Know More about F.C.C. Catalysts

Seminar Report

Submitted as part of coursework for

CL 618 (Catalysis and Surface Chemistry)

By

Rupsha Bhattacharyya

Roll No.: 09302009

M. Tech 1st Year

Department of Chemical Engineering

Indian Institute of Technology, Bombay

2010

Abstract

Fluidized Catalytic Cracking (FCC) is the principle gasoline upgradation technology practiced in

oil refineries round the world. The catalysts used in this process are zeolites of various kinds, tailor-made

for a particular application. The report describes the various interesting physical and chemical features of

these catalysts that have made them irreplaceable in the FCC applications. The catalytic reaction

mechanisms are also briefly discussed. The hundreds of varieties of zeolites commercially available and

used in refineries all over the world make it necessary to understand the relation between the

composition, structure and properties of these catalysts. As these catalysts are quite expensive and they

also have special additives mixed with them for accomplishing special functions, it is important to

provide a proper catalyst management system that takes care of catalyst recovery, replacement and loss

minimization. The major catalyst developers of the world have come up with various interesting catalyst

formulations for meeting the demands of refiners. Some of these proprietary catalysts are also mentioned

in this report. The current research being carried to gain deeper understanding of these catalysts and

predicting their performance is also alluded to in this work.

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Table of Contents

Abstract 2

Chapter 1: Introduction 5

Chapter 2: Typical Components of FCC Catalysts

2.1: Introduction 6

2.2: Zeolites 6

2.2.1: Zeolite Structure and Chemistry 7

2.2.2: Zeolite nomenclature 8

2.2.3: Features of Zeolites suitable for catalysis

2.2.4: Zeolites used as FCC catalysts 9

2.3: Matrix 9

2.4: Fillers and Binders 10

Chapter 3: Major Physical Characteristics of FCC Catalysts

3.1: Introduction 11

3.2: Attrition resistance 11

3.3: Pore Size Distribution and Pore Volume 11

3.4: Surface Area 11

3.5: Particle Size Distribution 12

3.6: Thermal and Hydrothermal Stability 12

3.7: Crystallinity 12

3.8: Microactivity test (MAT) for FCC Catalysts 12

Chapter 4: Synthesis of FCC Catalysts

4.1: Introduction 13

4.2: Conventional Zeolites 13

4.3: Ultrastable Zeolites (USY) 13

4.4: In-situ process 13

Chapter 5: Effects of FCC Catalyst Composition on its Properties

5.1: Introduction 14

5.2: Effect of Zeolite Content of the Catalyst 14

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5.3: Effect of Additives 14

5.3.1: ZSM-5 Additive 15

5.3.2: CO Promoter 15

5.3.3: SOx Additive 15

5.3.4: Metal Passivators 15

Chapter 6: Catalytic Cracking Reaction Mechanisms

6.1: Introduction 16

6.2: Mechanism of Catalytic Cracking Reactions 16

Chapter 7: Catalyst Management in FCC Units

7.1: Introduction 18

7.2: Addition of Fresh Catalyst 18

7.3: Spent Catalyst Stripping 18

7.4: Minimizing Catalyst losses 18

7.5: Preventing Excessive Coking Tendencies 19

Chapter 8: Recent Developments in FCC Catalyst Technology

8.1: Introduction 20

8.2: Demetallization Procedures 20

8.3: Modelling of FCC Catalyst Pore Structures 20

8.4: Advanced Kinetic Models 21

8.5: Special Catalysts for Short Contact Time Applications 22

8.6: Some other Special Purpose FCC Catalysts 23

Chapter 9: Conclusion 24

References 25

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Chapter One

Introduction

1.1 Background

Fluid catalytic cracking (FCC) is one of the most important operations carried out in a modern

petroleum refinery and its principle aim is the upgradation of low-value petroleum feed stock and its

conversion to more valuable liquid fuels like gasoline or petrol. Ever since the development of the

catalytic cracking processes by Eugene Houdry in the 1920s, it has become the mainstay of the refineries

as far as their economics are concerned. Today almost all the catalytic cracking practiced in refineries is

done through FCC and this goes onto show how valuable a process it is for any refinery. There are

several licensed technologies available from licensers like Shell Oil Company, UOP, ABB Lummus and

Kellogg Brown and Root [1]. The mechanical design of the FCC units vary from licenser to

licenser .The key difference in the operation of these units comes from the different FCC catalyst

formulations used by different refineries at different points of time as per the required product

distribution. In the 1960s the introduction of zeolite cracking catalysts marked a revolution in the

refining industry. Ever since then several kinds of zeolites have been commercially synthesized and

applied as FCC catalysts. Today such catalysts are used in all FCC units. They have increased the

profitability of the FCC units dramatically and have led to the diversification of the catalyst as well as

the major products from the FCC unit.

Catalytic cracking demands the use of acid catalysts. In the initial stages of the history of catalytic

cracking acid-leached clays and artificial or natural silica-alumina catalysts were used [2]. In the 1960s

they were replaced almost entirely by the zeolite catalysts and this led to great increase in the conversion

of the feedstock to gasoline as well as lowered the formation of coke and dry gas. Presently about 140

kinds of zeolites have been synthesized artificially along with about 40 naturally occurring types.

Several kinds of moderate to high temperature gas phase catalytic reactions are performed using them.

Several treatment operations carried out on the zeolites enable them to be used according to the

particular product (e.g. olefins like propylene, or gasoline with higher octane number rating) desired.

In this report attempt is being made to present some of the details regarding the synthesis,

characterization and use of the zeolites for FCC. In addition some modern developments and current

research pertaining to further modification and improvement of these fascinating catalysts also presented

here.

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Chapter Two

Typical Components of FCC Catalysts

2.1 Introduction

The catalysts typically used in FCC units consist of fine powders with an average particle size of 75

microns. The major components of these catalysts are discussed in the following sections.

2.2 Zeolites

The active catalytic ingredients in the FCC catalysts are zeolites. Zeolites are crystalline alumino-

silicates with a negatively charged macromolecular inorganic framework. They are micro porous and

hence are extensively used as commercial adsorbents. The first artificial synthesis was carried out in the

1960s and ever since then they have been applied extensively as industrial catalysts, most notably in the

petroleum and petrochemical sectors. FCC units are almost exclusively using these catalysts. The porous

structure of the zeolites enables them to accommodate a large variety of cations like Na+, Ca+, etc. These

positive ions are rather loosely held and can readily be exchanged for others especially H+ ions in a

contact solution. Natural zeolites form where volcanic rocks and volcanic ash layers react with alkaline

groundwater. They are mined in several parts of the world for commercial purposes. Along with it they

are also synthesized by a variety of techniques as discussed in this report.

2.2.1 Zeolite Structure and Chemistry

An enormous variety of zeolites have been identified and categorized and understanding their

structures and its relation to their properties is essential for selecting the right kind of zeolite for a given

purpose. Zeolites have a very well defined lattice structure. Its basic units consist of silica and alumina

tetrahedra. Each tetrahedron consists of a silicon or aluminum atom at the centre of the tetrahedron with

oxygen atom at the 4 corners. The pore diameters can vary over some limits in the different types of

zeolites, with 8 oA being a typical value and reversibly adsorbed water [1]. For a given zeolite, of course

the pore sizes are very nearly uniform. This is an important property which is exploited while designing

catalysts. The internal surface area is of the order of 600 m2/gm or even more. The framework of the

zeolites contains freely mobile and exchangeable cations and reversibly adsorbed water. The general

formula used to represent the zeolites is as follows:

Mex/z [(AlO2) x (SiO2) y]. nH2O

where Me stands for a cation with a positive charge z. Silicon is in a +4 oxidation state and hence a

tetrahedron consisting of silicon has neutral charge [2]. But aluminum is in +3 oxidation state and so

each tetrahedron containing aluminum has -1 negative charge .To compensate for the charge there are

other cations found in the zeolite structure and these are mainly obtained when the zeolite is being

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synthesized using some alkaline solution. Often the sodium ions from the sodium hydroxide solution are

used in the balancing of the charges but the zeolite made by this route is not hydrothermally stable. The

ammonium ion is frequently used to replace the sodium ions which on drying leaves behind H+ ions

which provide the acid sites( both Bronsted acid and Lewis acid sites) necessary for cracking reactions.

The Bronsted sites can further be exchanged with rare earth elements like lanthanum and cerium in order

to further tailor their properties.

(a) (b) (c)

Fig 1: Different types of Zeolites-(a) Commercially synthesized Zeolites, (b) Naturally occurring

Zeolites, (c) Computer generated structure of a typical Zeolite

(Source: (a) www.himfr.com, (b) and (c) www.wikipedia.org)

2.2.2 Zeolite nomenclature

A large variety of zeolites are already known and many more with interesting properties are being

synthesized. Hence it is necessary to follow a systematic procedure of naming them. 98 structures were

included in the 1996 issue of the Atlas of zeolite Structure Types and several more have been approved

by the Structure Commission of the International Zeolite Association (IZA). Some synthetic zeolites

have been named after the isostructural natural minerals like Faujasite, Mordenite or Ferrierite while

some others are known by the academic or industrial laboratories where they were first synthesized (e.g.

ZSM from Mobil or EU from Edinburgh University). For a common nomenclature policy each zeolite

framework has been assigned a three letter mnemonic. Some examples are provided below:

Fig 2: Mnemonics for different Zeolites

(Source: Collection of Simulated XRD Powder Patterns for Zeolites, Elsevier, 2001)

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2.2.3 Features of Zeolites suitable for catalysis

There are several reasons why zeolites have become the preferred catalyst for many reactions of

industrial importance. These are briefly described below:

a) Thermal Stability: One of the most important reasons behind the selection of zeolites as catalysts

for high temperature reactions is their good thermal stability. Most of the zeolites remain unaffected by

temperatures as high as 650oC. The structural collapse of the specially prepared high silica zeolites

becomes significant only at temperatures as high as 1000oC. Thus they are the most suitable for a

reaction scheme like fluid catalytic cracking where high temperatures and other harsh operating

conditions prevail.

b) Acidity: Zeolites exhibit much higher acidity than the earlier amorphous silica-alumina catalysts.

The acidity of the zeolites is due to the presence of both Bronsted as well as Lewis acid sites. The

Bronsted acid sites develop as the bridging hydroxyl groups consisting of hydroxyl protons associated

with the framework oxygen ions bonded to negatively charged tetrahedrally coordinated framework

aluminum. The maximum density of these acid sites is related to silicon to aluminum ratio in the zeolite.

As the density of the aluminum ions increases the acid site density decreases. The Bronsted acid sites are

generated in the zeolites by ion exchange.

The Lewis acid sites are formed upon heating the proton forms above 500 oC where cleavage of

water from the zeolite surface occurs.

The solid state acidity of zeolites is described by parameters like the following [2]:

1. The nature or type of the acid sites

2. The density or concentration of the acid sites

3. The strength distribution of the acid sites

4. The location of the acid sites within the zeolite framework

5. The geometric distribution of the acid sites over the zeolite crystals

An important parameter governing the total potential acidity per unit cell in the zeolite framework is

the Unit Cell Size (UCS), measured in Ao [1] .The negatively charged aluminum atoms are sources of

active sites in the zeolite .Silicon atoms do not possess any activity. The UCS is related to the number of

aluminum atoms per cell ( NAl) by

NAl = 111* (UCS-24.215)

c) Shape selectivity: The dimensions of the pores within the zeolite frameworks are often similar to

the sizes of the molecules participating in the reactions catalyzed by them. This creates a strong

influence on the selectivity of such reactions. There may be reactant shape selectivity, product shape

selectivity and transition state shape selectivity. The first two kinds of selectivity are usually based on

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the mass transfer effects rather than on chemical kinetics while the last category is based on the intrinsic

features of a reaction mechanism.

d) Concentration effects: The catalytic reactions are often involved with an adsorption step

whereby the reactants get attached to specific sites on the zeolites and then participate in the reaction.

The pore size distribution the specific pore volumes and the hydrophilicity or the hydrophobicity of the

zeolites can all affect the reactions and their selectivity. These properties can be fine-tuned while

synthesizing zeolites and therefore zeolites suitable for very specific purposes can be created.

2.2.4 Zeolites used as FCC catalysts

The zeolites typically applied as FCC catalysts are Type X, Type Y and ZSM-5. Types X and Y

possess similar crystal structures but the X zeolite has a lower silica-alumina ratio, which makes it less

stable. ZSM-5 is a versatile zeolite that helps in increasing the yield of olefins from FCC. The NaY

zeolites that are synthesized extensively are often ion-exchanged with rare earth elements like lanthanum

or cerium that form bridges with two or three acid sites in the zeolite framework. This protects the acid

sites and also makes the zeolites hydrothermally stable. Now-a-days as the demand for gasoline of

higher and higher octane rating is increasing, ultrastable Y or USY zeolites are being used to enhance

olefinicity and hence the octane number of gasoline. These catalysts have higher silica to alumina ratios

than the conventional Y zeolites.

2.3 Matrix

The matrix in the FCC catalyst is often considered to be that part of the catalyst other than the

zeolites. It may or may not have catalytic activity. Very often it does possess sufficient catalytic activity

towards some components of the feedstock and in this case they are described by the term ‘active

matrix’. Usually the matrix consists of substances that were used as FCC catalysts before the

development of zeolites e.g. acid leached clays and synthetic alumina-silica gels. In the majority of the

FCC catalysts the zeolites is embedded in the matrix.

The matrix serves several important physical functions such as:

A) Binding: One of the major functions of the matrix or certain components of it is to bind the zeolites

particles together in the form of microspheroidal catalyst particles after the spray drying operation.

B) Allowing diffusion: The matrix with its pore structure allows the diffusion of the required

hydrocarbons to the actual catalytic sites for reaction. A sufficiently stable pore structure of the matrix

will prevent collapse or sintering of the catalyst particles under adverse hydrothermal conditions of

operation of the FCC unit.

C) Diluting medium: This serves to moderate zeolite activity and avoids over-cracking that leads to

excessive coke formation and gas production.

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D) Heat transfer: The matrix functions as the storage of thermal energy in the FCC unit. It allows heat

transfer during cracking and regeneration operations and prevents catalyst damage due to excessive

temperatures.

The matrix consists of two main components-firstly a synthetic component like amorphous silica,

silica-alumina gels or silica-magnesia gels which serve as the binder and also exhibit catalytic properties.

The other component is some clay, natural or chemically modified, like kaolinite, halloysite or

montmorillonite. The clays provide mechanical stability.

Depending on their catalytic behaviour, the matrices may be classified as low activity, medium

activity or high activity matrices. The activity depends on the surface area of the fresh matrix the number

of acid sites and the presence of steam-stable pores in the size range of 50-150 Ao. The matrix usually

cracks some of the largest molecules in the feedstock and reduces them to such forms when thy can

easily enter the pores of the main zeolite catalysts. The highly active matrices can help in the bottoms

upgradation by enabling the cracking of the heaviest components of the residue; they can improve the

quality of light cycle oils (LCO) and also helps to increase the resistance of the zeolites towards

poisoning by metal deposition. The matrix is capable of trapping the metal contaminants and rendering

them passive [5].

A synergistic effect is often noted between the zeolite catalyst and the active matrix and this enables

us to have higher yields of the desired products.

2.4 Fillers and Binders

The functions of the fillers and binders incorporated into the FCC catalysts are often similar to those

performed by the matrix. Sometimes additional fillers like kaolin may be provided for physical integrity

and as a more efficient fluidizing medium [1]. The binder performs the all important function of holding

the catalyst, the matrix and the filler glued together. This is especially important when the catalyst

contains a higher amount of zeolites. The filler and binder minimize the production of catalyst fines in

the reactor-regenerator system and help to control the catalyst losses.

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Chapter Three

Major Physical Characteristics of FCC Catalysts

3.1 Introduction:

Several properties of zeolites have to be considered when selecting them as catalysts for FCC. Some

of these properties govern the catalytic behaviour of the zeolites while some others affect the mechanical

design and operation of the FCC unit itself [3]. These properties are briefly discussed below:

3.2 Attrition Resistance:

During the operation of the FCC unit the high gas flow rates and the harsh temperature often cause the

attrition of the catalyst particles and produce fines. Not all of the fines may be recovered by the cyclone

separators before the products can be separated. This leads to catalyst losses and causes emission of

particulate matter into the environment [3]. It also governs the rate at which fresh catalyst make-up must

be provided to the unit. As the particle size distribution is affected by this, the conditions of fluidization

itself might change.

An increase in the zeolite content of the catalyst, reduction of the zeolite crystal size and better

dispersion of the zeolite within the matrix leads to improved attrition resistance. The nature and quantity of

the binder provided to the catalyst also affects the attrition resistance. For example the boehmite form of

alumina is more effective in reducing attrition than gibbsite or bayerite. The type, particle size and the

morphology of the clay used as filler has a role to play in the attrition of the catalyst. Kaolin has been

found to increase the attrition resistance. The conditions maintained during spray drying of the catalyst

affects its future attrition resistance.

Catalyst attrition resistance is indicated by the value of the attrition index which is obtained from

standard methods.

3.3 Pore Size Distribution and Pore Volumes:

The pore size distribution has a major role to play in the catalyst properties of zeolites. If the pores are

too small, then they have a greater tendency to get clogged by coke and they also exhibit greater

diffusional resistance. If the pores are too large they provide a lower surface area for a given volume and

hence lower the efficacy of the catalyst and lead to enhanced attrition. The pore size distribution is also an

important parameter for the catalyst matrix as well. The shape of the pores is the most important property

governing the shape selectivity of the catalyst which is the most important characteristic used in many

applications.

3.4 Surface Area:

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The surface area of the catalyst comes from the zeolite and also the matrix. It ranges from over 800

m2/gm for conventional Y zeolites and is around 600 m2/gm for USY zeolites. The reduction in surface

area is due to the hydrothermal treatment given to the zeolites to enhance their stability which affects their

crystallinity and brings about changes in their pore structure. During the steam stripping step carried out

before regeneration of these catalysts the pore structure might collapse and thereby there is loss of surface

area. Catalysts in which the matrix is also active generally have higher surface areas.

3.5 Particle Size Distribution:

Most of the FCC catalysts have particle size ranging from 60 to 80 µm. The actual size distribution

depends heavily on the conditions prevailing in the spray drying step of catalyst manufacture. Fines

generated during the fluidized catalytic cracking often change the size distribution of the catalyst. This

might call for changes in the gas velocities to enhance good fluidization in the riser section and might also

make it necessary to make the operation of the cyclone separators somewhat flexible. Particulate emission

might also result from the fines.

3.6 Thermal and hydrothermal stability

The chemical nature of the zeolites, especially the silica-to-alumina ratio, crystallinity, ion-exchange

and residual sodium ions in the zeolite affect its stability. Maintaining the catalyst stability is essential to

maintaining its activity and selectivity. Rare earth exchanged zeolites are exceptionally stable. Stable

catalysts retain their pore structure during regeneration also.

3.7 Crystallinity

The crystalline nature of the FCC catalysts as determined from X-ray diffraction studies is an

indication of the zeolite content. The X-ray studies of the fresh catalyst and the equilibrium catalyst can

provide information about structural loss and the catalyst stability.

3.8 Microactivity Test (MAT) for FCC Catalysts:

The standard Microactivity test of the FCC catalysts is a valuable tool to evaluate the properties of

equilibrium catalysts from the FCC unit [4]. Since the operation of FCC units are cyclic the catalysts

undergo major changes in their properties from their fresh state and all these evaluations are carried out

after they have stabilized i.e. they have become equilibrium catalysts [6]. At the heart of this test lies a

fixed bed reactor into which the hot gas oil sample is injected. The catalyst activity is reported as the

conversion of the 2210C material. The conversion depends on the catalyst-to-oil ratio, feed space velocity

and other factors. The coke forming and gas forming tendencies of the catalyst as well as the above

mentioned physico-chemical properties of the catalysts cam be obtained from this useful test. It helps in

ascertaining the conditions to which the catalyst has been exposed. This test is simple and not much time

consuming.

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Chapter Four

Synthesis of FCC Catalysts

4.1 Introduction:

Modern processes used to manufacture FCC catalysts are of two types- (a) Incorporation method in

which the catalyst and the matrix are synthesized separately and then mixed with the fillers and binders,

and (b) ‘in-situ’ processes whereby the zeolite catalytic component is allowed to grow within preformed

microspheres of some kaolin type of particles. A general description of the processes is presented in the

following paragraphs.

4.2 Conventional Zeolites:

To prepare ordinary catalysts like REY, REHY or HY, firstly NaY zeolite is produced by the

digestion of a mixture consisting of silica, alumina and caustic soda for about 10 hours at prescribed

temperatures till crystallites start forming. After separating the crystals rare earth exchange of the

zeolites may be carried out either before or after its incorporation into the matrix element. The sodium

level of the zeolites is generally reduced to improve their hydrothermal stability by rare earth exchange

or by treatment with ammonium salt solutions to impart H+ ions into the matrix. The rare earth

exchanged catalysts are generally more active in increasing the yield of gasoline from the gas oils or the

resid fractions if their content in the catalyst is over 10 % by weight.

4.3 Ultrastable Zeolites (USY):

The preparation of these catalysts is accomplished by replacing some of the aluminum ions in the

framework by silicon. Steam calcination of the zeolite at a temperature of 700-800 oC is generally carried

out for this purpose. Apart from this acid leaching, chemical extraction and chemical substitution are

also employed to reduce the alumina content of zeolites. All the components of the catalyst are mixed to

form slurry and that is finally spray-dried to obtain USY zeolite catalyst. The residual alumina

precipitated within the pores of the zeolite during the synthesis can affect the catalyst properties

significantly. Octane boosting catalyst additives like ZSM-5 zeolites are also incorporated within the

catalyst matrix before the spray drying operation. The preparation of ZSM-5 is done in a manner similar

to that of any general zeolite material.

4.4 In-situ processes:

In this method the slurries of various grades of kaolin are first spray dried to form microspheres

which are then hardened at 700OC. These spheres are then digested with caustic soda or sodium silicate.

Thus the catalyst, the active matrix and the filler and binding agents develop simultaneously within the

microspheres. This process was developed by Engelhard.

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Chapter Five

Effects of FCC Catalyst Composition on its Properties

5.1 Introduction:

A wide variety of zeolite catalysts, having a range of useful properties are being used in different FCC

units round the world. These catalysts are tailor-made for specific purposes like selective cracking of the

feedstock to produce more olefins or more gasoline as per the requirements. Several catalysts have

additives like ZSM-5 mixed with them to enhance the octane number of the gasoline produced or to

control the emission of SOx, NOx or CO from the regenerator. In general, as the yield of gasoline

increases its octane number decreases, and this makes it necessary to add more expensive octane boosting

agents to it. As conversion of the feedstock rises, the yields of both gasoline and LCO pass through a

maximum and then decrease. But coke formation and gas production show a rising trend as conversion

rises.

5.2 Effect of Zeolite Content of the Catalyst:

An increase in the zeolite content of the catalyst leads to an increase in the catalytic activity as

expected. For a given conversion, as zeolite input increases the gasoline and LCO yields also increase

while hydrogen, C3-C4 olefins and coke yields decrease. The decrease in the unit cell size of the catalyst

arising out of the increase in the silica-to-alumina ratio leads to decrease of catalytic activity. Though the

USY catalysts are less active compared to conventional REY zeolites, they retain their activity under more

severe operating conditions. For a higher silica-to-alumina ratio there is greater production of LPG

fractions, lower formation of coke and lower gasoline selectivity [3]. The unit cell size thus has a major

role to play in dictating the selectivity exhibited by the zeolite. Apart from this the decrease in the unit cell

size of the zeolite results in an increase in the RON and MON values of the gasoline. This is predominant

at very low pore sizes in the region of highly acidic sites [3]. The presence of Na+ ions in the de-

aluminated zeolite framework inhibits the formation of high octane gasoline. This is probably due to the

neutralization of the acid sites by the residual free Na+ ions. Even the preparation method of the zeolites

has been found to have an effect on the catalytic activity and hence the final product distribution from the

FCC unit.

5.3 Effect of Additives:

Several kinds of additives are often added to the zeolite catalysts used in FCC units in order to change

the product distribution or to comply with increasingly stringent environmental regulations. Some of these

are discussed below:

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5.3.1 ZSM-5 additive: This is a special kind of high silica zeolite which is a member of the pentasil

family. It is widely used as an octane boosting additive in FCC. It may be incorporated into the catalyst

during the preparation or it may be added as separate particles. About 1-3 % of ZSM-5 added to the FCC

catalyst can raise the RON of the gasoline produced by up to 3 units [3]. The pore geometry of this

additive is such that it prevents the formation of cyclic compounds which lead to subsequent coke

formation.

Fig 3: Computer generated image of ZSM-5 showing its pore structure

(Source: www.3dchem.com)

5.3.2 CO Promoter: This additive consists of metals from the platinum group, present in the

concentration of 300 to 800 ppm and distributed over a support, and its use is to facilitate the conversion

of CO produced during catalyst regeneration to CO2. Though more uniform burning of the coke is

accomplished the additive tends to raise the temperatures during regeneration and also the production of

NOx. Typically 1 to 2.3 Kg of the promoter is added per ton of the fresh catalyst [1].

5.3.3 SOx Additive: The coke deposited on the FCC catalyst contains sulphur in the form of organic

compounds and during regeneration it forms SO2 and SO3. These acidic gases are finally discharged into

the atmosphere along with the flue gases. Keeping in mind the harmful environmental effects of the SOx it

has become necessary to control their emission from the FCC units. The additive is usually a metal oxide

which is directly mixed with the catalyst and it absorbs and bonds chemically with the SO3 in the

regenerator. This stable sulphate species is carried to the riser section where it is reduced to H2S and the

metal oxide. An excess of oxygen is needed in the regenerator so that the additive may provide the full

benefits.

5.3.4 Metal Passivators: Metals like Nickel, Vanadium and Sodium are often present in the feed to the

FCC unit and they have a detrimental effect on the FCC catalyst as they poison the active sites. To prevent

this metal passivators like Antimony are used. The passivators are injected into the feed and they form an

alloy with the nickel. This greatly reduces the dehydrogenation reactions and the resultant production of

dry gas and hydrogen.

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Chapter Six

Catalytic Cracking Reaction Mechanisms

6.1 Introduction:

FCC operations underwent a dramatic change with the introduction of zeolite catalysts and the

replacement of the conventional amorphous silica-alumina catalysts. Zeolites or artificial faujasites are

several hundreds of times more active that the conventional FCC catalysts used before 1960s and the

product distribution obtained from their usage is also quite different. With the development of newer

varieties of catalysts, greater control over catalyst selectivity and hence the main product from FCC has

been made possible. The typical reactions occurring in an FCC unit are tabulated below:

Feedstock Component Reaction Products ObtainedParaffins Cracking Lower Paraffins and

Olefins

Cracking LPG OlefinsCyclization Naphthenes

Isomerization Branched HydrocarbonsH-Transfer Paraffins

Condensation, Coke

Naphthenes

Cracking OlefinsDehydrogenation Cyclo-olefins, Aromatics

Isomerization A variety of Naphthenes

Aromatics

Side-chain Cracking Unsubstituted AromaticsTransalkylation Different Alkyl aromatics

Dehydrogenation, Poly-aromatics

6.2 Mechanism of Catalytic Cracking Reactions:

Catalytic cracking reactions generally proceed through the formation of Carbenium ions (R-CH 2+).

Another type of carbocation that may also be formed is the Carbonium ion (CH5+) by the addition of a

proton to the paraffin molecule. The proton is obtained from the catalyst Bronsted acid site. But this

species is not at all stable and the acid sites on the catalysts are usually not strong enough for the formation

of many such species.

On the other hand, a carbenium ion is formed either by the addition of a positive charge to an olefin or

by the removal of hydrogen and two electrons from a paraffin. These reactions are shown below:

(a) R-CH=CH-CH2-CH2-CH3 + H+(a proton at Bronsted site) R-C+H-CH2-CH2-CH3

(b) R-CH2-CH2-CH2-CH3(removal of H- at Lewis acid site) R-C+H-CH2-CH2-CH3

The tertiary carbocations are most stable, followed by the secondary and primary species. So during the

cracking process the primary and tertiary species tend to rearrange themselves into the tertiary forms and

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hence skeletal isomerization leading to the formation of branched hydrocarbons is one of the major

reactions taking place in the FCC. This gives us gasoline having higher octane numbers. Some olefins are

also produced due to typical thermal cracking mechanisms.

The main reaction is cracking which proceeds by the mechanism of Beta Scission. Here the

hydrocarbon is split at that carbon atom which is at the beta position with respect to the positive charge

bearing carbon atom. This is due to lower energy required to break this bond than the bond at the alpha

carbon. Short chained hydrocarbons have been found to be less reactive than long chained ones and hence

stable carbenium ions are not formed from the short chained species. Initially the products of beta scission

are an olefin and a new carbenium ion. The carbenium ion goes on to participate in a series of chain

reactions which lead to the cracking of the larger molecules and which stop with a typical chain

termination step. Cracking is a mono-molecular process and is favoured by high temperatures as it is an

endothermic process.

Hydrogen transfer reactions are also common during catalytic cracking. These are bimolecular

reactions which typically involve an olefin with another olefin or a naphthene. In case of two olefins, both

of them are adsorbed on active sites in the catalyst and one of them becomes the paraffin while the other is

converted to a cyclo-olefin as hydrogen is transferred from one species to another. The cyclo-olefin further

reacts to form a cyclo-di-olefin which finally rearranges to form aromatics. In case of the naphthenes, they

are the hydrogen donors and they react with olefins to produce paraffins and aromatics. The rare earth

exchanged zeolites enhance hydrogen transfer reactions as they protect adjacent acid site through the

formation of bridges. Hydrogen transfer reactions consume some of the olefins thereby increasing the

stability and yield of the gasoline produced i.e. they reduce the tendency of olefin rich gasoline to

overcrack. But hydrogen transfer reactions also have certain negative aspects like lower octane number of

the gasoline produced and higher amounts of LCO in the products from the FCC unit.

If the feed to the FCC unit contains metal poisons like nickel or vanadium they tend to promote

dehydrogenation reactions as well. This leads to the formation of compounds with increasing carbon to

hydrogen ratio which in turn leads to coke formation and deactivation of the catalyst. Unsaturated species

like olefins, di-olefins and polycondensed aromatic compounds are very reactive and they ultimately

polymerize to form coke. The amount of coke formed is crucial to the operation of the FCC unit as during

the regeneration step it is the combustion of this coke that produces the heat necessary to vaporize the feed

and allow the endothermic cracking reactions to be carried out. The riser temperature also has a role to

play in controlling the coke formation. Its optimum value is somewhere between 4500C and 5500C.

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Chapter Seven

Catalyst Management in FCC Units

7.1 Introduction:

The zeolite catalysts used in FCC is the heart of the whole operation. It is therefore necessary to

understand the effects of the catalyst on the actual operation of the unit. Catalyst management basically

involves recovery of the catalyst and minimizing losses, replacement of the spent catalyst from time to

time, ensuring proper catalyst circulation between the reactor and regenerator and stripping hydrocarbons

from it before regeneration. The importance of these operations is discussed in the following paragraphs.

7.2 Addition of fresh catalyst:

Regeneration of catalyst, no matter how efficient cannot restore its activity to the level of fresh

catalyst. Hence it is imperative to add new catalyst from time to time. It is necessary to maintain proper

activity and selectivity and compensate for catalyst losses due to some inefficiency in the recovery

cyclones. Also the metals deposited on the catalyst cannot be allowed to increase beyond some level for

which some of the catalyst must be withdrawn and replaced by fresh catalyst periodically. This will also

depend on the quality of the feedstock. The amount of catalyst added is equal to the amount lost and

withdrawn. But sometimes low activity catalyst may be provided as replacement in order to prevent

excessive activity which in turn leads to coking. Catalyst replacement is also necessary to ensure that the

particle size distribution in the FCC unit is properly maintained, as the particles tend to be reduced to fines

in the harsh operating conditions of the FCC. Replacement does not always ensure the same levels of

activity as was prevailing before the addition of new catalyst. Generally if the cyclone separators are

working properly about 80% replacement efficiency is obtained.

7.3 Spent catalyst stripping:

Before the catalyst enters the regenerator it must be freed of the hydrocarbons remaining on it.

Generally steam is used as the stripping agent. The optimum steam rate is governed by the mechanical

design of the riser, the catalyst circulation rate, the porosity and the surface area of the equilibrium

catalyst. The stripping steam rate is optimized by adjusting it to such a value for which the regeneration

temperature is minimized while maintaining the heat balance. Poor stripping leads to formation of

excessive coke in the subsequent cycles which in turn elevates the regenerator temperature.

7.4 Minimizing catalyst losses:

Catalyst losses occur when there is excessive production of fines due to attrition or when the reactor

or regenerator cyclones are not working properly. The replaced catalysts often have different physical

properties e.g. they may be softer than the older catalyst and thereby they contribute to the losses. To

18

control the losses it is necessary to monitor the performance of the cyclones. The size distribution of the

equilibrium catalyst as well as the fines needs to be considered. The typical size distribution of the catalyst

is shown in Figure 4. While smaller particles lead to easy fluidization, the requirements for preventing

excessive catalyst losses are quite the opposite. Hence there are optimum size ranges for the catalyst

particles. The cyclone performance can be affected if it has suffered some kind of mechanical damage like

the development of perforations in the diplegs or erosion of the internal lining. Sudden surges in the

vapour rates in the reactor unit could lead to entrainment of even smaller particles. Too high or too low

catalyst levels in the disengagement section of the reactor can also lead to the malfunctioning of the

cyclones, thereby leading to catalyst losses.

Figure 4: Particle size distributions of equilibrium FCC catalyst

(Source: Albemarle FCC Manual-The Role of Catalyst in FCC Troubleshooting)

7.5 Preventing Excessive Coking Tendencies:

Coke formation is an important part of the FCC operation and the heat balance depends on the

exothermic reactions involving the combustion of the coke in the regenerator. But excessive coking

diminishes catalyst life and in general affects the working of the unit drastically. Coking tendencies

depend on the nature of the feed especially its metal content. To minimize these problems it is necessary

to avoid having cold spots in the system, minimizing of heat losses from the transfer lines connecting the

reactor and regenerator and improvement of the feed-catalyst mixing system by introducing high

efficiency feed nozzles.

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Chapter Eight

Recent Developments in FCC Catalyst Technology

8.1 Introduction:

As the demand for gasoline increases worldwide, the importance of FCC as the main fuel upgradation

technique will also continue to rise. Hence a lot of research work is being carried out round the world to

gain a fundamental understanding of the action of the FCC catalysts on the cracking reactions and then

synthesizing newer and improved catalysts for this process. In this section an attempt is made to describe

some of the recent work done in the field of FCC catalysts and their importance.

8.2 Demetallization Procedures:

Most of the crude being processed in refineries round the world now is heavy crude which has a high

concentration of metals like nickel, vanadium, iron, chromium, cadmium, barium etc. These elements tend

to poison the acid sites on the catalyst and thereby they lead to catalyst loss. To remove these metals from

the catalyst, pyrometallurgical and hydrometallurgical techniques are used. Recent developments in this

field have lead to more efficient techniques of demetallization. Upto 67% of the lost activity is reportedly

regained after these new demetallization techniques [4]. Effective demetallization reduces the requirement

for fresh catalyst and thereby leads to savings for the refiners.

8.3 Modelling of FCC Catalyst Pore Structures:

Designing newer and more effective FCC catalysts needs a model of the structure of the FCC

catalysts. There have been several attempts at visualizing the porous structure of the zeolites. The simplest

assumption says that the zeolite pores are uniformly distributed among the pores of the support, as

depicted in Fig. 5. But electron microscopic studies indicate that the pores form a highly entangled,

interconnected network and it is modeled as a stochastic network. This is shown in Fig. 6.

Figure 5: Simple parallel bundle model for a supported zeolite catalyst.

(Source: R Mann, Catalysis Today, 18 (1993) pg. 516)

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Figure 6: A 10 x 10 x 10 3-dimensional stochastic pore network model in an FCC catalyst

(Source: R Mann, Catalysis Today, 18 (1993) pg. 518)

These stochastic pore models can be used for the study of reaction and diffusion effects and they have

been used in the study of coking phenomena also, with the coking on the catalyst and coking on the

support being analyzed separately. Validation of these pore models can be done by the pressurized

penetration of a low melting allow into the actual catalyst sample and then allowing it to solidify,

thereby providing a measure of the volume available.

8.4 Advanced Kinetic Models:

The reactions taking place in an FCC unit are quite complex. To understand the role of the FCC cat -

alysts on the various reactions of such a complex network, several models have been proposed and their

validity has been checked by comparison with actual product distributions. One such study was carried

out by Hongjun et. al. [8] where the aromatization reactions of FCC gasoline were studied. A new com-

plex reaction network with nine lumped kinetics models was proposed for the aromatization reaction of

FCC gasoline. In the network, the aromatization reaction species were firstly lumped into n paraffins, i-

paraffins, olefins, aromatics, coke, and some lower hydrocarbons. Three main type reactions among

these lumped components were considered in the aromatization reaction network, such as paraffin dehy-

drogenation and cyclization, paraffin isomerization and cracking to low carbon hydrocarbon. All the re-

actions were modeled as first order, irreversible reactions and catalyst deactivation effects were also in-

corporated. The performances of FCC gasoline and catalyst were evaluated in a confined fluidized bed

reactor with piston flow and its reaction is controlled by the reaction dynamics and the effect of external

diffusion. The various parameters in the material balance equations were obtained by optimization using

the Levenberg-Marquardt technique and the theoretical predictions were found to be close to the actual

data obtained from the refineries. The reaction network is modeled as follows:

21

Figure 7: A web model of the aromatization reactions in FCC

(Source: Hongjun et. al, Catalysis Communications, 7 (2006) pg. 556)

8.5 Special Catalysts for Short Contact Time Applications:

The modern day FCC units are all designed in such a way that the maximum

cracking reactions take place in the riser section within a very short contact time.

This makes it necessary to develop catalysts for short contact time cracking (~ 1 to

3 seconds) of even the heavier fractions and resids to produce the primary

products and also olefins which are the principle feed stocks for the polymer

industries. Engelhard Corporation has developed and commercialized the NapthaMax™ catalyst

for this purpose. Studies have also been carried out at the research laboratories of Akzo Nobel Catalysts,

Amsterdam. The catalysts have been developed in such a way that they use a patented matrix technology

called Distributed Matrix Structure (DMS) with specially developed PyroChem Plus zeolites to achieve

optimized porosity and high catalytic activity. Typically the active matrix causes pre-cracking of the

heavier feedstocks which then diffuse into the pores of the zeolites to reach the actual catalytic cracking

sites. But in the DMS architecture the zeolites are highly dispersed over the matrix and hence even the

pre-cracking takes place on the zeolite surfaces. This allows for better selectivity of the catalyst and

eliminates the need for a secondary diffusion step to reach the active sites. Excessive coke and dry gas

formation are minimized. Though the overall porosity of the new and the conventional catalysts may be

the same yet their morphologies are quite different. In these proprietary catalysts the pores are located

much more uniformly thereby allowing easy diffusion of the pre-cracked species. These catalysts are

also evaluated in a manner different from the conventional Microactivity test using a fixed bed reactor.

A fixed fluid bed unit is used for these tests with which a reasonable simulation of the short contact time

riser reactors can be achieved.

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a) b) Figure 8: SEM images of two kinds of FCC catalysts-(a) DMS Matrix, (b) Conventional Zeolite

(Source: Mc Lean J B and Stockwell D M, NaphthaMaxTM Breakthrough FCC Catalyst

Technology for Short Contact Time Applications, NPRA 2001 Annual Meeting, New Orleans)

The figures above illustrate the differences in the structure of the short contact time catalyst and the

conventional zeolites catalyst.

8.6 Some other Special Purpose FCC Catalysts:

The demand for reformulated gasoline is on the rise worldwide. Octane boosting additives

(oxygenates) like methyl-tertiary-butyl ether (MTBE) or TAME are synthesized using iso-olefins

(isobutylene and iso-amylene respectively) and the main source of these hydrocarbons is the FCC unit of

a refinery. Catalyst manufacturers like Engelhard Corporation has come up with special varieties of

catalyst to maximize the production of specific hydrocarbons like the iso-olefins through the use of

proprietary catalyst formulations like IsoPlus™ catalyst. It is claimed that more than 50 % increase in

the yield of iso-olefins have been obtained by using these catalysts over that from conventional catalysts.

Figure 9: Relative Increase in the yield of Iso-butylene with the use of IsoPlus catalysts in FCC

(Source: Mc Lean J B and Witoshkin A, Isoolefins for Oxygenate Production using IsoPlus™ , NPRA

1993 Annual Meeting, Texas )

23

Chapter Nine

Conclusion

FCC technology is the most important gasoline upgradation technology that refiners round the world

have at their disposal and they are continually seeking to improve the performance of these units by

incorporating newer and better catalyst formulations in their operations along with modifications in the

process design. A large variety of catalyst formulations are available from Engelhard Corporation (now

BASF), UOP, Akzo Nobel, Lummus etc to suit specific purposes like coke and gas yield minimization,

enhanced production of olefins or rise in the octane number of the gasoline produced. Though it appears

that none of these new catalysts have had as dramatic an impact on the economics of the petroleum

refineries in general and the FCC process in particular as had happened with the introduction of zeolites

as the hydrocarbon cracking catalysts, yet each of the new variety of catalyst has enhanced the

profitability of the refining operations to some extent. Further developments in this field are possible.

As environmental regulations become more and more stringent the emission of SO x, NOx and CO

along with the FCC regenerator flue gases also needs to be controlled. This has given a boost to the

development of FCC co-catalysts and catalyst additives.

Various new procedures of catalyst synthesis have also developed and sometimes minor changes in

the processing methodology can yield catalysts with very different properties and hence final product

distribution is influenced by this factor as well. Catalyst properties are continually improving with

greater silica-to-alumina ratio in the zeolites and better control over the distribution of the acidity and

crystallinity. There has been increased emphasis on the development of FCC catalysts for resids

upgradation and bottoms product cracking without too much of the problem of catalyst fouling. The use

of additives like ZSM-5 is expected to increase steadily because of their inherent ability to crack or

isomerize low octane straight chain paraffins, even more so after the patent on this additive expires. In

brief customized FCC catalysts will continue to be developed to meet the various requirements of

refiners.

Catalyst evaluation techniques also need to be modified and several improved versions of the

conventional Microactivity test are already in place. While MAT used a fixed bed pilot plant reactor,

these newer tests evaluate the equilibrium FCC catalysts in fluidized or partially fluidized beds which

produce more realistic results. The modeling of cracking reactions and theoretical prediction of catalyst

behaviour is also an area of active research now.

FCC catalysts have witnessed a long history of progress and the trend will hopefully continue in the

future as well.

24

References

[1] Fluid Catalytic Cracking Handbook, Reza Sadeghbeigi, 2nd Edition, Gulf Publishing Company,

Houston, Texas, 2000

[2] Basic Principles in Applied Catalysis (edited by M Baerns), Springer-Verlag, 2004

[3] Octane enhancing Zeolite FCC Catalysts (Scientific and Technical Aspects), Julius Scherzer, Marcel

Dekker.Inc, New York and Basel, 1990

[4] Fluid Cracking Catalysts (edited by Mario L. Occelli and Paul O’Connor), Marcel Dekker .Inc, New

York, 1998

[5] Matrix Effects in Catalytic Cracking, Lance D. Silverman et.al, Engelhard Corporation, Edison, New

Jersey, paper presented at the 1986 NPRA Annual Meeting at Los Angeles, California

[6] Passamonti et. al., Catalysis Today, 133–135 (2008) 314–318

[7] Mann R, Catalysis Today, 18 (1993) 509-528

[8] Hongjun et. al. Catalysis Communications, 7 (2006) 554–558

[9] Mc Lean J B and Stockwell D M, NaphthaMaxTM Breakthrough FCC Catalyst Technology For Short

Contact Time Applications, NPRA 2001 Annual Meeting, New Orleans

[10] Mc Lean J B and Witoshkin A, Isoolefins for Oxygenate Production using IsoPlus™ , NPRA 1993

Annual Meeting, Texas

[11] www.himfr.com

[12] www.wikipedia.org

[13] www.3dchem.com

[14] www.catalysts.basf.com

[15] www.thefccnetwork.com

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