PERFORMANCE EVALUATION OF THE WARRI REFINERY FLUID CATALYTIC CRACKING UNIT

EMEMERURAI OGHENEOVO JOELHND (PETROLEUM REFINING TECHNOLOGY), PETROLEUM TRAINING INSTITUTE, EFFURUN; PGD (CHEMICAL ENGINEERING), RIVERS STATE UNIVERSITY OF SCIENCE AND TECHNOLOGY, PORT HARCOURT).

PG. 1997/09263

A THESIS SUBMITTED TO THE DEPARTMENT OF CHEMICAL/PETROCHEMICAL ENGINEERING, FACULTY OF ENGINEERING, RIVERS STATE UNIVERSITY OF

SCIENCE AND TECHNOLOGY, NKPOLU-OROWORUKWO, PORT HARCOURT,

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD

OF THE DEGREE OF MASTER OF TECHNOLOGY (M.TECH) IN CHEMICAL

ENGINEERING.

SEPTEMBER, 2001

ABSTRACT

The evaluation of the Warri Refinery Fluid Catalytic Cracking Unit (WRFCCU) was done to determine the extent to which its designed functions had been carried out. In this work operating data and mathematical correlations were used to evaluate the unit. The approach adopted is quite distinct from the use of process models such as the three lump cracking model and the regenerator model, which require computer application. The evaluation was done using the following indices: Onstream availability, gasoline selectivity, driveability index of gasoline, hydrogen-methane ratio in fuel gas, hydrogen content of coke, carbon on regenerated catalyst, and ratio of equilibrium catalyst surface area and kinetic conversion (S.A/K). Other indices are ratio of carbon monoxide and carbon dioxide in the regenerator flue gas (CO/CO2), metal contamination of catalyst, and catalyst fluidization properties which determine the ease with which the catalyst can be made and maintained in a fluid state. It was observed that the unit had a large downtime due to the non-availability of fresh catalyst and the failure of the spent catalyst plug valve. The low availability of Vapour Recovery Unit of the FCC plant resulted in the flaring of gasoline estimated to cost about N344137200 with its concomitant environmental pollution. The calculated average driveability index of FCC gasoline of 616.250C was higher than the specified value of 5000C. There was an indication of metal contamination of the circulating catalyst since the H2/CH4 ratios were above the optimum of 0.6. This was confirmed with such parameters as Nickel equivalent and contamination index. The catalyst stripper performed well as indicated by the SA/K number. Finally, the fluidization parameters showed that the circulation of catalyst within the F.C.C converter was smooth.

CERTIFICATION

This is to certify that this work was done under my supervision by Mr. Ememerurai O.J

of the Masters programme in Chemical Engineering, Chemical / Petrochemical

Engineering Department, School of Post Graduate studies, Rivers state University of

Science and Technology (RSUST), Nkpolu-Oroworukwo, Port Harcourt.

DEDICATION

This thesis is dedicated to all persons who earnestly and genuinely seek for knowledge,

for their reward will be enlightenment.

ACKNOWLEDGEMENT

I would like to express my gratitude to all those who helped in one way or the other to

the completion of this thesis and, indeed, my Masters degree in chemical engineering.

Specifically, my sincere thanks go to my course mates, Miss Mavis Omovo and Miss

Gladys Agbone for their understanding and brotherly love. My thanks also go to the rest

of my course mates and all staff of chemical/petrochemical engineering department of

the university. I am also indebted to my thesis supervisor, Dr Samuel Ebika Ovuru, for

his advice and corrections. I also salute all the lecturers who imparted their knowledge

in chemical engineering and related fields to me during the Masters programme. They

are Dr. E.O Oboho, Dr H.A. Ogoni, Dr S.A. Amadi, Dr E.N Wami and Prof. M.F.N.

Abowei. Others are Dr. I.E. Douglas and Dr. Koumakou both of marine engineering

department of the Rivers State University, Port Harcourt. The same special thanks go to

Engr. A. S. Ladenegan and Messrs Daniel Alao and Stephen Idiata of the Warri

Refining and Petrochemicals Company (WRPC), Ekpan for lending me some books and

supplying some of the data used for the thesis. I also appreciate the contributions of

Messrs E.N Umoh, Jonas Siloko, Agrippa Emu (my nephew) and Mrs. Odiere De-Ayes

of the Postgraduate school, Rivers State University Port Harcourt. Finally and most

importantly, I thank God for his love and guidance throughout the period of my studies.

God bless every one! Amen.

TABLE OF CONTENTS

Content page

Title page i

Abstract ii

Declaration iii

Certification iv

Dedication v

Acknowledgement vi

Table of content vii

List of Figures ix

List of Tables xi

Nomenclature xiii

CHAPTER 1: INTRODUCTION

1.1 Significance and objective of the evaluation 3

1.2 Process description of the WRFCCU 4

1.2.1 Feed pre-heat 5

1.2.2 Riser, Disengager, and Catalyst stripper 5

1.2.3 Catalyst regenerator 6

1.2.4 Main fractionator 8

1.2.5 The Vapour Recovery Unit (VRU) 9

1.2.6 Treatment unit 12

CHAPTER 2: LITERATURE REVIEW 15

CHAPTER 3: PERFORMANCE EVALUATION OF THE

WARRI REFINERY FCC UNIT 34

3.1 Onstream availability or time efficiency of the unit 34

3.2 Gasoline selectivity 35

3.3 Driveability index of gasoline 36

3.4 Hydrogen/methane ratio in fuel gas of WRFCCU 36

3.5 Hydrogen content of coke (H/C) and CO/CO2 ratio 37

3.6 Carbon on regenerated catalyst, kinetic

Conversion and S.A/K ratio 40

3.7 Effect of metals on product distribution 40

3.8 Evaluation of catalyst fluidization properties 40

CHAPTER 4: DISCUSSION OF RESULTS OF THE

EVALUATION 45

CHAPTER 5: CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion 51

5.2 Recommendations 53

REFERENCES 54

APPENDICES 73

LIST OF FIGURES

Figure Page4.1 Relationship between ASTM 50 and 90% temperatures

And driveability index of FCC Gasoline in 1996 58

4.2 Hydrogen-methane ratio in FCC fuel gas 59

4.3 hydrogen make and metal index 60

4.4 Hydrogen content of coke or stripper efficiency 61

4.5 Relationship between net heating value of coke and hydrogen Content of coke 62

4.6 Relationship between catalyst activity, coke on regenerated Catalyst and SA/K Number 63

4.7 Extent of metal contamination of WRFCCU catalyst 64

4.8 Comparison of contamination index of equilibrium catalyst With a minimum of 900 65

4.9 Particle size distribution (PSD) of WRFCCU E-Cat 66

4.10 Umb/Umf, collapse time, and particle diameter relationship of WRFCCU E-Cat 67

4.11 Umb/Umf, collapse time, and average particle size relationship Of WRFCCU catalyst Calculated using Engelhard’s correlation 68

4.12 Effects of catalyst properties on the ratio of collapse time and height of minimum Fluidization 69

A.1. The converter (reaction section) of the Warri Refinery FCC unit 74

A.2. The Vapour Recovery Unit (VRU) section of the WRFCCU 75

A.3. LPG-H2S Absorber circuit of the WRFCCU 76

A.4. Fuel Gas-H2S Absorber circuit of the WRFCCU 77

A.5. The LPG Merox Unit of the WRFCCU 78

A.6. The Gasoline Merox Unit of the WRFCCU 79

A.7 Log-normal probability chart to evaluate the catalyst FractionLess than 40, 45 and 80 μm for catalyst sample A 80

A.8. Log-normal probability chart to evaluate the catalyst fraction Less than 40, 45 and 80 μm for catalyst sample B 81

A.9. Log-normal probability chart to evaluate the catalyst fraction Less than 40, 45 and 80 μm for catalyst sample C 82

A.10. Log-normal probability chart to evaluate the catalyst fraction

Less than 40, 45 and 80 μm for catalyst sample D 83

A.11. Log-normal probability chart to evaluate the catalyst fraction Less than 40, 45 and 80 μm for catalyst sample E 84

A.12. Log-normal probability chart to evaluate the catalyst fraction Less than 40, 45 and 80 μm for catalyst sample F 85

A.13. Log-normal probability chart to evaluate the catalyst fraction Less than 40, 45 and 80 μm for catalyst sample G 86

A.14. Log-normal probability chart to evaluate the catalyst fraction Less than 40, 45 and 80 μm for catalyst sample H 87

LIST OF TABLES

Table Page4.1 Contribution of downtime hours for 1996-1998 70

4.2 Driveability indices of design feed gasoline 71

4.3 S.A/K numbers from literature 72

B.1 Comparison of design and operating data of the Warri Refinery FCCU in 1993 88

B.2 WRFCCU Flue Gas analysis in 1993 89

B.3 Air required for coke burning in FCC regenerator 90

B.4 Troubleshooting guide using metals content of equilibrium catalyst 91

B.5 Data base used for correlations of FCC catalyst particle density 92

B.6 Coefficients for correlation (prediction) of particle density Using equation (2.37) 93

B.7 The WRFCCU onstream availability data for 1996 94

B.8 The WRFCCU onstream availability data for 1997 95

B.9 The WRFCCU onstream availability data for 1998 96

B.10 Hours of Gas flaring by the WRFCCU in 1996 97

B.11 Hours of Gas flaring by the WRFCCU in 1997 98

B.12 Hours of Gas flaring by the WRFCCU in 1998 99

B.13 Gasoline selectivity or efficiency of the WRFCCU in 1996 and 1997 100

B.14 Driveability index of WRFCCU gasoline in 1996 101

B. 15 Hydrogen-Methane ratio in WRFCCU fuel gas 102

B. 16 Calculation of standard cubic feet of hydrogen per barrel

Of feed and metal index 103

B.17 WRFCCU regenerator flue gas analyses and Hydrogen content of coke calculations 104

B.18 Net heating values of coke (NHVC) in WRFCCU Regenerator 105

B.19 Relationship between coke on regenerated catalyst (CRC),Catalyst activity, and S.A/K ratio 106

B.20 Metal contamination of WRFCCU catalyst 107

B.21 Physical properties of WRFCCU equilibrium catalyst 108

B.22 Average particle diameter (dp) of WRFCCU Equilibrium catalyst 109

B.23 Calculation of cumulative weight percentage of catalyst Particles Less than the minimum in each size range 110

B.24 Fraction of catalyst particles less than 40, 45 and 80 μm For Catalyst samples A through H 111

B.25 Particle density, Umb/Umf ratio, F-prop and collapse time of WRFCCU equilibrium catalyst for the regenerator 112

B. 26 Calculation of Umb/Umf ratio, collapse time, and height of Minimum fluidization (Hmf) 113

B.27 Calculation of Umb/Umf ratio, and collapse time for the WRFCCU Converter standpipe 114

NOMENCLATURE

A Air rate to regenerator, Kg/hr

ABD Apparent Bulk density

API American Petroleum Institute

APS Average particle size, micron

ASTM American Society for testing and materials

bbl Barrel

C Amount of carbon burned in regenerator, Kg/hr

CA Atomic weight of carbon

CB Carbon black (petrochemical product)

CCR Catalyst circulation ratio

CDU Crude distillation unit

CO Mole % of carbon monoxide

CO2 Mole % of carbon dioxide

C/O Catalyst-oil ratio

CRC Carbon (Coke) on regenerated catalyst wt %

Cu Amount of copper in E’cat

DCO Decanted oil

dp Diameter of particle, m

DI Driveability index of gasoline / Davison index of E’Cat

E’Cat Equilibrium catalyst

F40 Weight fraction of catalyst fines less than 40 microns.

F45 Weight fraction of catalyst fines less than 45 microns.

F80 Weight fraction of catalyst fines less than 80 microns.

FFR Fresh feed rate

F-Prop Raterman fluidization factor

g Acceleration due to gravity, ms-2

GLN Eff Gasoline efficiency

h Height of catalyst bed surface above regenerator air distributor m.

H Amount of hydrogen burned in regenerator, Kg/hr

HA Atomic weight of hydrogen

HD Intercept of extrapolated collapse curve at t=0, m

Hmf` Height of minimum fluidization

H/C Hydrogen content of coke

HCO Heavy cycle oil

HFAU Hydrofluoric acid alkylation unit.

HGO Heavy gas oil

HN Heavy naphtha

HPS High pressure steam

K Kinetic conversion

LCO Light cycle oil, m3

LPG Liquefied petroleum gas

MAT Micro activity test

MEROX Mercaptan oxidation

MEA Monoethanolamine

MH2 Mass flow rate of hydrogen

Mol. Frac. H2 Mole fraction of hydrogen

Mol. Wt Molecular weight

Mol. Wt. H2 Mole weight of hydrogen

N.E Nickel equivalent of equilibrium catalyst, ppm

NHVC Net heating value of carbon

Ni Amount of Nickel in catalyst, ppm

O.N Octane number of gasoline

PSD Particle size distribution of catalyst

PPM Parts per million

PV Pore volume cm3/g

QA Quantity of aeration medium required

QE Amount of entrained steam entering regenerator from standpipe

RE Rare earth content of catalyst wt %

RCPV Regenerated catalyst plug valve (16-PV-2)

RVP Reid vapour pressure, Kg/cm2

ROM Research octane number

SA Surface area of catalyst, m2/g

Scf Standard cubic feet

S.G Specific gravity at 600F

SCPV Spent catalyst plug valve 16PV1

T10, T50, T90 10, 50 and 90 volume % ASTM temperatures, respectively

Ud Velocity of gas, m/s

Ude Deaeration velocity, m/s

Umb Minimum bubbling velocity, m/s

Umf Minimum fluidization velocity, m/s

Uo Gas superficial velocity, m/s

Up Solids superficial velocity, m/s

USL Slip velocity

UQCC Ughelli quality control centre

V volume of air in regenerator, vanadium content of catalyst

VH2 Volume of hydrogen

VP Volume of particles

VOC Volatile organic compound

VRU Vapour recovery unit

VPGY Volume percent gasoline yield

WP Weight of particles, Kg

WGC Wet gas compressor

WPCY Weight percent coke yield

WRFCCU Warri refinery fluid catalytic cracking unit

WRPC Warri refining and Petrochemicals Company

Wt% Weight percent

Xi Weight fraction of catalyst of a given size range

X1 ABD or CBD

X2 (X1)2

X3 Non zeolite mole fraction of AI2O3 in catalyst

X4 (X3)2

X5 (X3)3

X6 Mole fraction of alkali oxides

X7 Weight fraction of alkali oxides

X8 Natural logarithm of APS

Μ Micron

Hcomb Heat of combustion

τc Collapse time of catalyst

μg Gas viscosity, kg/ms

ρg Gas density, kg/m3

ρp Density of particle kg/m3

EQUIPMENT

A Air cooler

B Boiler

C Column, fractionators, absorber

D Drum, separator

E Heat exchanger, cooler, condenser

H Heater, furnace

CHAPTER ONE

INTRODUCTION

The primary function of the Fluid catalytic cracking unit (FCCU) of the Warn

Refinery is to augment or increase the entire refinery gasoline pool or yield with

respect to the crude oil fed to the crude distillation unit (CDU) of the refinery. The

unit converts high boiling petroleum fractions called gas oil (which boils in the

range of 650-1050°F) to high-value high-octane gasoline. The gas oil used in the

refinery is the heavy vacuum gas oil from the vacuum distillation unit.

The other functions of the Warn Refinery Fluid Catalytic Cracking Unit

(WRFCCU) include:

(i) Production of C4 and C3 liquefied petroleum gases (LPG). The C4LPG

contains a higher percentage of hydrocarbons having four carbon atoms

and serves as feed to the hydrofluoric acid Alkylation unit (I-IFAU) of the

refinery where it is split into normal butane and isobutane. The latter is

charge to the HFAU reactor where it is reacted with light olefins in the

presence of hydrofluoric acid to produce a motor fuel called alkylate. The

alkylate has a very high octane number and is thus a very valuable motor

fuel blending component. The C3 LPG, which must be above 73 mole

percent purity with respect to the propylene content, is the feed to the

polypropylene plant of the petrochemical complex of the Warn Refining

and Petrochemicals Company (WRPC), Ekpan. The LPG produced also

forms a part of the domestic gas production of the entire refinery fuels

plant.

(ii) Production of ethane and lighter, which constitute the fuel gas (FG) used

for heating purposes in the refinery process heaters

or furnaces.

(iii) Production of stripped slurry oil or decanted oil, which serves as, feed to

the carbon black (CB) plant of the petrochemical

complex.

(iv) Production of Light cycle oil (LCO) that is a diesel-blending component.

(v) Removal of sulphur in the form of hydrogen sulphide (H2S) and

mercaptans from the fuel gas, LPG and gasoline. Sulphur compounds

present in the feed may form the acid gas, H2S, which react with

hydrogen. The H2S is either burnt in the catalyst regenerator, liberating

sulphur dioxide (SO2) or is sent to the main column form where it escapes

to the absorber column and debutanizer column. The absorber off gas and

the debutanizer overhead are then treated with monoethanolamine (MEA)

in the vapor recovery unit (VRU) of the plant. The mercaptans that are still

left in the LPG and those in gasoline stream are removed by the use of

caustic soda (NaOH) in the LPG and gasoline merox (MERcaptan

OXidation) units respectively.

In order to perform the above functions, many complex phenomena must be

encountered in the process: the reactions and reacting species are numerous while the

cracking catalyst deactivates very quickly and must therefore be reactivated or

regenerated. This thesis is therefore undertaken to evaluate the performance of

WRFCCU. The evaluation is meant to examine or study the products produced by the

WRFCCU and the catalyst used by the process based on historical data that are

available. This will give a moving picture of how the WRFCCU has performed in the

past. However, in order to perform the evaluation it is necessary to state the period that

will be covered. Based on available data, most of the evaluation will be based on the

three-year period of 1996 to 1998. However, when no data is available on any

parameter within this period data may be taken from literature for uniform comparison.

1.1 SIGNIFICANCE AND OBJECTIVE OF THE EVALUATION

The main objective of the evaluation is to establish the efficiency and effectiveness of

the WRFCCU operations. The significance of the results obtained from the evaluation is

enumerated as follows:

(i) They will enable marketers and people engaged in petroleum business to

assess the extent to which the WRFCCU and indeed the WRPC and

customers objectives have been met.

(ii) The evaluation will enable the comparison of operating data with design

values.

(iii) The information contained in this work especially the

equilibrium catalyst data may be useful for modeling and

simulation studies

(iv) The extent of environmental pollution attributed to low time efficiency of

the gas plant (Vapour recovery unit, VRU) would be pointed out.

(v) The evaluation may also sensitize academic interest to carry out

evaluation of this type in other refineries and processing plants in Nigeria.

No comprehensive evaluation of the WRFCCU has been done previously.

(vi) The disadvantages associated with analyzing the WRFCCU equilibrium

catalyst abroad would also be evident from the evaluation.

1.2 PROCESS DESCRIPTION OF THE WRFCCU

The Warn Refinery’s FCC process is of the Kellogg orthoflow “F” model and was

commissioned in 1978. It employs a micro-spheroidal zeolite based catalyst, which

fluidizes when properly aerated. The catalysts consumed presently by WRFCCU (at the

time of writing this thesis) are manufactured by Grace Davison of Germany and Akzo

Nobel of the Netherlands. The orthoflow riser reactor of the WRFCCU is shown in

Figure A.1. The regenerator and the riser arrangement is called the converter. In order

to have a thorough picture of the WRFCCU operation, the process description is divided

into six separate sections. These include the following:

(i) Feed preheat

(ii) Reactor (Riser)

(iii) Regenerator

(iv) Main fractionator

(v) Vapour recovery unit

(vi) Treatment facilities

1.2.1 Feed Pre-Heat

The atmospheric heavy gas oil and vacuum gas oil blend from the tank farm is received

into a surge drum which also serves as a device to separate any water that may be

present in the feed through a manually operated drain valve. From the surge drum, a

centrifugal pump transports the feed through four separate heat exchangers and a

preheater furnace. The heating fluids in the four heat exchangers are streams from the

main fractionator and these serve to maximize process heat recovery. These streams

include heavy naphtha, light cycle oil, heavy cycle oil and slurry bottoms. The preheat

train normally heats the feed from about 80Oc (in the feed surge drum) to about 320Oc at

the preheater furnace outlet.

1.2.2 Riser, Disengager and Catalyst Stripper

The riser reactor is a part of the converter section (Figure A.1) that also includes the

disengager, regenerator, catalyst stripper and other ancillary equipment. From the

preheater furnace, the preheated gas oil feed enters the riser near the base where it is

contacted with hot regenerated catalyst from the catalyst regenerator. The net heat of

the reaction, which takes place at about 520OC, is endothermic and the circulating

catalyst provides it.

For an intimate contact between the catalyst and oil feed, the latter is sprayed by

dispersion steam to reduce the hydrocarbon partial pressure. This increases the

availability of the feed to the reactive acid sites of the catalyst. The superficial velocity at

the riser is in the range of 5-l2 m/s (16- 40 ft/s) and steam injected at the base of the

riser helps to maintain this velocity of oil and catalyst reaction mixture.

The reaction mixture (products and catalyst) passes through a horizontal crossover into

the disengager, which is also called a reactor. The reactor (disengager) serves as a

disengaging space for the separation of spent catalyst and vapour as well as the

reactor’s cyclone housing. The set of cyclones attached to the riser known as rough-cut

cyclones separate a greater percentage of the catalyst from the hydrocarbon vapours.

The remaining catalyst particles are separated from the vapours by 4 cyclones. These

cyclones collect the catalyst and return it to the stripper though the use of dip-legs and

flapper valves that work like a check (flow safety) valve. The product vapours then flow

to the main fractionator column for product recovery.

The spent catalyst with adsorbed coke, a carbonaceous material, in its active sites is

stripped of any entrained hydrocarbon material in a stripper by means of steam, which

flows in countercurrent with the catalyst in upward motion.

From the stripper, the catalyst flows through the spent catalyst standpipe to the

regenerator. In the standpipe, aeration steam is injected for smooth catalyst flow and

generation of the necessary hydrostatic head. A plug valve PV-1 is directly coupled to

the base of the standpipe, which controls the flow of spent catalyst into the first stage of

the catalyst regenerator.

1.2.3 Catalyst Regenerator

The regenerator has two main functions. These are the restoration of the catalyst

activity and the supply of the heat to crack the feed. The regeneration is done in two

stages and partial combustion mode is used. In the regenerator first stage, about 85

percent of the adsorbed coke are burnt off the catalyst with the help of air from a

centrifugal air blower K-01. The air blower supplies sufficient air velocity and pressure to

maintain the catalyst bed in a fluidized state. The air enters the regenerator through an

air distributor or grid located near the bottom of the vessel.

A dividing wall separates the second stage of the regenerator from the first and slots on

the diving wall and the open top direct catalyst to the second. After regeneration, the

catalyst, which has regained much of its activity, is circulated back to the riser by the

opening of the regenerated catalyst plug valve (RCPV), PV-2 and thus repeating the

cycle.

When the flue gas that results from the burning of the coke leaves the dense phase of

the regenerator, it entrains catalyst particles. These particles are recovered by six sets

of primary and secondary cyclones and are returned to the regenerator via cyclone dip-

legs. The flue gas devoid of much of the catalyst particles is passed through an orifice

chamber S-01 where the pressure is reduced from about 2.1 kg/cm2g (regenerator

pressure) to about 0.9 kg/cm2g.

Two seal pots arrangement is provided to direct the flue gas either to the stack or the

co-boiler. When the CO-boiler unit is operational seal pot A is filled with water to a level

at which flue gas cannot escape to the stack. The level of water in seal pot B will then

be maintained at a low level to allow flue gas to the CO-boiler where high pressure

steam is generated by the burning carbon monoxide (resulting from the partial

combustion mode in the regenerator) and 10% auxiliary fuel. However, at the time of

writing this thesis, the CO-boiler has not been operational and thus flue gas is directed

to the stack by maintaining high water level in seal pot B.

1.2.4 Main Fractionator

The purpose of the main fractionator is to desuperheat and recover liquid products from

the hydrocarbon vapours coming from the disengager. The fractionation is

accomplished by condensing and revapourising hydrocarbon components as the vapour

flows upward through the column trays. The hydrocarbon vapours in the disengager are

routed to the base or baffle zone where the temperature of the vapour is reduced and

the heavier components condensed. The cooled slurry bottoms serves to wash down

catalyst fines entrained in the vapours. The recovered heat from the main column

bottoms carried by the slurry is used to preheat the fresh feed in a heat exchanger and

for generation of medium pressure steam in a kettle-type heat exchanger. In all, the

following products/streams are fractionated.

i. Unstabilized gasoline and light gases (overhead vapours)

ii. Heavy naphtha (H.N)

iii. Light cycle (LCO)

iv. Heavy cycle oil (HCO)

v. Slurry oil

The H.N, LCO, HCO, and slurry oil are used for pre-heating the gas oil feed in the four

separate heat exchangers, which has been mentioned in section

1.2.1. The H.N and HCO and a part of the slurry oil are used as pump around streams

only and are not withdrawn as products. A part of the slurry oil is also injected into the

feed to the riser as recycle while the remaining part, stripped of its lighter hydrocarbons

to meet flash point specifications is sent to storage as decanted oil (DCO).

The LCO is the only side-cut that leaves the unit as a product. It is withdrawn from the

main column and routed to a steam stripper where it is stripped for flash point control or

specification.

The overhead vapour contains fuel gas, LPG and unstabilised gasoline. The overhead

vapour is cooled and partially condensed in an air-cooled exchanger.

1.2.5 The Vapour Recovery Unit (VRU)

The function of the VRU (Figure A.2) is to separate the unstabilised gasoline and light

gases into fuel gas, C3 LPG, C4 LPG and gasoline. The partially condensed overhead

stream from the main fractionator overhead air-cooled condenser contains vapours,

hydrocarbon liquid and water. These flow to an overhead receiver, which operate at

pressure of about 0.9 kg/cm2g (12.8 psig). The hydrocarbon liquid is split into two parts.

A part is sent to the top of the main column as reflux while the remainder is joined to the

hydrocarbon vapour going to a secondary condenser where further condensation takes

place. The sour water flows to a hydrocarbon skimmer. The outlet of the secondary

condenser also contains hydrocarbon liquid, water and vapour and is received in a net

overhead receiver. The sour water joins the one from the main overhead receiver to the

hydrocarbon skimmer. The hydrocarbon liquid from the net overhead receiver is

pumped to an absorber column while the vapour flows to the wet gas compressor. The

vapours contain some gasoline, C3 and C4 components and fuel gas.

The wet gas compressor is a two stage centrifugal machine, which is employed to raise

the pressure of the gas from about 0.9 kg/cm2g to about 12.5 kg/cm2g. The compressor

is driven by a multi-stage steam turbine that uses high-pressure steam as the motive

fluid. The vapours from the compressor’s first stage at about 4 kg/cm2g are condensed

and flashed in an inter-stage drum. The liquid portion is pumped to the second stage

discharge of the compressor. The vapour from the inter-stage drum flows to the second

stage where the pressure is raised to about 12.5 kg/cm2g and 120OC. The second stage

discharge (vapour) and the liquid from the inter- stage drum are cooled and condensed

and are collected in a high pressure separator (HPS) from where vapours flow to the

absorber column to contact the hydrocarbon liquid from the net overhead receiver.

1.2.5.1 The Absorber Column

The absorber column consists of primary and secondary absorber sections, each

having 20 trays. However, only the primary absorber is being used presently. The

absorbent liquid for the primary absorber and secondary absorber are (gasoline and

LPG mixture) from the net overhead receiver and Heavy naphtha, respectively. When

the HPS vapour is contacted with the absorbent liquid, C3’s and heavier components

are absorbed. The mixture is then pumped back to the HPS. The liquid portion of HPS

content is pumped to the light hydrocarbon stripper. The C2 and lighter that are not

absorbed become the absorber off gas that goes to the treatment unit for H2S, removal.

1.2.5.2 Light Hydrocarbon Stripper Column

The HPS liquid consists mostly of C3s and heavier hydrocarbons. However, it also

contains some C2’s, H2S and entrained water. The stripper column removes these with

heat supplied by heavy naphtha in a re-boiler. The vapour that is generated in the

reboiler rises through the column and strips the lighter fractions from the descending

liquid. The stripped hydrocarbon liquid leaves the bottom and goes to the debutanizer

column.

1.2.5.3 The Debutanizer Column

This debutanizer column separates the C3 and C4s from the gasoline. The overhead

product is a mixture of C3s and C4’s while the bottom product is

stabilized gasoline. The heat for separating these streams comes from heavy cycle oil

in a reboiler. The overhead product is totally liquefied in an air-cooled condenser and

water condenser. A portion of the overhead LPG is pumped and returned to the tower

as reflux. The remainder is sent to a sweetening unit (the MEA section and LPG merox)

to remove H2S and other organic sulphur compounds like mercaptans. From the LPG

merox the LPG is split into C3 LPG and C4 LPG in a depropanizer column. The C3’s are

then processed as petrochemical feedstock and C4’s as Alkylation feed component as

explained in section 1.1.

The debutanized gasoline is cooled first by pre-heating the debutanizer feed in a heat

exchanger and again by an air-cooled condenser before flowing to the Gasoline merox

for sulphur removal.

1.2.6 Treatment Unit

The fuel gas, LPG and gasoline produced contain some impurities, which are acidic in

nature. Examples of these impurities are hydrogen sulphide (H2S), carbon dioxide

(CO2), mercaptans (R-SH), phenol (ArOH), and naphthenic acids (RCOOH). Other

impurities such as carbonyl and elemental sulphur may also be present in these

streams. There are two sections in which the treatment is done: the monoethanolamine

(MEA) section and the LPG and gasoline merox. The LPG merox is downstream of the

monoethanolamine section where we have the LPG and Fuel gas absorbers.

1.2.6.1 LPG and Fuel Gas Absorbers

In the LPG/H2S absorber (Figure A.3), the LPG from the debutanizer overhead receiver

flows to the bottom to be contacted by lean-MEA also from the MEA regenerator. The

treated LPG then flows to the LPG merox unit via a MEA separator where entrained

MEA is removed.

Similarly, the sour off gas (fuel gas) from the top of the high-pressure absorber column

(Figure A.4) flows to the fuel gas/H2S absorber where the bulk of the H2S is removed.

The gas flows to the bottom of the column where it is contacted with a counter current

flow of a cooled-lean MEA from the MEA regenerator. The treated (sweet) fuel gas

leaves the top of the column to the refinery fuel gas system.

The rich- MEA streams from both contactors and then from the MEA separator (if any)

then flow to the regenerator surge drum where dissolved hydrocarbons are separated

from the Rich- MEA solution. The rich MEA then flows to the MEA regenerator through

two filters, which remove dirt, rust, iron sulphide and other materials.

In the MEA regenerator, the rich MEA solution is heated to reverse the acid- base

reaction that takes place in the H2S absorbers. The heat is supplied by low-pressure

steam in a reboiler. The hot lean MEA is pumped from the bottom of the regenerator

and exchanges heat with the rich MEA in a rich MEA- lean MEA exchanger before it is

pumped to an air-cooler before entering the H2S absorbers in separate streams to

complete the cycle.

The sour gas containing small amounts of amine leaves the top of the amine

regenerator and flows through an air-cooled condenser. It is then received in an

overhead accumulator. The sour gas from the top of the accumulator is sent to blow-

down (flare) while the condensed liquid is pumped to top of the regenerator as reflux.

12.6.2 LPG Merox Unit

The LPG/H2S absorber is not effective for the removal of mercaptans. The LPG merox

unit is shown in Figure A.5. The process involves both sweetening and extraction

processes using caustic soda (sodium hydroxide), catalyst, and air. The LPG from the

MEA section flows to a vertical drum, where an 8° Be (8 degrees Baume) caustic is

used to wash it. From the caustic wash drum the LPG flows to the caustic extractor

column where caustic (20O Be) is used to extract mercaptans form the LPG. The LPG

leaves at the top to a sand-filter from where solid particles are removed. The LPG flows

to the depropanizer feed drum under pressure.

The spent caustic from the extractor bottom flows to the oxidizer. In the oxidizer an

oxidizing catalyst and air are used to convert the extracted mercaptans to disulphides.

The reactions take place according to the following equations:

(1.1)

(1.2)

The mixture of caustic and disulphides is transferred to a separator from

where the caustic is pumped back to the extractor column. The solvent containing

disulphide oil is sent to the fuel oil tank of the power plant unit. The gaseous by-

products, namely nitrogen and excess oxygen are vented to atmosphere.

1.2.6.3 The Gasoline Merox Unit

The gasoline merox unit is shown in Figure A.6. The gasoline from the debutanizer column

bottom flows to a mixer column after having being cooled. In the mixer, the gasoline is

contacted with both air and caustic soda. The reaction mixture then flows to a caustic settler

where the gasoline is separated from the caustic and the latter is recycled to the mixer. The

gasoline leaves at the top to a sand filter to remove solid particles. The treated gasoline then

flows to storage.