Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining

Подождите немного. Документ загружается.

varying amounts of contaminants such as sulphur, nitrogen and metals

particularly in the higher boiling fractions. These differences in feed com-

position and in contaminants affect the operating conditions required to

obtain desired yields. To protect the catalyst, feed pre-treatment by hydro-

treating is required in order to remove contaminants and improve cracking

characteristics and yields.

Gas oil from residue and conversion processes (predominantly coking)

can be fed to catalytic cracking units. They must be hydrotreated before

catalytic cracking to separate aromatics and remove sulphur. The principal

limitation on charge stocks are the Conradson Carbon Residue (CCR) and

metal contaminants. The effect of Conradson carbon is to form a deposit on

the catalyst. This deposit could be beyond the burning capacity in the

regenerator. For atmospheric residue, it is desulphurised first in the

ARDS unit. Vacuum residue must also desulphurised and may be deas-

phalted before used in the FCC. Special RFCCs are designed to handle such

heavy feeds.

The presence of organometalic compounds tends to deposit metals in

the desulphurisation process; therefore a guard reactor should be used.

Nitrogen tends to poison the catalyst by neutralising its acid sites. However,

the FCC process is unaffected if the nitrogen content level is controlled

below 0.2%. Some possible feedstocks are atmospheric distillates, coking

distillates, visbreaking distillates, VGO, atmospheric residue (desulphurised)

and vacuum residue (desulphurised, deasphalted). Typical feedstock proper-

ties are given in Table 8.1. In addition, products with their corresponding

yields and characteristics are shown in Table 8.2.

Table 8.1 Feedstock properties of FCC unit (Tominaga and Tameki, 1997)

Desulphurised

vacuum gas oil

Atmospheric

residue

Speciﬁc gravity (15/4



C) 0.896 0.889

API 26.3 27.5

Gas oil fraction (GO), wt% (boiling point

< 343



VGO fraction (VGO), wt% (boiling point

343–538



88.5 52.5

Vacuum residue fraction (VR), wt%

(boiling point > 538



4.5 43.5

Conradson Carbon Residue (CCR), wt% 0.2 4.2

Sulphur, wt% 0.4 0.11

Nitrogen, wt% 0.064 0.19

Nickel (Ni), wppm 0.26 17

Vanadium (V), wppm 0.15 0.5

Fluidised Catalytic Cracking 201

8.4. Fluidisation

When a fluid flows upward through a packed bed of catalyst particles

at low velocity, the particles remain stationary. As the fluid velocity

increases, the pressure drop increases. Upon further increases in velocity, a

balance of pressure drop times the cross-sectional area equals the gravita-

tional forces on the particles’ mass. Then the particles begin to move. This is

the minimum fluidisation velocity as shown in Figure 8.2. The stable

fluidisation starts at a certain pressure drop. As the velocity increases, the

bed expands and bed porosity increases while the pressure drop remains

practically unchanged. This is the practical region which includes all modes

of fluidisation such as bubbling, slugging and pneumatic transport (Bonifay

and Marcilly, 2001). These modes are shown in Figure 8.2.

8.5. FCC Reactions

The main reaction in the FCC is the catalytic cracking of paraffin,

olefins, naphthenes and side chains in aromatics. A network of reactions

occurring in the FCC is illustrated in Figure 8.3. The VGO undergoes the

desired ‘primary cracking’ into gasoline and LCO. A secondary reaction also

occurs, which must be limited, such as a hydrogen transfer reaction which

lowers the gasoline yield and causes the cycloaddition reaction. The latter

could lead to coke formation (needed to provide heat for catalyst

regeneration).

Table 8.2 FCC products

Products Characteristics

Yield

(wt%)

Dry gas + H

)+H

S must be removed 3–5

LPG: C

Petrochemical feedstock 8–20

Gasoline Main product, good octane

number

35–60

Light cycle oil (LCO) Rich in aromatics, high

sulphur content, diluent

for fuel

12–20

Heavy cycle oil (HCO) + slurry Very rich in aromatics, slurry

of solids, (mainly catalyst

coke)

10–15

Coke Consumed in regenerator 3–5

202 Chapter 8

High velocity Low velocity

Fixed bed Bubbling

Minimum

fluidization

Dilute phase

Pneumatic

transport

Slugging

Pressure loss

Linear gas

velocity

Fluidized bed

Minimum velocity

for fluidization

Pneumatic

transport

Fixed bed

Figure 8.2 Modes of fluidisation

, C

Heavy

Gas oil

Gasoline

LCO

Residue

Coke

Primary Cracking

Gases

Dehydrogenation

Primary

Cracking

Secondary

Cracking

Cycloaddition

Dehydrogenation

Oligomerization

Figure 8.3 FCC reactions network

Fluidised Catalytic Cracking 203

8.5.1. Primary Reactions

Primary cracking occurs by the carbenium ion intermediates in the follow-

ing steps:

(a) Olefin is formed first by the mild thermal cracking of paraffin:

! CH

þ CH

 CH

ðÞ

 CH ¼ CH

ð8:1Þ

(b) Proton shift:

 CH

ðÞ

 CH ¼ CH

þ H

O þ

 O  Si !

Zeolite

 CH

ðÞ

 CH

þ HO  Al  Si½



Carbenium ion

ð8:2Þ

Carbon–carbon scission takes place at the carbon in the position beta to the

carbenium ions and olefins.

ðCH

 CH

! CH

 CH ¼ CH

 CH

ð8:3Þ

The newly formed carbenium ion reacts with another paraffin molecule and

further propagates the reaction.

The chain reaction is terminated when (a) the carbenium ion losses a

proton to the catalyst and is converted to an olefin; or (b) the carbenium ion

picks up a hydride ion from a donor (e.g. coke) and is converted to paraffin.

Beside paraffins, other hydrocarbons which are formed by primary cracking

include the following:

1. Olefins – smaller olefins

 CH ¼ CH  CH

 CH

 CH ¼ CH  CH

þ CH

 CH ¼ CH

ð8:4Þ

2. Alkylaromatics – Dealkylation

-CH

+ CH

= CH

(8.5)

3. Alkylaromatics – Side chain cracking

-CH

+ CH

= CH

-CH

(8.6)

204 Chapter 8

Hydrogen transfer plays a key role in the gas oil cracking process. It reduces

the amount of olefins in the product, contributes to coke formation, and

thereby influences the molecular weight distribution of the product.

Through intermolecular (bimolecular) hydrogen transfer, highly reactive

olefins are converted to more stable paraffins and aromatics as in the

following reaction:

þ C

! 3C

2nþ2

þ C

2m6

Olefin Naphthene Paraffin Aromatic

ð8:7Þ

Further loss of hydrogen to olefins by aromatics or other hydrogen-deficient

products results in more paraffins and coke

2n6

or C

2m2

!

H loss

Coke

Aromatics Cyclo-olefins Alkylation; condensation

Polymerization

ð8:8Þ

!

H addition

2nþ2

Olefins Paraffins

ð8:9Þ

8.5.2. Secondary Reactions

Gasoline formed from primary cracking can undergo further secondary

cracking, which is generally caused by hydrogen transfer mechanisms such

as isomerisation, cyclisation and coke formation.

Isomerisation

R  CH

 CH

Secondary

Carbenium ion

R  CH



 CH

Tertiary

Carbenium ion

ð8:10Þ

The final product is the transformation of paraffins and olefins to iso-

paraffins.

Cyclisation

-CH-(CH

)

-CH=CH-CH

(8.11)

Fluidised Catalytic Cracking 205

The final result would be the cyclisation of olefins to naphthenes and

possibly further cyclisation to coke.

The main reactions in the FCC reactor can be summarised as follows:



Paraffins

Thermal catalytic cracking

Paraffin cracking ! Paraffins + Olefins



Olefins

The following reaction can occur with olefins:

Olefin cracking ! LPG olefins

Olefin cyclisation ! Naphthenes

Olefin isomerisation ! Branched olefins + Branched paraffins

Olefin H-transfer ! Paraffins

Olefin cyclisation ! Coke



Naphthenes

Naphthene cracking ! Olefins

Naphthene dehydrogenation ! Aromatics

Naphthene isomerisation ! Restructured naphthenes



Aromatics

Aromatics (side chain) ! Aromatics + Olefins

Aromatic transalkylation ! Alkylaromatics

Aromatic dehydrogenation ! Polyaromatics ! Coke

8.6. Thermodynamics of FCC Reactions

The key reaction in cracking is b-scission, which is not equilibrium

limited; therefore, thermodynamics are of limited value in either estimating

the extent of a reaction or adjusting the operating variables. Cracking of

relatively long-chain paraffins and olefins can go up to 95% completion at

cracking conditions.

Certain hydrogen transfer reactions act in the same way. Isomerisation,

transalkylation, dealkylation and dehydrogenation reactions are intermedi-

ate in attaining equilibrium. Condensation reactions, such as olefin poly-

merisation and paraffin alkylation, are less favourable at higher temperatures.

Examination of the equilibrium constants and heat of reaction of these

reactions are shown in Table 8.3 (Sadeghbeigi, 2000). It is apparent that

the type and magnitude of these reactions have an impact on the heat

balance of the unit. For example, a catalyst with less hydrogen transfer

characteristics will cause the net heat of reaction to be more endothermic.

Consequently, this will require a higher catalyst circulation and possibly a

higher coke yield to maintain the heat balance.

206 Chapter 8

Table 8.3 Typical thermodynamic data for idealised reactions of importance in catalytic cracking

Log K

equilibrium constant

Heat of reaction

BTU/mole

950



F850



F950



F980



Cracking n-C

! n-C

2.04 2.46 – 32,050

1-C

! 2C

1.68 2.1 2.23 33,663

Hydrogen transfer 4C

! 3C

12.44 11.09 – 109,681

cyclo-C

+ 3 l-C

! 3n-C

11.22 10.35 – 73,249

Isomerisation l-C

! trans-2-C

0.32 0.25 0.09 4,874

n-C

! iso-C

0.2 0.23 0.36 3,420

o-C

(CH

)

! m-C

(CH

)

0.33 0.3 – 1,310

cyclo-C

! CH

-cyclo-C

1 1.09 1.1 6,264

Transalkylation C

+ m-C

(CH

)

! 2C

0.65 0.65 0.65 221

Cyclisation l-C

! CH

-cyclo-C

2.11 1.54 – 37,980

Dealkylation iso-C

-C

! C

0.41 0.88 1.05 40,602

Dehydrogenation n-C

! l-C

2.21 1.52 – 56,008

Polymerisation 3C

! l-C

––1.2 –

Parafﬁn alkylation l-C

+ iso-C

! iso-C

– – 3.3 –

Fluidised Catalytic Cracking 207

The occurrence of both exothermic and endothermic reactions contri-

butes to the overall heat balance. Overall, the reaction is quite endothermic,

and heat must be supplied to the system by the combustion of coke during

catalyst regeneration.

The high volume of products caused by the cracking of larger mole-

cules, requires low operating pressure (1–5 bars). The high endothermic

nature of cracking reactions requires that the reactor operates at high

temperatures 480–550



C (869–1022



F).

8.7. FCC Catalyst

The main catalyst which is used in a FCC reactor is the zeolite type. It

is in a powderform with an average particle size of 75 mm and an average

surface area of 800 m

/g. It has a crystalline structure of aluminosilicates.

A matrix is added to the zeolite which acts as a binder and filler.

8.7.1. Zeolite

The main active component in the catalyst is the Y-Zeolite. It is a crystalline

structure of aluminosilicates which has the Y-faujasite structure

(Figure 8.4). The highest pore size in the Y-faujasite structure is 8



which is called the super cage. It can allow some C

–C

mono-, di- and

tri-nuclear aromatics present in the VGO to pass.

In the cracking of long chain paraffins, another type of high silica zeolite

is added. This zeolite is called ZSM-5 and is used to improve octane number

Sodalite cages

Super cage

Hexagonal

prisms

Figure 8.4 Structu re of Y-faujasite

208 Chapter 8

and it is composed of zig-zag channel systems (Scherzer, 1990). Figure 8.5

shows how hexane will easily enter the catalyst pore. Then hexane cracks

into propane, while iso-hexane and benzene do not enter the pores. Thus

the unreacted stream is enriched with iso-paraffins and aromatics, which

contribute to an increase in the octane number.

8.7.2. Matrix

The matrix is added to the zeolite to increase the body of the catalyst and

add some improved properties. Three types of substances constitute the

matrix:

1. A binder is added as a glue which is then added to the catalyst to provide

cohesion for zeolite particles.

2. A filler is added to make up the body of the catalyst. It is usually a clay

(Kaoline). The function of the filler and binder is to provide physical

integrity (density and attrition resistance).

3. Additives, such as a small amount (ppm) of metal, are added to the

catalyst to promote the combustion of CO to CO

in the regenerator.

Metallic oxides are added to fix SO

on the catalyst. The sulphur is

recovered as H

S in the reactor. An addition of 5% ZSM-5 zeolite will

lead to an increase of one research octane number (RON).

8.8. FCC Configuration

The basic configuration of the FCC unit is a reactor (riser) and a

regenerator. The catalyst is circulated between them where it is deactivated

in the riser and regenerated in the regenerator. There are two basic types of

FCC units in use today. The ‘Side-by Side’ type is one in which the reactor

C-C-C-C-C-C

C - C- C

C - C

C-C-C

Figure 8.5 Schematic representation of shape-selective cracking with ZSM-5 zeolite

(Scherzer,1990)

Fluidised Catalytic Cracking 209

and regenerator are separate vessels adjacent to each other. The Stacked or

Orthoflow type reactor is mounted on the top of the regenerator. There

have been many developments and modifications over the last few years to

improve both performance and/or efficiency, or to develop special purpose

FCCs. The basic two types of FCC are shown in Figure 8.6.

8.9. Process Description

The process flow diagram for the side-by-side FCC unit is shown in

Figure 8.7. Steam and VGO heated up to 316–427



C (600–800



F) are fed

to the bottom of the riser, which is a long vertical pipe. The regenerated hot

catalyst at 649–760



C (1200–1400



F) is also fed to the bottom of the riser.

The riser is the main reactor in which the endothermic reactions take place.

The residence time in the riser is 2–10 s. At the top of the riser, the gaseous

products flow into the fractionator, while the catalyst and some heavy liquid

hydrocarbon flow back in the disengaging zone. Steam is injected into the

stripper section, and the oil is removed from the catalyst with the help of

some baffles installed in the stripper. The oil is stripped in this way from the

catalyst and the spent catalyst is sent to the regenerator at a temperature of

F = Feed, P = Products, S = Steam, FG = Flue gas, A = Air

Reactor

Regenerator

Side–by–Side type Stacked type

Figure 8.6 FCC type configuration

210 Chapter 8