UNIT II – CRACKING - 9

Cracking, Thermal Cracking, Vis-breaking, Catalytic Cracking (FCC), Hydro Cracking,

Coking and Air Blowing of Bitumen

I, Cracking

Cracking heavy petroleum cuts is a process in petroleum refining that breaks down heavy hydrocarbon
molecules into lighter molecules. This process is important for producing gasoline and diesel fuel.

Cracking process Description

Catalytic cracking Petroleum vapor passes through a catalyst bed, which breaks down heavier
fractions into lighter, more valuable products.

Fluid catalytic Heavy gas oil is heated to a high temperature and pressure, then comes into
cracking contact with a hot catalyst, which breaks down the long-chain molecules into
shorter ones.

Hydrocracking Heavy petroleum cuts are cracked in the presence of hydrogen and a catalyst at
high pressure and temperature to produce gasoline, jet fuel, and light gas oils.

Some things to know about cracking heavy petroleum cuts include:

 Temperature and time - These are two of the most important factors in the thermal evolution of
organic materials.
 Exposure to magnetic induction - Exposing heavy oils to magnetic induction before cracking can
improve the cracking yield and degradation rate.
 By-products – Cracking produces by-product gases that are more economically valuable than those
produced by thermal cracking.

Cracking Methodologies
1. Thermal methods
a. Thermal cracking
b. Steam cracking
2. Catalytic methods
a. Fluid Catalytic cracking
b. Hydrocracking

Reactions
A large number of chemical reactions take place during the cracking process, most of them based on free
radicals. The main reactions that take place include

Initiation
In these reactions a single molecule breaks apart into two free radicals. Only a small fraction of the feed
molecules actually undergo initiation, but these reactions are necessary to produce the free radicals that drive
the rest of the reactions. In steam cracking, initiation usually involves breaking a chemical bond between
two carbon atoms, rather than the bond between a carbon and a hydrogen atom.
 CH3CH3 → 2 CH3•

Hydrogen abstraction
In these reactions a free radical removes a hydrogen atom from another molecule, turning the second
molecule into a free radical.
 CH3• + CH3CH3 → CH4 + CH3CH2•

Radical decomposition
In these reactions a free radical breaks apart into two molecules, one an alkene, the other a free radical.
This is the process that results in alkene products.
 CH3CH2• → CH2=CH2 + H•

Radical addition
In these reactions, the reverse of radical decomposition reactions, a radical reacts with an alkene to form
a single, larger free radical. These processes are involved in forming the aromatic products that result when
heavier feedstocks are used.
 CH3CH2• + CH2=CH2 → CH3CH2CH2CH2•

II, Thermal cracking

Thermal cracking is a process that uses heat and pressure to break down long-chained hydrocarbons into
shorter-chained hydrocarbons. It can be used to convert low-value feedstock into lighter products, such as
gasoline, biodiesel, and liquefied petroleum gas (LPG).

Here are some details about thermal cracking:

 History - William Merriam Burton, a chemist for Standard Oil Company (Indiana), invented thermal
cracking in 1913.
 Process - Thermal cracking involves heating organic substances in the absence of oxygen. The
process is governed by free radicals, which are reactive species with unpaired electrons but no
electronic charge.
 Sub-categories - Thermal cracking can be classified into three sub-categories based on operating
conditions: flash pyrolysis, conventional pyrolysis, and fast pyrolysis.
 By-products - The earliest process involved heating heavier oils in pressurized reactors, which
yielded gaseous by-products that were initially used as illuminating gas or fuel.
 Yield - Thermal cracking favors alpha-olefins and also produces large amounts of methane, ethane,
and propane.
The fluid catalytic cracking process (FCC) is one of the most important units for a refiner focused on
gasoline production. Refineries can cash in on the benefits of opportunity crudes and maximize profitability
by upgrading bottoms product to produce more - higher octane gasoline compared to basic thermal cracking.

The three main steps in fluid catalytic cracking FCC in the right order are:

Catalytic cracking consists of three major processes namely Reaction, Regeneration, and Fractionation. As
depicted in Fig. 4.10 at the reactor's entrance (referred to as the riser), a fluidized-bed (or fluid-bed) of
catalyst particles is brought into contact with the gas oil feed and injected steam.

III, VISBREAKING

Visbreaking is essentially a mild thermal cracking operation at mild conditions where in long chain
molecules in heavy feed stocks are broken into short molecules thereby leading to a viscosity reduction of
feedstock. Now all the new visbreaker units are of the soaker type. Soaker drum utilizes a soaker drum in
conjunction with a fired heater to achieve conversion.

Visbreaking is a non-catalytic thermal process. It reduces the viscosity and pour point of heavy
petroleum fractions so that product can be sold as fuel oil. It gives 80 - 85% yield of fuel oil and balance
recovered as light and middle distillates. The unit produces gas, naphtha, heavy naphtha, visbreaker gas oil,
visbreaker fuel oil (a mixture of visbreaker gas oil and vsibreaker tar).

A given conversion in visbreaker can be achieved by two ways:

1. High temp., low residence time cracking: Coil visbreaking.
2. Low temp., high residence time cracking: Soaker visbreaking.

Coil cracking uses higher furnace outlet temperatures [885–930°F(473–500°C)] and reaction times
from one to three minutes, while soaker cracking uses lower furnace outlet temperatures [800–830°F (427–
443°C)] and longer reaction times. The product yields and properties are similar, but the soaker operation
with its lower furnace outlet temperatures has the advantages of lower energy consumption and longer run
times

The feed is introduced into the furnace and heated to the desired temperature. In the furnace or coil
cracking process the feed is heated to cracking temperature (474– 500°C) and quenched as it exits the urnace
with gas oil or tower bottoms to stop the cracking reaction. In the soaker cracking operation, the feed leaves
the furnace between 800 and 820°F (427–438°C) and passes through a soaking drum, which provides the
additional reaction time, before it is quenched. Pressure is an important design and operating parameter with
units being designed for pressures as high as 750 psig (5170 kPa) for liquid-phase visbreaking and as low as
100–300 psig (690– 2070 kPa) for 20–40% vaporization at the furnace outlet. For furnace cracking, fuel
consumption accounts for about 80% of the operating cost with a net fuel consumption equivalent of 1–1.5
wt% on feed. Fuel requirements for soaker visbreaking are about 30–35% lower. Many of the properties of
the products of visbreaking vary with conversion and the characteristics of the feedstocks.
However, some properties, such as diesel index and octane number, are more closely related to feed
qualities; and others, such as density and viscosity of the gas oil, are relatively independent of both
conversion and feedstock characteristics.

IV, Fluid catalytic cracking (FCC),

A type of secondary unit operation, is primarily used in producing additional gasoline in the refining
process.

Unlike atmospheric distillation and vacuum distillation, which are physical separation processes,
fluid catalytic cracking is a chemical process that uses a catalyst to create new, smaller molecules from
larger molecules to make gasoline and distillate fuels.

The catalyst is a solid sand-like material that is made fluid by the hot vapor and liquid fed into the
FCC (much as water makes sand into quicksand). Because the catalyst is fluid, it can circulate around the
FCC, moving between reactor and regenerator vessels (see photo). The FCC uses the catalyst and heat to
break apart the large molecules of gas oil into the smaller molecules that make up gasoline, distillate, and
other higher-value products like butane and propane.
 After the gas oil is cracked through contact with the catalyst, the resulting effluent is processed in
fractionators, which separate the effluent based on various boiling points into several intermediate products,
including butane and lighter hydrocarbons, gasoline, light gas oil, heavy gas oil, and clarified slurry oil.

The butane and lighter hydrocarbons are processed further to separate them into fuel gas (mostly
methane and ethane), propane, propylene, butane, and butene for sale, or for further processing or use. The
FCC gasoline must be desulfurized and reformed before it can be blended into finished gasoline; the light
gas oil is desulfurized before blending into finished heating oil or diesel; and the heavy gas oil is further
cracked in either a hydrocracker (using hydrogen and a catalyst) or a coker. The slurry oil can be blended
with residual fuel oil or further processed in the coker.

Carbon is deposited on the catalyst during the cracking process. This carbon, known as catalyst coke,
adheres to the catalyst, reducing its ability to crack the oil. The coke on the spent catalyst is burned off,
which reheats the catalyst to add heat to the FCC process. Regeneration produces a flue gas that passes
through environmental control equipment and then is discharged into the atmosphere.

Catalyst is used in the FCC unit

A modern FCC catalyst has four major components: crystalline zeolite, matrix, binder, and filler.
Zeolite is the active component and can comprise from about 15% to 50%, by weight, of the catalyst.
Faujasite (aka Type Y) is the zeolite used in FCC units.

Metals are in the FCC catalyst

Other metals such as iron, copper, calcium and magnesium are also present in FCC feeds, however
nickel, vanadium and sodium are usually present in much higher concentrations. The main effects of these
three contaminants are dehydrogenation, coke formation and catalyst deactivation.

Advantage of FCC

One of the biggest advantages of the FCC process is the flexibility to process all kinds of streams
that are normally sent as complex mixtures, where the mixing processes are not always efficient enough to
ensure a completely homogeneous blend.
V, HYDROCRACKING

Although hydrogenation is one of the oldest catalytic processes used in refining petroleum, only in recent
years has catalytic hydrocracking developed to any great extent in this country. This interest in the use of
hydrocracking has been caused by several factors, including

(1) the demand for petroleum products has shifted to high ratios of gasoline and jet fuel compared with the
usages of diesel fuel and home heating oils,
(2) by-product hydrogen at low cost and in large amounts has become available from catalytic reforming
operations, and
(3) Environmental concerns limiting sulfur and aromatic compound concentrations in motor fuels have
increased.

The fresh feed is mixed with makeup hydrogen and recycle gas (high in hydrogen content) passed
through a heater to the first reactor. If the feed has not been hydrotreated, there is a guard reactor before the
first hydrocracking reactor. The guard reactor usually has a modified hydrotreating catalyst such as cobalt-
molybdenum on silica-alumina to convert organic sulfur and nitrogen compounds to hydrogen sulfide,
ammonia, and hydrocarbons to protect the precious metals catalyst in the following reactors. The
hydrocracking reactor(s) is operated at a sufficiently high temperature to convert 40 to 50 vol% of the
reactor effluent to material boiling below 400°F (205°C). The reactor effluent goes through heat exchangers
to a high-pressure separator where the hydrogen-rich gases are separated and recycled to the first stage for
mixing both makeup hydrogen and fresh feed. The liquid product from the separator is sent to a distillation
column where the C4 and lighter gases are taken off overhead, and the light and heavy naphtha, jet fuel, and
diesel fuel boiling range streams are removed as liquid sidestreams. The fractionators bottoms are used as
feed to the second-stage reactor system. The unit can be operated to produce all gasoline and lighter
products or to maximize jet fuel or diesel fuel products.

The bottoms stream from the fractionator is mixed with recycle hydrogen from the second stage and
sent through a furnace to the second-stage reactor. Here the temperature is maintained to bring the total
conversion of the unconverted oil from the first-stage and second-stage recycle to 50 to 70 vol% per pass.
The second-stage product is combined with the first-stage product prior to fractionation.

Both the first- and second-stage reactors contain several beds of catalysts. The major reason for
having separate beds is to provide locations for injecting cold recycled hydrogen into the reactors for
temperature control. In addition, redistribution of the feed and hydrogen between the beds helps to maintain
a more uniform utilization of the catalyst.

When operating hydrocrackers for total conversion of distillate feeds to gasoline, the butane-and-
heavier liquid yields are generally from 120 to 125 vol% of fresh feed.
Hydrocracking Catalyst
There are a number of hydrocracking catalysts available and the actual composition is tailored to the
process, feed material, and the products desired. Most of the hydrocracking catalysts consist of a crystalline
mixture of silica-alumina with a small uniformly distributed amount of rare earths contained within the
crystalline lattice. The silica-alumina portion of the catalyst provides cracking activity while the rare-earth
metals promote hydrogenation. Catalyst activity decreases with use, and reactor temperatures are raised
during a run to increase reaction rate and maintain conversion.

Process Variables
The primary reaction variables are reactor temperature and pressure, space velocity, hydrogen
consumption, nitrogen content of feed, and hydrogen sulphide content of the gases.

VI, Coking and Air Blowing of Bitumen

The industrial production of coke from coal is called coking. The coal is baked in an airless kiln, a
"coke furnace" or "coking oven", at temperatures as high as 2,000 °C (3,600 °F) but usually around 1,000–
1,100 °C (1,800–2,000 °F).

Coking is a process that involves heating coal or crude oil to produce a porous, carbon-rich material called
coke:

 Coal coking

Coal is heated in the absence of oxygen to temperatures above 600 °C, driving off volatile components and
leaving behind coke. Coke is a hard, strong, porous material with a high carbon content. It's used in the
electric power and industrial sectors.

 Petroleum coking
Crude oil is heated in a refinery to treat the bottom of the oil, or vacuum residue. This process involves
rejecting carbon as solid coke, converting the residue to gas, distillates, and coke, and maximizing the yield
of distillate products.

Coking can also refer to:

 A physical process that occurs in refineries at temperatures greater than 900°F and pressures slightly
higher than atmospheric.

 Two types of coking processes: delayed coking and fluid coking.

Use of coking

 To make steel in a blast furnace, coal must first be turned into coke. Coke has a dual role in the
steelmaking process. First, it provides the heat needed to melt the ore, and second, when it is burnt, it
has the effect of 'stealing' the oxygen from the iron ore, leaving only the pure iron behind.

What are 5 uses of coke?

The following are the uses of coke:

 It is used to reduce iron oxide to produce iron.

 It can be used in households as a clean fuel, relatively free of smoke and impurities.

 In the past, it was often used in kitchen stoves.

 Coke is also used to produce water gas.

 It is used as a reducing agent in smelting iron ore.

Two types of coking processes exist—delayed coking and fluid coking. Both are physical processes that
occur at pressures slightly higher than atmospheric and at temperatures greater than 900 oF that thermally
crack the feedstock into products such as naphtha and distillate, leaving behind petroleum coke.

Delayed coking and fluid coking are both refinery processes that use heat and cracking to produce petroleum
coke and other products:

 Delayed coking

Uses multiple large reactors, called coke drums, to hold and crack the feedstock. The coke is deposited as a
solid in the coke drums, and is removed by hydraulic cutting.

 Fluid coking

Uses a reactor and a burner to circulate coke particles and transfer heat to the reactor. The coke is formed as
a fluidized solid, and some of it is burned to heat the reactor. The remaining coke is collected as a fluid coke
product.

Here are some differences between the two processes:

 Coke yield

Fluid coking produces less coke and more liquid products than delayed coking.

 Feedstock
Delayed coking is often used to process heavy crudes. Fluid coking can process heavier VDR.

 Coke type

Petroleum coke produced by delayed coking can be used as filler for manufacturing anodes for the
electrolysis of alumina.

 Coke structure

Fluid coke has an onion skin structure, while delayed coke is a sponge coke.

Coking is an important part of refinery economics, especially as the quality of crude oil declines and demand
for transportation fuels increases.

Air-blowing is a process that uses heated air to increase the stiffness of vacuum bottom (VB) bitumen. The
process involves:

 Blowing air- Air is blown through the bitumen at high temperatures using an air distributor at the
bottom of a column.

 Oxidation - The bitumen consumes oxygen as the air rises through it, oxidizing light compounds in
the VB.

 Mixing - The air also agitates and mixes the bitumen, increasing its surface area and the rate of
reaction.

 Adding steam and water - Steam is sprayed into the vapor space above the bitumen to suppress
foaming and dilute the oxygen content of waste gases. Water is sprayed in to cool the vapors and
prevent after-burning.

 Collecting - The blown bitumen flows into a surge drum, then through heat exchangers, and finally
to storage.

Air-blowing is a time-consuming process that can be sped up by using solid catalysts like FeCl. The process
can also be modified with additives like acidic compounds to promote intermolecular interactions.
The resulting blown bitumen, also known as oxidized bitumen or blown asphalt, has a higher penetration
index and altered chemical structure than regular bitumen. These properties make it ideal for waterproofing
and roofing, as well as other applications where these properties are important.