1 

| P a g e  
 

Module 3: Liquid Fossil Fuel (Petroleum)

Lecture 22: Secondary Processing

IIT Kharagpur NPTEL Phase – II Web Courses 
 
2 | P a g e  
 
Keywords: FCC, RFCC, catalyst regeneration, feed atomisation

3.4.1.3 Catalytic cracking

Introduction

Catalytic cracking is the process where low-valued high boiling feedstocks are cracked into

value-added products by using a catalyst. Previously there were two types of catalytic cracking

practiced in the refinery, moving bed catalytic cracking and fluid bed catalytic cracking. But

eventually, fluid bed catalytic cracking proved to be advantageous and superior over moving bed

and hence the later one is no longer used in the refinery. Fluid Catalytic Cracking (FCC) and

Resid Fluid Catalytic Cracking (RFCC) are the two important cracking operations used in the

petroleum industry where high boiling gas oils (>3500C) and residues are converted into high

octane gasoline, light distillate oil and liquefied petroleum gas. FCC is a continuous process

where atomized feedstock is brought in contact with fluidized catalyst in a reactor. The heavy

feedstock is cracked to lighter products at temperature above 5000C and the catalyst is

deactivated. The products are separated from the catalyst and then the catalyst is sent to a

regenerator. In regenerator, the residual carbon on the surface of the catalyst powder is oxidized

at a high temperature (over 6000C) and regenerated. Regenerated catalyst is recycled to the

reactor.

In the early 1960’s, M.G.Kellogg and Phillips Petroleum began the development of residual oil

cracking. It was clear that high carbon residue of these feedstocks would result in high coke yield

compared to gas oil unit at the same conversion. Moreover, these were contaminated with high

metal content such as nickel and vanadium, which resulted in deposition of metal on catalyst

which in turn increased the yield of light gases, especially hydrogen. Residual oil cracking got

IIT Kharagpur NPTEL Phase – II Web Courses 
 
3 | P a g e  
 
significance when crude oil price was increased and availability of light crude was decreased, a

demand for producing light transportation fuel was grown high, at the end of 1970’s. In response

to this pressure, two new residual oil cracking technologies- the ‘UOP residual oil cracking

technology’ and the ‘Total catalytic cracking technology’ were developed to compete with heavy

oil cracking. Both of these technologies cope with the problem of excess coke production by

using two stage regeneration. Depending on the level of contamination by metal and alphaltene,

the residual oil feeds may be deasphalted or hydrotreated.

The common feedstocks of modern catalytic cracking are: gas oils from different origin (such as,

atmospheric column, vacuum column, coker unit and thermal cracker), solvent deasphalted oil,

lube-oil extract and bottom product of hydrocracker fractionators. FCC catalysts are mainly high

silica to alumina Y-zeolites, ion exchanged by either rare-earth element or ammonium-ion. The

catalyst is a fine solid having particle diameter in the range of 60-70 m and at this size, the

catalyst particles are easily fluidized by hydrocarbon vapours in the riser reactor. The catalyst

particles are of high attrition resistance to prevent the fracture due to high speed vapour and

steam jets.

Process description

There are four major sections in the FCC plant: converter, flue gas train, main fractionator and

vapour recovery section. Converter can be subdivided into riser reactor, catalyst stripper and

regenerator. Fig. 1 shows the FCC converter. Cracking reaction occurs in the riser reactor. In this

part, preheated feedstock along with recycled feed is atomised at the bottom of the reactor. When

the atomised feed comes in contact with the hot catalyst powder, it vapourises and cracks into

smaller molecular weight compounds. The feed vapour carries the catalyst powder up to the riser

IIT Kharagpur NPTEL Phase – II Web Courses 
 
4 | P a g e  
 
as a dilute suspension. With the progress of the reaction, coke is continuously deposited on the

catalyst powder and by this way, the catalyst loses it activity very soon. The feed atomizer is a

conical shaped piece consisting of a number of flat fan-shaped spray nozzles. These spray

nozzles atomize the feed in such a way that uniform coverage, to prevent bypassing of the

catalyst powder, and maximum penetration of the feed into the catalyst particle occur.

Fig. 1. Fluidised catalytic cracking unit

The catalyst after getting deactivated by the coke deposition is separated from the reaction

products by cyclone separators located in the disengager, at the end of the riser. The separation

of catalyst particle and reaction mixture stops the cracking reaction automatically. Product

vapours after separation flow through the disengager overhead line to the vapour recovery

section. In this section, product vapours are quenched and sent to the main fractionator. The

IIT Kharagpur NPTEL Phase – II Web Courses 
 
5 | P a g e  
 
product vapours are not allowed to stay long in the disengager line, as at this place thermal

cracking of the products may occur to produce low valued gases. The spent catalyst after getting

separated from the products is sent to a two-stage steam stripper. Steam strips off the

hydrocarbon contamination of the catalyst particles from the surface and within. This way, steam

stripping minimizes the hydrocarbon carry over by the catalyst particles to the regenerator, which

in turn imparts the advantages of lower regeneration temperature and lesser probability of

catalyst deactivation due to sintering at high temperature. Hence, catalyst regeneration should be

done at such an environment which can retain the catalyst activity and selectivity to a maximum

possible extent.

After steam stripping, the coked catalyst particles are sent to regenerator where coke is removed

by oxidation with air at high temperature. Air is supplied by high volume air blower which

discharges air at low pressure. Air is passed counter-current to the catalyst. Spent catalyst is

charged at the top of the regenerator and distributed uniformly. Air is introduced from the

bottom. Combustion of coke produces mainly carbon dioxide, although some amount of carbon

monoxide can also be expected due to incomplete combustion. The carbon monoxide content of

flue gas is maintained below 200 ppm. The amount of coke deposition on the catalyst depends on

the type of feedstock charged to FCC. The lighter feed produces lesser coke while heavy feed

deposits higher amount of coke on catalyst. Moreover, the feed containing asphaltene and metals

is responsible to produce more coke. Asphaltene is a precursor of coke production and metals

catalyse dehydrogenation reaction of the feed compounds. Both of these aggravate the

production of extra coke on the catalyst surface. This situation occurs in RFCC unit.

After regeneration, the catalyst powders are cooled to the reactor temperature by the use of a

catalyst cooler. High pressure steam is generated here. The heat from the flue gas obtained from

IIT Kharagpur NPTEL Phase – II Web Courses 
 
6 | P a g e  
 
the regenerator is recovered by a flue gas expander in the form of shaft work, i.e, mechanical

energy and also by generating superheated steam. After heat removal, the flue gas is sent to a

scrubbing unit and/or an electrostatic precipitator to remove SOx and particulate matter as per

environmental laws.

The product vapours obtained from the disengager is condensed and the condensed liquid is

separated into different cuts in the main fractionator. The products obtained from the fractionator

are wet gas and unstabilised gasoline as the top product, light cycle oil as middle product and a

heavy bottom product. The cycle oil is striped to remove light ends. Unstabilised gasoline is sent

to a debutanizer to remove C4 cuts and thus the gasoline is stabilized.

The last unit of the whole operation is vapour recovery section, where C2 to C4 cuts are

recovered from wet gas and gasoline. C3 and C4 cuts are utilized as LPG components. Sometimes

a depropaniser is used to separate propane and propylene and a gasoline splitter is used to

separate light and heavy gasoline according to the requirement.

IIT Kharagpur NPTEL Phase – II Web Courses 
 
7 | P a g e  
 
Reference

1. Modern petroleum technology, Downstream, ed. by Alan G, Lucas, Vol 2, 6th edition, IP, John

Wiley & Sons Ltd., 2001.

2. Fluid Catalytic Cracking Technology and Operations, Joseph W. Wilson, Pennwell Books,

1997.

3. Fluid Catalytic Cracking Handbook, R. Sadeghbeigi, 3rd Edition, Elsevier, 2012.

IIT Kharagpur NPTEL Phase – II Web Courses