SPRING 2019

CL 4003 PETROCHEMICALS AND REFINERY ENGINEERING

Lecture 23

Department of Chemical Engineering

Birla Institute of Technology Mesra, Ranchi1

Catalytic Cracking

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✓ Catalytic cracking works with high-molecular-weight

hydrocarbons located in a boiling range above approximately

350 °C.

✓ It breaks them up into lower-molecular-weight hydrocarbons,

mainly consisting of a gasoline cut ranging from C5+ to 200 °C,

at low pressure on catalyst at a temperature of some 500 °C.

✓ It is today the leading refining conversion process.

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Introduction

✓ Catalytic cracking is much more rapid and selective than

thermal cracking.

✓ It allows lower operation severity, thereby considerably reducing

secondary reactions that produce gases, coke and heavy

residues at the expense of gasoline.

✓ Moreover, the gasoline produced is of much better quality (the

stability and octane numbers are superior by far). As a result,

the process quickly became widely used in refineries.

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Introduction

✓ The first attempts to reduce the molecular mass of heavy

petroleum cuts date back to 1912. They were followed sometime

around 1920 by the development of the McAfee batch cracking

process with AlCl3, as a catalyst, which was to be used for 14

years in the Gulf refinery in Port Arthur. In 1923, a French

engineer named Eugene Houdry launched a study that led to

the fixed bed catalytic cracking process. The first unit started

up in 1936 with a natural clay based catalyst (montmorillonite).

In 1940, the natural catalyst was replaced by a more active and

selective silica-alumina based synthetic one. 5

Introduction

✓ The fixed bed Houdry process used 3 reactors working

alternately in reaction then regeneration with intermediate

purges. Switching back and forth quickly between phases made

the process complex and expensive, and research was soon

undertaken to improve on it.

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Introduction

Research work, intensified by the demand for gasoline during the

Second World War, started giving results in the early forties. The

following new technologies were developed:

✓ The fluidized bed process, or FCC (Fluid Catalytic Cracking).

The first PCLA unit (Powdered Catalyst Louisiana) was

commissioned in May 1942 in the Esso refinery in Baton Rouge,

with a catalyst whose clay base was ground up into powder.

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Introduction

✓ The moving bed process. The first TCC (Thermofor Catalytic

Cracking) then Houdriflow units started up at more or less the

same time in 1943.

✓ The most efficient technology, FCC, gradually gained

ground the world over.

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Introduction

Fluidized Catalytic Cracking

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✓ The typical feed going into the FCC is the vacuum distillate

(VGO or vacuum gas oil), whose initial boiling point is 350-

380°C and end point is approximately 550-560°C.

✓ However, the refiner very often adds other stocks with a

comparable molecular weight that he wants to upgrade from

various conversion units such as visbreaking, coking and

deasphalting.

✓ Ever since the early eighties, the tendency has been toward

heavier feeds by the addition of varying amounts (10 to 50%

generally) of atmospheric residue (AR). 10

FCC: Introduction

✓ These feeds are converted in a few seconds in the FCC reactor on a

solid acid catalyst (fine fluidized powder).

✓ The cracking product yield and quality obviously depend on the

characteristics of the processed feed, the operating conditions

(480°C < T < 550°C, 1 bar <P < 3 bar, catalyst and feed flow rate)

and the catalyst.

✓ A very wide range of products can be obtained, ranging from light

gases (C4-) to very heavy fractions (HCO: 350-55O °C, slurry: 550

°C+) and even coke. Usually the most valued product is gasoline

with an average yield of some 50 wt% in relation to the feed.

HCO: Heavy cycle oil; Slurry: bottom of the fractionation column that can contain catalyst fines. 11

FCC: Introduction

Feeds and Products

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✓ FCC feeds are characterized by a number of properties that

govern the yields, the catalyst deactivation rate and the

operating conditions.

✓ The simplest property that directly influences yields is the feed’s

specific gravity. For a given distillation range it indicates the

degree of saturation of the molecules. For instance, low specific

gravity is evidence of high hydrogen content and the feed’s

potential to be readily converted into high added-value products

such as gasoline and liquefied gases.

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Characteristics of Feeds

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Examples of FCC feeds

✓ In contrast, high specific gravity is evidence of high aromaticity,

the feed’s resistance to cracking and its potential to give heavy

aromatic oils such as LCO, HCO and slurry. The same

relationships are found when the feed is characterized by the

aniline point test which measures its aromaticity.

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Characteristics of Feeds

✓ Conradson carbon is the main indicator of the presence of

residue. Generally an increase in Conradson carbon also means

an increase in the asphaltene and metal (e.g. nickel and

vanadium) content. High Conradson carbon is synonymous

with increased coke yield and regenerator temperature. The

presence of metals at the same time causes more catalyst to be

consumed to maintain the same activity.

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Characteristics of Feeds

✓ Other properties influence the thermal balance directly and the

yields indirectly. They are the distillation range and the

viscosity. These two properties affect the degree of feed

atomization and vaporization in the reactor. High viscosity and

an overly high distillation end point explain the production of

additional unwanted coke which leads to the increased

regenerator temperature and lower conversion.

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Characteristics of Feeds

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Typical FCC yields during maximum gasoline production (%wt)

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Product Characteristics

The catalytic cracking process consists of four sections:

• reaction section,

• product fractionation section,

• flue gas treatment section,

• catalyst handling section,

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Description of the Process

• This section is the heart of the unit and, whatever the

technology, it is basically made up of a reactor and a

regenerator.

• The catalyst is kept in a fluidized state and circulates

continuously like a liquid between the reactor and the

regenerator.

• Typically, the catalyst runs a complete cycle in less than 15

min, i.e. the reaction, the separation of reaction products from

the catalyst, catalyst stripping and regeneration.

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Reaction Section

• The fundamental operating principle of the FCC is based on the

thermal equilibrium achieved constantly between the reactor

and the regenerator.

• Catalyst circulation is the energy vector, it provides the energy

required to vaporize the feed in the reactor and to make the

endothermic cracking reaction occur.

• The energy comes from the regenerator where coke is burned.

The coke is laid down on the catalyst in the reactor and

deactivates it, so when the spent catalyst comes from the

reactor its activity must be restored by eliminating the coke. 22

Reaction Section

• The thermal balance depends on the characteristics of the feed

processed.

• For feeds with low Conradson carbon, the coke yield is too low

to meet the unit's needs. Energy must be supplied to the system

by a feed preheater.

• For feeds containing a greater proportion of residues, the energy

available from coke combustion may prove to be excessive and

heat will have to be exported to make the process workable.

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Reaction Section

• Catalyst circulation is generally controlled by two slide valves

controlled by two main regulators.

• The valve regulating the flow rate of regenerated catalyst that is

fed into the reactor is controlled by reactor outlet temperature.

• The valve regulating the flow rate of spent catalyst to the

regenerator is controlled by the catalyst level in the stripper.

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Control Systems

In all modern FCC plants, the reactor consists of several

component parts, each with a very distinct function.

✓ The actual reactor is in fact a usually vertical pipe (riser), whose

internal diameter is approximately 1 m. At the riser foot, very

hot catalyst (680 to 750 °C) returning from the regenerator is

mixed with the liquid feed that has been finely atomized by

injectors (typically, the ratio of catalyst/feed mass flow rates

is 5 to 6). As a result, the feed is vaporized and cracked,

causing a sudden expansion in volume which accelerates the

mixture to superficial velocity close to 15-20 m/s. 25

Reactor and Regenerator

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Reaction and catalyst handling section

✓ The residence time of the hydrocarbons in the riser is

approximately 2 s and the reaction temperature at the top of

the reactor is kept in 500-530 °C range. The riser is connected

to the disengager, where solids and the gases are separated.

✓ The top of the riser generally features a primary reaction

product-versus-catalyst separation system. Separation must be

as efficient as possible, because post-riser cracking is

detrimental to reaction product quality and yields.

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Reactor and Regenerator

✓ The gaseous products are then routed to a cyclone separator

system for final separation of entrained catalyst fines by

centrifuging. The vapor coming out of the cyclones is then sent

to the primary fractionation column while the recovered solids

are sent to the catalyst stripper.

✓ The catalyst coming from the primary separation system and

from the cyclones then flows into the stripper where the catalyst

residence time is approximately 1-2 min.

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Reactor and Regenerator

✓ Here steam contacts the spent catalyst counter-currently in

order to desorb and recover the hydrocarbons entrained by the

catalyst.

✓ Stripping efficiency is very important, because any unrecovered

hydrocarbons will subsequently be burned as coke in the

regenerator. This will only raise the regenerator temperature

needlessly and cause a loss in yield. The stripped catalyst is

then sent into the regenerator under level control.

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Reactor and Regenerator

✓ In the regenerator, the air required for coke combustion is

carefully distributed. The more coke laid down on the catalyst

during the reaction, the higher the regenerator equilibrium

temperature.

✓ Most present-day FCC plants work on the basis of total coke

combustion, with the air flow rate regulated so that excess

oxygen from 0.5 to 2.0 mol% is present in the flue gases.

✓ The catalyst generally remains less than 10 min in the

regenerator and returns to the reactor with a residual carbon

content of less than 0.1%.30

Reactor and Regenerator

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