitrogen purity is less then chances of damaging catalyst increases.
4.4.10. Fuel Gas System
FG supply header is of 2” size. Fuel Gas is mainly used for flare header purging and maintaining the pressure of feed surge drum 73-V-01.
4.4.11. Flare System
Relief line from safety valves, Dry gas seal system of the compressor, Purge gases etc. are collected in the common 28” common flare header.
4.4.12. Closed Blow-Down System
Drains from various equipments are collected in two nos of Closed Blow-down headers (4” size each) and combined to form 6” header leaving the battery limit.
4.5 WASTE EFFLUENT FROM ISOMERIZATION SECTION 4.5.1. Spent Caustic from Scrubber section:
Spent caustic is one of the effluents leaving the unit, and it is sent to the ETP unit. The spent caustic is removed from the bottom of the Caustic Scrubber 73-C-03 on an intermediate basis of estimated at 7 days.
Quantity, Tons/week 23
Temperature, oC 45-50
Water 87.3
Also, a small amount of spent water that is purged periodically from the scrubber water wash section is combined with the spent caustic. This purge water contains entrained caustic and dissolved light hydrocarbons.
4.5.2. Oily water:
Normally there is no sour water effluent leaving this unit, as the hydrocarbons and the hydrogen feed streams contain only traces of water. However the maximum effluent discharge from Regenerant Degasser is approximately 16 Kg/hr based on the maximum
effluent water content specified for the hydrocarbon feed, H2 make-up and scrubber off
SECTION – 5
PRE-COMMISSIONING PROCEDURE
5.1 GENERAL
As the new unit nears completion, there is a large amount of preparatory work, which should be performed by the operating crew. A planned check of the unit will not only set the foundation of a smooth start-up, but will also provide a firm basis for acquainting operators with the equipment. Start-up is a critical period and the operator must know exactly the operation of all equipments.
Some of the pre-commissioning works can be carried out simultaneously along with construction. But, care in the organisation of this work is necessary so that it will not interfere with construction work. It is most important to plan schedule and record with checklists and test schedules all the preliminary operation and to co-ordinate the construction programme.
5.2 PRE-COMMISSIONING ACTIVITIES
The material in this section gives general guidelines for preparing a unit for start-up. Some sections need to be expanded to give specific directions (water flushing procedure, inertising procedure for example); this shall be prepared by commissioning personnel prior to start of the pre-commissioning/start-up.
5.3 INSPECTION / CHECKING
Sections of the unit should be checked out as soon as the contractor completes work in those areas. Immediately following inspection of those areas, punch lists which indicate the deviations from the design specifications should be written and distributed to the contractor. In this manner mistakes in construction can be found and corrected early. Inspection of the plant can be basically divided into the following areas:
- Vessels including reactor
- Piping - Heaters - Exchangers - Pumps - Compressors - Instrumentation - Catalyst/Chemical Inventory
Inspection of vessel, column, heaters etc.
Inspection of the interior of the vessels, columns, heaters and other equipments not normally accessible during operation should be made to ensure that they are complete, clean and correctly installed. Tray assemblies in columns should be checked with reference to the engineering drawings to detect any defect in assembly or construction and to ensure cleanliness. Packings, if any, are to be done after internal inspection and flushing. The vessels are to be checked with reference to engineering drawings. The demisters are to be fitted after internal cleaning and water washing.
In heaters, the burner assemblies should be checked for easy operation of air registers,
contour of the burner throat, debris material etc. The heater coils supports to be checked for proper installation.
Piping and accessories will be checked against drawings and specifications. Piping support and hangers will be inspected to ensure that all anchorages are firm. Valves will be checked for proper packing and mounting direction and accessibility for operation and maintenance. Spring supports, if any, to be checked for the cold setting and later for hot settings while plant is in operation. Check for completion of welding work, especially on small bore piping and socket weld valves.
b) Instruments
All instrument tapings for pressure, level and flow should be clear and thermo wells should not foul with the internals. These should be checked prior to box up of the equipment. Instruments will be checked, starting from the controller and proceeding logically through the control loop. Cascade control system will be checked from the impulse point of primary loop. Operating crew should check proper mounting of control valves. Control valves responses should be checked for controller outputs. The shutdown systems of the equipments should be checked by simulating the various conditions in the control circuits. c) Relief Valves
Relief valves will be set in the shop and mounted before the system pressure test. Block valves ahead and after relief valves will be checked for lock open or lock close position as per P&ID. Relief valves will be checked against specifications.
d) Rotary Equipment
All rotary equipment such as pumps, compressors, turbines etc. are to be checked for bearings, internals and free movement. The auxiliaries, control systems on this equipment should be thoroughly inspected.
e) Drainage System
5.4 PREPARATION FOR PRE-COMMISSIONING
- Check the unit for completion of mechanical work against P&ID.
- Check list points are liquidated. Any pending point should not affect pre-commissioning operation.
- Remove all construction debris lying around in the unit and clean up the area.
- Install blinds as per master blind list.
- Safety valves should be kept blinded during flushing and re-installed afterwards.
These should be shop tested and set at the stipulated values.
- Ensure that underground sewerage system is in working condition. Clear plugging, if any. Check by flushing with water.
- Check that communication between units, control room, offsites and utilities are
complete and in working condition.
- Ensure that the required lube oil, grease and other consumable are available in
the unit.
5.5 PRE-COMMISSIONING OPERATION
Prior to the commissioning of the plant there are several pre-commissioning operations that must be conducted to prepare the plant for the actual start-up; these are:
1. Commissioning of utilities
2. Final inspection of vessels
3. Flushing
4. Functional test of rotating equipment
5. Service and calibrate instruments
6. Vessel Loading
7. Tightness test
8. Purge and gas blanketing
9. Dry out of Reactor section
It is important that these operations be carried out as thoroughly as possible to help achieve a smooth and trouble-free start-up and later steady normal operation. A discussion detailing the major items to monitor in each of these operations follows. The above outline may be expanded somewhat as follows:
5.5.1. Commissioning of Utilities
The various utility lines should be tested and placed into service as soon as the construction schedule allows. Pressure tests should be carried out on all steam condensate, air, fuel gas, flare, and nitrogen lines as is done on all process lines.
A. Steam Network
Network shall be blown through completely from battery limit with a strong steam flow in order to clean the lines. The following steps are recommended:
- Check network, all equipment will be disconnected to avoid entry of flushed
material.
- Drain all the low points. If necessary open steam trap inlet flanges.
- Open slowly battery limit valve and let the temperature rise in the header, slowly
and steadily.
- Check support of fixed points and expansion loops.
- When line is hot, blow it through completely with a strong steam flow.
- Close battery limit valve and prepare another network. When the blowing is
satisfactory, reconnect all equipment and remount steam traps. Recharge header as above.
- To gauge the effectiveness of the steam blowing (and the amount of scale left in
the lines), target plates should be installed at the blow down points. The lines should be repeatedly blown down until virtually unmarked target plates are obtained. Condensate lines should be continually checked and traps removed and cleaned if plugged.
Note: The following precautions to be taken while blowing / commissioning steam header:
- To drain the low points of the lines before and during heating period in order to
avoid water accumulation, this causes hammering.
- To open drain / vent during cooling period to prevent vacuum formation
- To isolate the instruments, remove orifice plates and control valves; to re-install
the orifice plates and control valves after blowing is over. B. Cooling Water and Service Water:
Network shall be cleaned from battery limit with a strong water flow. All equipment will be disconnected at the inlet and reconnected when lines are cleaned. Control valves and orifice plates will be removed and re-installed, after the lines become clean. When system has been flushed, charge the lines to the operating pressure.
The following precautions to be taken:
- To open vents at high points in order to expel air from equipment and piping
- To open the battery limit valve, slowly and steadily.
C. Instrument and Plant Air:
Network shall be blown through completely from battery limit with strong flow of air in order to clean and dry the lines. All joints and connections shall be checked for tightness with soap solution. Header and branch lines will be blown through with a high flow rate of air. During all tests, the instruments and control valve shall be carefully isolated from the system.
D. Fuel Gas Networks:
Networks shall be blown through from battery limit with a strong air flow in order to clean the lines. During the operations, orifice plates and control valves shall be removed.
