Cracking Process in Refinery

This inquiry will investigate the effect on product yields of the following recycles to the FCCU riser: 1.1 Sponge Column Off-Gas SCOG Often referred to as dry gas, SCOG is a light hydro

Executive Summary

Caltex Australia operates an Oil refinery at Lytton on the mouth of the Brisbane River TheFluidised Catalytic Cracking Unit (FCCU) has the ability to recycle both selected product andwaste streams to the reactor-riser The objective of this individual inquiry was to investigatethe yield effects on product, of recycling two streams to the riser Sponge Column Off Gas(SCOG) and Hydro-treated Light Cycle Oil (HT-LCO)

Literature suggested that a stream rich in hydrogen would passivate nickel and heavy metalcontamination on the catalyst and result in a decrease in SCOG production SCOG wasrecycled to the Wye junction on the riser HT-LCO is rich in olefinic material and wasrecycled to the riser to act as mixed temperature control (MTC) liquid increasing conversion

to favour gasoline and liquid petroleum gas (LPG)

Test runs, using reactor mix sampling (RMS) technique, were performed over two days (April

9thand 10th) and Caltex laboratory technicians performed subsequent analyses

It resulted that SCOG did not behave in the same manner as reported in literature, resulting in

an increase in SCOG and coke production HT-LCO as MTC liquid was a success as anincrease in both gasoline and LPG gas was recorded

This inquiry concludes that the recycle of SOCG to the riser is not a practice recommendedfor the Lytton refinery The recycle of HT-LCO to the riser is a practice highly recommendedespecially when demand for gasoline and LPG is higher than that of diesel

Further investigation should be conducted into the recycle of Light Catalytic Naphtha as liftgas in the riser as an un-planned unit shut down prevented the stream from being researched

Contents Page

Section Page

1.0 Introduction 1

1.1 Sponge Column Off Gas 1

1.2 Hydro-treated Light Cycle Oil 1

1.3 Experimental Objective 1

2.0 Background 2

2.1 Refinery Process Description 2

2.1.1 Crude Distillation Products 2

2.2 FCCU Process Description 4

2.2.1 Reactor/Regenerator 5

2.2.1.1 Reactor – Riser 5

2.2.1.2 Catalyst Deactivation 6

2.2.1.3 Regenerator 6

2.2.1.4 Mixed Temperature Control Liquid 7

2.2.1.5 Catalyst Addition 7

2.2.2 Main Fractionator 8

2.2.2.1 Fractionator Overheads 8

2.2.2.2 Light Catalytic Naphtha (LCN) 8

2.2.2.3 Heavy Catalytic Naphtha (HCN) 8

2.2.2.4 Un-treated Light Cycle Oil (UT-LCO) 8

2.2.2.5 Clarified Oil (CLO) 8

2.2.3 Product Treatment 8

2.2.3.1 Sponge Column Off Gas (SCOG) 8

2.2.3.2 Propane/Propylene (C3) and Butane/Butylene (C4) 8

2.3 Catalytic Cracking Theory 10

2.3.1 Reaction Mechanisms 10

2.3.2 Thermal Cracking 10

2.3.3 Catalytic Cracking 10

2.3.3.1 Catalytic Cracking Initiation 10

2.3.3.1.1 Carbonium Formation 10

2.3.3.1.2 Carbenium Formation 11

2.3.3.2 Catalytic Cracking Propagation Mechanisms 11

2.3.3.2.1 Primary Cracking Reactions 11

2.3.3.2.2 Secondary Cracking Reactions 12

2.3.4 Catalyst Properties 12

2.3.4.1 Zeolite 13

2.3.4.2 Matrix 13

2.3.4.2.1 Matrix Binder and Filler 13

2.3.4.2.2 Active Alumina 13

3.0 Literature Review 14

3.1 FCCU History 14

3.2 Recycling SCOG to the Riser 15

3.2.1 SCOG – U.S Patent 15

3.2.1.1 SCOG – U.S Patent Description 15

3.2.1.2 SCOG – Australian Patent Description 15

3.2.1.3 SCOG – U.S Patent Catalyst 16

3.2.2 SCOG – U.S Patent Testing 16

3.2.3 SCOG – U.S Patent Results 17

3.2.4 SCOG – Literature Summary 17

3.3 Recycling HT-LCO to the Riser 17

3.3.1 Split Feed Technology 17

3.3.2 HT-LCO as MTC Liquid 18

3.3.2.1 HT-LCO Chemistry 18

3.3.3 HT-LCO – Summary 19

4.0 Methodology 20

4.1 Test Runs 20

4.1.1 Normal Sampling 20

4.1.2 Reactor Mix Sampling (RMS) 20

4.1.2.1 RMS Analysis 21

4.2 Inquiry Test Run Sampling 21

4.2.1 Other Samples 21

4.2.2 Analysis Performed 21

4.2.3 Process Instrumentation Error 22

4.3 Plan of Testing 22

4.3.1 Crude Composition 23

4.3.2 Labour 23

4.3.3 Test Run Schedule 23

4.4 Implementation 24

4.4.1 Problems 24

5.0 Results 25

5.1 Mass Balance 25

5.2 SCOG Test Run Results 25

5.2.1 SCOG – Process Variables 25

5.2.2 SCOG – Normal Sampling 26

5.2.2.1 Fractionator Column Dynamics 27

5.2.3 Hysys Processing of RMS Results 28

5.3 HT-LCO Test Run Results 29

5.3.1 HT-LCO – Process Variables 29

5.3.2 HT-LCO Normal Sampling 29

6.0 Discussion 31

6.1 SCOG Test Run 31

6.1.1 Error 31

6.1.1.1 Instrument Calibration 31

6.1.1.2 RMS Technique 31

6.1.2 Plant Data vs Literature Data 31

6.1.2.1 MAT Testing 31

6.1.2.2 Residence Time 31

6.1.2.3 Catalyst to Oil Ratio 32

6.1.2.4 Catalyst Formulation 32

6.1.2.5 SCOG 32

6.1.3 Fractionator Column Dynamics 32

6.2 HT-LCO Test Run 32

7.0 Conclusions and Recommendations 33

7.1 SCOG 33

7.2 HT-LCO 33

7.3 LCN 33

References 34

List of Figures

Title Page

Figure 1 – Refinery Flow sheet 3

Figure 2 – Crude Distillation Unit 4

Figure 3 – Basic FCCU Flow sheet 4

Figure 4 – Reactor-riser 5

Figure 5 – Temperature Profile for MTC Addition 7

Figure 6 – Main Fractionator 9

Figure 7 – Reactor Mix Sampling 21

Figure 8 – SCOG/HT-LCO black box analysis 25

Figure 9 – Feed rate SCOG Test Run 26

Figure 10 – Reactor Temperature SCOG Test Run 26

Figure 11 – LCN Yield – SCOG Test Run 27

Figure 12 – LCO Yield – SCOG Test Run 27

Figure 11 – CLO Yield – SCOG Test Run 27

Figure 14 – Reactor Pressure – SCOG Test Run 28

Figure 15 – Feed rate HT-LCO Test Run 29

Figure 16 – Reactor Temperature HT-LCO Test Run 29

Figure 17 – LCN yield shift - HT-LCO Test Run 30

Figure 18 – Polymerisation Unit (C3’s) yield - HT-LCO Test Run 30

List of Tables Title Page Table 1 – Catalytic Cracking : Reactants and Products 12

Table 2 – Test Conditions 16

Table 3 – Patent Results 17

Table 4 – Diesel and LCO analysis 18

Table 5 – Laboratory Test Methods and Repeatability 22

Table 6 – ASTMD3507 Repeatability 22

Table 7 – SCOG RMS Results 28

1.0 Introduction

The focus on the optimistion of the petroleum refinery has intensified over the pastdecade with an increasing demand for transportation fuels as well as tighteningenvironmental restrictions The fluidised catalytic cracking is one of the moreimportant processes in the refinery (upgrading low-value feed stock) The FCCU atCaltex Lytton Refinery (herein referred to as the Lytton refinery) has several locationswhere recycled product can be injected into the reactor/riser in addition to normalfeed The ability to recycle these products has recently been installed and the effect ofthese activities has not yet been investigated This inquiry will investigate the effect

on product yields of the following recycles to the FCCU riser:

1.1 Sponge Column Off-Gas (SCOG)

Often referred to as dry gas, SCOG is a light hydrocarbon mixture (comprising ofmainly H2, CH4, C2H6, C2H4, N2 etc) and relatively high concentrations of H2S(1000ppm sulphur) Nickel is present in most crudes, plating out on the FCCUcatalyst Nickel acts as a catalyst poison catalysing reversible dehydrogenationreactions, resulting in elevated hydrogen gas yields and a loss of valuable product.Sulphur is a temporary poison for nickel-catalysed dehydrogenation reactions Anexcess of hydrogen will favour dehydrogenation reactants via Le Chatelier’sprinciple of equilibrium (Wilson, 1997) as well as reducing the nickel to a metallicstate promoting selective carbonisation of contaminated sites (rendering theminactive) Pre-treating the catalyst with a gas containing hydrogen and sulphurbefore contacting the catalyst with the feed should render the nickel inactive.However, sulphur, in high concentrations, can also act as a catalyst poison andexcess gas recycle can overload product treatment compressors This inquiry willinvestigate whether recycling SCOG to the riser at the Lytton refinery will improveproduct yield by the above said mechanisms

1.2 Hydro-treated Light Cycle Oil (HT-LCO)

Light cycle oil (LCO) is a product of the FCCU used as a blend stock for diesel.Hydro-treating the LCO saturates olefinic (and aromatic) material By recyclingthis paraffinic/naphthenic stream to the riser it is proposed that the yield of

gasoline and LPG will increase

1.3 Experimental Objective

The purpose of this inquiry is to investigate the effects on product yields of

recycling HT-LCO and SCOG to the riser To observe yield shifts with respect to asingle changing variable (e.g feed rate, MTC rate or reactor temperature) a steptest is often conducted The system is held at steady state with no recycle andproduct yields are recorded A recycle stream is then added, at a specified time andrate, in a single step The system returns to steady state and the new product yieldsare recorded The yield shift observed is then attributed to the change in variable

In order to observe these shifts a material/mass balance must be closed around theFCCU To understand the implications of such a change a detailed background intothe process and supporting theory is also presented in this inquiry (see note*)

Note*: In the original thesis proposal a third variable (Light Catalytic Naphtha)was scheduled to be tested on the 26th of April 2001 Due to an un-planned FCCUshut down, this trial was aborted and the scope of the inquiry was re-defined to

Page 2 of 36include a detail background investigation.

2.0 Background

To understand the impact of yield changes within the FCCU it is important to havesufficient background knowledge of the refinery process itself as well as crackingmechanisms and catalyst properties

Crude oil is found in deposits deep below the earth’s surface as a result ofcarbonaceous (plant and animal) material decomposing over millions of years (hencethe term fossil fuel) The major components of crude oil are hydrocarbons (paraffins,olefins, naphthenes and aromatics) in the range CH4to material with boiling point inexcess of 750°C, sulphurous compounds, and traces of metals such as Ni, Va, Fe, Cuetc Crude Oil is refined/processed into a variety of fuels to meet the energy demands

of today’s society The operational aim of a refinery is to produce high yields ofgasoline (30°C – 185°C) kerosene (jet fuel) (160° C- 230°C) and diesel distillate(230°C – 330°C) and to minimise the production of fuel oil (> 360°C) and dry gas (C2

and lighter), (see Figure 1 – Refinery Flow sheet over page).

2.1 Refinery Process Description

Crude oil is drawn from wells and transported to the refinery where it undergoesprimary atmospheric distillation in a crude distillation unit (CDU), separatingproducts on a boiling point/component basis Crudes processed at the Lyttonrefinery are purchased primarily from South East Asia and Australia and are low insulphur to meet state regulations on sulphur content of salable fuels (<500ppm ondiesel)

2.1.1 Crude Distillation Products

Dry gas (C2 and lighter) is fed to the refinery fuel gas header and is also sold toneighbouring Ammonia plant Wide-range naphtha is split into two streams,light and heavy fractions Light Naphtha is isomerised into poly-gasoline(increasing the octane) over a platinum-based catalyst The heavy fraction isprocessed into gasoline blend stock isomerising and dehydrogenating thehydrocarbons over a platinum/rhenium/tin-based catalyst Kerosene is sold as jetfuel, meeting stringent quality requirements Straight-run diesel is also storedand blended with other diesel distillates (including light cycle oil) before sale.Reduced crude can be up to 50% of the CDU feed stock and can only be used aslow-value fuel oil The reduced crude from the distillation unit is sent to thefluid catalytic cracking unit (FCCU) for upgrading to more valuable products.Other refineries often pass the reduced crude through a vacuum gas unit toextract the heaviest components, but the FCCU under investigation can processvery heavy feed stocks and the vacuum gas unit is not employed

Page 4 of 36

Figure 1 –Refinery Flow sheet

Figure 2 – Crude Distillation Unit

2.2 FCCU Process Description

The FCCU has three main sections:

Clarified Oil

Heavy Cycle Oil

Light Cycle Oil

Cat Naptha Product Vapour to Fractionator

Air

CO / CO2

Reduced Crude

Spent Catalyst

Regenerated Catalyst

Crude Oil Feed

To Reformer (catalytic hydrogen reformation)

Wide-Range Naphtha

Wye

reaction and vaporise the feeds At the top of the riser, the feed, catalyst andproduct mixture enters the riser termination device (RTD) The RTDcomprises of cyclones separating the particulate catalyst and the super-heatedproduct vapour The catalyst falls into the stripper while the vapour flows out

of the top of the reactor

2.2.1.2 Catalyst Deactivation

During the course of the catalytic cracking reactions that occur (see section2.3.1) in the reactor/riser, the catalyst is coated in coke The coke restrictsaccess to the catalyst active sites by blocking pores as well as coating thesurface of the particle The blocked pores may also entrain hydrocarbon.High-pressure steam is injected through steam rings in the stripper, fluidisingthe bed of catalyst stripping excess hydrocarbon from both the catalyst poresand the coke Coked catalyst then flows to the regenerator where it iscombusted with air to regenerate the catalyst providing the heat required forthe reactions in the riser

of coke is an exothermic reaction and a lower regenerator bed temperaturemay be maintained by only partially combusting the coke on the catalyst Thetemperature of the regenerator bed is controlled using air addition to maintainthe concentration of CO in the flue gases within a desired range Thisensures that the carbon on regenerated catalyst (CRC) remains within anacceptable level while also lowering the catalyst temperature The hotregenerated catalyst then flows (regulated by a large slide valve) to thebottom of the riser to a section called the Wye Here the hot catalyst isfluidised, into the region of pneumatic transport, by steam injected to theWye Mid-way up the riser the feed is injected and the process continues

Page 8 of 36

2.2.1.4 Mixed Temperature Control Liquid

Another way to increase the catalyst to oil ratio in the riser is the addition of

a quenching fluid after the introduction of feed to the riser called mixedtemperature control liquid or MTC liquid This increases the feed mix zonetemperature requiring more catalyst There is two ways the addition of MTCincreases conversion:

1 Increase Catalyst to Oil interface (surface/contact) area

2 Increase reaction temperature to promote faster kinetics

Heavy cycle oil (HCO) from the fractionator bottom pump around is mostcommonly used at the Lytton refinery as MTC This also promotes a change

in fractionator temperature dynamics and can lead to the decrease in CLOproduction (raises the boiling cut point of LCO) (Woodford 2001)

Feed

Injection

18kg/sec Hot Catalyst

MTC Injection

25kg/sec Hot Catalyst

of fresh catalyst every day in the order of 1-2% of catalyst inventory

2.2.2 Main Fractionator

The reactor overheads are sent to the separation stage of the FCCU The heated vapour undergoes separation in the main fractionator, a distillationcolumn comprising of regions of packing, shed decks and trays

super-2.2.2.1 Fractionator overheads

The fractionator overheads range from entrained gasoline to hydrogen gas.They are sent to the Product treatment for further separation

2.2.2.2 Light Catalytic Naphtha (LCN)

LCN is stored and blended with other gasoline blend-stocks HCN is used as

a swing-cut to supplement gasoline or diesel production depending ondemand

2.2.2.3 Heavy Catalytic Naphtha (HCN)

HCN is either stored or used as a blend stock in the hydro-treater

2.2.2.4 Un-treated Light Cycle Oil (UT-LCO)

UT-LCO drawn from the fractionator differs from the straight-run diesel(from the CDU) in composition Straight-run diesel is mostly saturatedhydrocarbons while the untreated LCO drawn from the fractionator has up to50% unsaturated 2-ring aromatic hydrocarbons which are undesirable, due totheir instability, in the product In order to blend these two components theLCO must be stabilised To combat this problem the untreated LCO issaturated with hydrogen in the LCO hydro-treater This breaks the unstable2-ring compounds into stable 1-ring compounds Hydro-treated LCO (HT-LCO) is no blended without diesel distillates

2.2.2.5 Clarified Oil (CLO)

CLO is drawn from the column, passed through a stripper, some is recycled

to the fractionator and the rest is sent to storage

2.2.3 Product Treatment

The fractionator overheads are passed through a product treatment section inwhich water is drawn off and gases are separated on a boiling point basisthrough a series of distillation columns

2.2.3.1 Sponge Column Off Gas (SCOG)

SCOG comprising of C1's, C2's, H2, H2S, N2 and other gases feeds therefinery fuel-gas header

2.2.3.2 Propane/Propylene (C 3 ) and Butane/Butylene (C 4 )

C3's and C4's are separated from the fractionator overheads and sent to thepolymerisation unit and alkylation units respectively for further treatmentinto Liquid Petroleum Gas (LPG) and gasoline blend stocks

4FC 033

03 FC

03 FC

03 HC

QUENCH

HCO RECYCLE

HCO

03 FC

03 FC

1 3 4 5

LCO to STG

03 FC

03 FC

03 LC LCO STRIPPED VAPOUR

OFFGAS to RFG

SPONGE OIL CIRCUIT RETURN

RICH SPONGE OIL

03 FC

03 LC

03 FC

CLO CIRC

03 FC

UPPER CIRC PA

07 FC

03 FC HCN

03 FC

03 FC

03 FC UPPER CIRC RETURN

03 FC SLOPS HCO P/A

03 FC OVHDS

REFLUX

03 FC

R GASOLINE

OVHD VAP

BOTTOMS / QUENCH

to CLO R/D FEED

SPONGE OIL STRIPPED WATER

Figure 6 – Main Fractionator (drawn by Alex Marshall 1997)

2.3 Catalytic Cracking Theory

When optimising a FCCU it is important that both the reaction mechanisms andconditions as well as catalyst properties are exploited as they are two of the mosteasily manipulated variables

2.3.1 Reaction Mechanisms

There are two types of cracking mechanisms that take place in the FCCU,thermal and catalytic cracking Thermal cracking takes place due to the elevatedtemperatures (425°C) produces light gases such as SCOG (which is unwanted)while the formation of gasoline, LPG and diesel distillates are promoted bycatalytic cracking

2.3.2 Thermal Cracking

Thermal cracking is initiated by the formation of a free radical Very reactivefree radicals are formed by the breaking of carbon-carbon/carbon-hydrogenbonds More energy is required to break carbon-hydrogen bonds hence thebreaking of carbon-carbon bonds is the more common mechanism of freeradical formation There is little difference in the energy required to breakprimary, secondary or tertiary carbon-carbon bonds therefore methyl or ethylfree radicals are equally as likely to form as longer chain free radicals (Wilson1997) The free radicals undergo alpha and beta scission resulting in high yields

of C1 and C2 Free radicals undergo little isomerisation (branching) Onedrawback to thermal cracking is that most olefins that are formed asintermediate products during thermal cracking polymerise and condense directly

to coke (Sadeghbeigi 1995) Thermal cracking should be avoided as the maincracking mechanism in the FCCU

2.3.3 Catalytic Cracking

Catalytic cracking is a generic term describing many complex reactions Theproducts of catalytic cracking vary greatly from those of thermal cracking andare more desirable Catalytic cracking reactions can be divided into two majorcategories, primary cracking and secondary rearrangement and recrackingmechanisms Catalytic cracking is dependent on feed and catalyst composition

as well as catalyst properties and health (Sadeghbeigi 1995)

2.3.3.1 Catalytic Cracking Initiation

Catalytic cracking begins with the formation of a positively charged carbonatom called a carbocation There are two types of carbocations, carbeniumions and carbonium ions

2 3

Page 12 of 36

2.3.3.1.2 Carbenium Formation

The Lewis and Bronsted acid sites on the catalyst are responsible for the

formation of carbenium (R-CH2+) ions, by either removing a hydrogen

and an electron pair from a paraffin (Lewis acid) or the donation of a

proton to an olefin (Bronsted acid)

Carbenium Formation

Bronsted acid mechanism

3 2

2 2

3 2

CH CH CH

Lewis acid mechanism

3 2

2 2

3 2

2

CH

Carbenium ions as formed by the above reactions can be primary,

secondary or tertiary Unlike thermal cracking, the nature of the alkyl

groups attached affects the stability of the carbenium ion One of the

benefits of catalytic cracking is that primary and secondary carbenium

ions rearrange to form tertiary ions in creasing the degree of branching in

the final product

2.3.3.2 Catalytic Cracking Propagation Mechanisms

According to Sadeghbeigi, the three dominant reactions of carbenium ions

that occur are:

1 Cracking of carbon-carbon bonds

2 Isomerisation

3 Hydrogen transfer

2.3.3.2.1 Primary Cracking Reactions

Most cracking that takes place catalytically is beta-scission cracking

splitting the C-C bond two bonds away from the positively charged

carbon atom The initial product of beta-scission cracking is an olefin and

a new carbenium ion

Beta Scission

R CH CH

H C CH CH CH

CH CH

CH CH

H C

The carbenium ion continues the chain reaction until the new carbenium

ion is small (C4 – C5) and stable whereby it hydrogen transfers its charge

to a larger molecule allowing it to crack

Hydrogen Transfer

2 3

2 2 3 3

2 2 3

2

As shorter chains are more stable than long hydrocarbon chains, the scission of catalytic cracking promotes chains in the order of 5 –10carbons (gasoline – diesel) The cracking mechanism is endothermic and

beta-is not limited by equilibrium but rather favoured at high temperatures(Sadeghbeigi 1995) The fact that the cracking happens beta to the ionicsite means that the smallest hydrocarbon that can be formed by betascission contains three carbon atoms Therefore, any hydrocarbon withless than three carbons can only be formed by thermal cracking (Wilson1997)

2.3.3.2.2 Secondary Cracking Reactions

Isomerisation occurs when a carbenium ion rearranges to form a tertiarycarbenium ion and is then involved in hydrogen transfer with a paraffin.The result is a mono-branched paraffin and a new carbenium ion Thisreaction is more common in catalytic cracking than in thermal cracking asthe tertiary carbenium ion is most stable and free radicals only yieldstraight chains Another secondary reaction is the cyclisation of straightchain olefins to form naphthene and then hydrogen transfer of naphthene

n-Paraffins Iso ParaffinsCyclisation

Page 14 of 36

FCCU catalysts are micro-spheroidal and are usually 10-150 microns indiameter The first commercial FCC catalyst was acid-treated natural clay.Introduction of synthetic amorphous silica-alumina materials led to thebreakthrough development of the structured X and Y zeolites in the early 1960s.Zeolites are crystalline alumina-silicates having regular pore structure(Sadeghbeigi, 1995) Modern day catalysts have four major components: Yfausjasite zeolite, binders, fillers and active alumina The last three compoundsare more commonly referred to as the catalyst matrix Catalyst distributorsAkzo-Nobel produce the equilibrium catalyst used at Caltex Lytton Refinery.Other catalyst companies include Grace-Davidson and Engelhard

2.3.4.1 Zeolite

Zeolite is tetrahedral in shape, with four oxygen atoms surrounding a centre

of silicon or aluminium This basic building block combines at the oxygensite to form a cage-like structure or molecular sieve The overall charge onthe structure is –1 and must be balanced by a cation to maintain electricalneutrality The activity and selectivity of the zeolite largely depends on thecation occupying the zeolite structure Most zeolites are synthesised insodium hydroxide solutions, resulting in NaY zeolite or soda zeolite Sodazeolite is unstable at regenerator conditions but the acid activity can be easilyrestored by exchanging sodium for hydrogen or rare earth materials such asLantheium (La3+) or Cerium (Ce3+) (Sadeghbeigi 1995) This type of catalyst

is referred to as equilibrium catalyst (E-cat) E-cat is used at the Lyttonrefinery

2.3.4.2 Matrix

The matrix comprises of three non-zeolitic ingredients vital to theperformance of the catalyst: binder, filler and active alumina

2.3.4.2.1 Matrix Binder and Filler

Binders act as the catalyst glue, holding the particle together based binders are catalytically neutral while some binders such as naturaland treated clays have some catalytic activity (Wilson 1997) Inconjunction with the filler, the binder is what gives the catalyst itsphysical and mechanical integrity (hardness, density, size distribution etc).Kaolin clay (Al2(OH)2, Si2O5) is the filler used to contain the activecatalyst ingredients

Silicon-2.3.4.2.2 Active Alumina

Used to modify the performance of the catalyst, the primary purpose ofactive matrix is to crack the larger molecules present in the FCCU feed(Wilson 1997) The large pores of active alumina can provide this serviceand greatly improve the yield of the FCCU especially ones that processheavy residual feeds

3.0 Literature review

3.1 FCCU History

The first references to thermal cracking were made as early as the 1880s due to aninterest in the conversion of animal oils to light lamp oils using high temperatures.The year 1910 saw the commercial introduction of a batch operated thermalcracking horizontal stills to produce gasoline from crude by Dr W M Burton ofthe Standard Oil Company of Indiana This was closely followed by thedevelopment of a continuous process in the early 1920’s by Clark and Dubbs(Grace, 1993)

In 1915 the Gulf Refining Company found that an aluminium chloride catalystcould crack heavy oils (Sadeghbeigi, p2) In the early 1920’s Frenchman EugeneHourdy discovered that catalysts could be regenerated by burning off theaccumulated carbon (Grace, 1993) These two discoveries led to the development

of catalytic cracking and saw the formation of a group Catalytic ResearchAssociates in 1938 (Wilson, 1997) The first commercial on-line FCCU using up-flow reactors was at Esso’s Baton Rouge refinery commissioned by Standard Oil

of New Jersey in 1942

Since then, much advancement has been made in the field of FCCU technology.These advancements vary from the discovery of new/improved catalysts and theuse of cyclones for catalyst recovery to antimony injection for the passivation ofheavy metals Substantial research has been conducted into the optimisation ofwhat is often referred to as the highest earning unit in the refinery Many aspects ofthe FCCU have been cited as possible reasons for poor unit performance including:

1 Poor catalyst health (including reduced surface area and metals contamination)resulting in product loss in the form of coke and hydrogen

2 Heavy “hard to crack” aromatic feeds resulting in high yields of CLO

3 Poor product separation in main fractionator (poor column dynamics) resulting

metals (nickel and vanadium) deposition Nickel promotes dehydrogenation

reactions on the catalyst and results in elevated yields of hydrogen and coke Dr

Chia suggested that the catalyst could be pre-treated in situ before coming in

contact with the feed to prevent the nickel from becoming catalytically active.Antimony pentoxide (SbO5) is used as an in situ FCCU additive to passivate metalssuch as nickel and vanadium The Lytton refinery currently uses this method ofheavy metal passivation however it is expensive One method of pre-treating thecatalyst before it is exposed to the feed is to fluidise the catalyst at the Wye with a

Page 16 of 36

stream other than fluidising steam The refinery has the capability (physically) torecycle LCN and SCOG to the Wye but the effects have never been investigated

3.2 Recycling SCOG to the Riser

Two patents have been registered in both the United States of America (US) andAustralia concerning the recycle of SCOG-like material to the riser to passivateheavy metals (including nickel) The recycling of SCOG to the Wye could alsohave a quenching effect similar to that of MTC liquid (see section 2.2.1.4) Not agreat deal of non-proprietary information exists about the use of SCOG as atreatment for metals contamination The passivation of nickel and other metalssuch as vanadium is briefly discussed as being one way to improve catalyst health

by Sadeghbeigi and Wilson but not to the extent found in the US and Australianpatents

3.2.1 SCOG – U.S Patent

United States Patent, 4,447,552 – “Passivation of metal contaminants oncracking catalyst was registered by inventors John C Hayes and CarmenCastillo and was filed in 1983 Hayes claims in the patent abstract that,

“A contaminating metal on a cracking catalyst used for the cracking ofhydrocarbons is passivated by contacting the catalyst with a hydrocarbon gas ormixture of gases comprising molecules of three carbon atoms or less atpassivation reaction conditions prior to the cycling of the catalyst to thecracking zone The cracking catalyst comprises crystalline alumino-silicatecontained in a substantially alumina-free inorganic oxide matrix.”

3.2.1.1 SCOG – U.S Patent Description

Hayes et al observed that in addition to the catalysis of dehydrogenationreaction, large quantities of nickel could even plate the catalyst and blockaccess to active cracking sites While it would be ideal to remove the nickelaltogether such procedures can be quite elaborate, time consuming and noteconomically feasible A simpler approach is to neutralise or passivate thenickel present on the catalyst (Hayes et al 1994) Hayes found that reactionconditions must be selected to achieve complete reduction of the nickel to thefree metallic state This state tends to promote the undesirable coke-makingreactions however they discovered that the catalyst acquires a condition forselective carbonisation It was apparent that the contaminating free metalactive sites that cause the formation of hydrogen and coke react withhydrocarbon gas and are carbonised, i.e coating the active site with a layer

of coke, preventing further reactions

It has been hypothesised that if this passivation could occur before thecatalyst was exposed to the feed, then no nickel-catalysed dehydrogenation

of feed would result It was also found that a hydrocarbon gas of three carbonmolecules or less was inert to the catalyst active sites Therefore injection of

a light hydrocarbon gas prior to catalyst contact with feed would selectivelycarbonise and deactivate the metal contaminated sites while the desirableacid catalyst site would remain unaffected (Hayes et al 1984)

3.2.1.2 SCOG - Australian Patent Description