Cracking Term Paper

8/8/2019 Cracking Term Paper

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CRACKING

October 12

2009Cracking is one of the most important processes in crude refining. It hasled to a rise in production of gasoline to fulfill the ever rising energy

demands. Here is a study dealing with the concepts of cracking & the story

of its development.

KrittikaRoll No.

07101016


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CRACKING

Introduction

Cracking is a petroleum refining process in which heavy-molecular weight hydrocarbons are

broken up into light hydrocarbon molecules by the application of heat and pressure, with or

without the use of catalysts, to derive a variety of fuel products. Cracking is one of the principal

ways in which crude oil is converted into useful fuels such as motor gasoline, jet fuel, and home

heating oil.

The early refiners discarded the low boiling fractions, the part we now call gasoline, and

recovered the more valuable, higher boiling, kerosene for lighting and heating. But the internal

combustion engine required a lower boiling fuel than kerosene because the fuel had to vaporize

in the engine cylinder, and electrical service replaced much of the demand for kerosene.

So turn of the last century chemists developed the refinery practice of "cracking"; heating the

crude to very high temperatures in the absence of air. They found they could produce additional

amounts of the gasoline fractions by what seemed to be a process of rupturing chemical bonds to

get smaller molecules.

In 1855, petroleum cracking methods were pioneered by American chemistry professor,

Benjamin Silliman, Jr., of Sheffield Scientific School (SSS) at Yale University.

The first thermal cracking method, the Shukhov cracking process, was invented by Russian

engineer Vladimir Shukhov, in the Russian empire, Patent No. 12926, November 27, 1891.

Eugene Houdry, a French mechanical engineer, pioneered catalytic cracking and developed the

first commercially successful process after immigrating to the United States. The firstcommercial plant was built in 1936. His process doubled the amount of gasoline that could be

produced from a barrel of crude oil.

This paper is an attempt to compile and present the various methods and practices applied to

cracking of hydrocarbons.

Chemistry ofCracking

Before we begin with the processes involved in cracking, let us first have a look at the reactionswhich lead to cracking & can hence justify its affectivity in producing lower molecular weight

hydrocarbons.

We know from our thermo chemistry and thermodynamics that we can determine a standardenthalpy and entropy of formation for any substance. These standards represent the properties of

formation of the substance, from the elements. And the higher the standard enthalpy, the less


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stable, more energetic the species is. Standard enthalpy and entropy are state property, unaffectedby the pathway by which it was generated.

Why Cracking Works - Why Hexane Cracks to Lower Molecular Weight Fragments: Letus propose an equation for hexane behavior on heating. Suppose hexane breakdown to give two

products, propane and propene. The balanced equation is:

C6H14(Hexane) --> C3H8 (Propane)+ C3H6(Propene)

H0 kcal.mol S0 cal/mol*K

n-hexane -167 +389

propane -104 +270

propene +21 +267

Enthalpy for the reaction:

(Change in enthalpy, products minus reactants)

H0 (propane+propene) - H0 n-hexane = H

-83kcal/mol - (-167kcal/mol) = 84kcal/mol

Entropy for the reaction:

(Change in entropy, products minus reactants)

S0 (propane+propene) - S0 n-hexane = S

537cal/mol*K - 389cal/mol*K = 148cal/mol*K

Free Energy:

G = H - T S

At standard temperature:

G0 = 84kcal - (298K)(148cal/K) = 44kcal

So at room temperature, the reaction is not spontaneous.

Let us lookhigher temperatures, let us say 400 degrees Celsius, where we might expect thecracking to occur, we find:

G0 = 84kcal - (673K)(148cal/K) = -16kcal

So the process of splitting at 400 digress Celsius is spontaneous.

A process can be non spontaneous at a low temperature because it requires heat.


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The same process can reach a temperature at which it will be spontaneous. Theendothermic heat properties are overcome by the increased entropy of the products. In

this reaction, we are making two molecules and that factor of increased disorder allowsthe cracking process to occur.

Types ofCracking

A. ThermalCracking

Thermal cracking is a refining process in which heat (~800C) and pressure (~700kPa) are usedto break down, rearrange, or combine hydrocarbon molecules. The first thermal cracking process

was developed around 1913. Distillate fuels and heavy oils were heated under pressure in largedrums until they cracked into smaller molecules with better antiknock characteristics. However,

this method produced large amounts of solid, unwanted coke. This early process has evolved into

the following applications of thermal cracking: vis breaking, steam cracking, and coking.

Depending upon the pressure & temperature employed and the characteristics of the feed,

thermal cracking is classifies as

LOW TEMPERATURE & HIGH PRESSURE PROCESS

The feed is residue from atmospheric distillation unit. Products are mainly gas, fuel oil and

gasoline. Temperature and pressure employed are 500C & 20 atm resp. This process is alsocalled vis breaking when heavy fuel oil is thermally cracked to reduce its viscosity so that it can

be properly atomized through the burners.

HIGH TEMPERATURE & HIGH PRESSURE PROCESS

This is also called the light oil thermal cracking process. When feed is naphtha, it is calledthermal reforming. The feed is of gas oil and products are gas and gasoline. Temperature is about

530C and pressure is 50 to 70 atm.

LOW PRESSURE & HIGH TEMPERATURE PROCESS

Feed- A residue from atmospheric distillation unit

T & P- >550C and 2-5 atm

Products- gasoline and gas rich in unsaturated hydrocarbons

HIGH TEMPERATURE & LOW PRESSURE PROCESS

It is also called as pyrolysis. Pressure and temperature employed are 1 atm and 700C. Products

are mainly gas rich in aromatic and unsaturated hydrocarbons.


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REACTIONS INVOLVED

Initiation reactions, where a single molecule breaks apart into two free radicals. Only a smallfraction 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 acarbon and a hydrogen atom.

CH3CH3 2 CH3

Hydrogen abstraction, where a free radical removes a hydrogen atom from another molecule,

turning the second molecule into a free radical.

CH3 + CH3CH3 CH4 + CH3CH2

Radical decomposition, where a free radical breaks apart into two molecules, one an alkene, the

other a free radical. This is the process that results in the alkene products of steam cracking.

CH3CH2 CH2=CH2 + H

Radical addition, the reverse of radical decomposition, in which a radical reacts with an alkeneto 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

Termination reactions, which happen when two free radicals react with each other to produceproducts that are not free radicals. Two common forms of termination are recombination, where

the two radicals combine to form one larger molecule, and disproportionation, where one radicaltransfers a hydrogen atom to the other, giving an alkene and an alkane.

CH3 + CH3CH2 CH3CH2CH3

CH3CH2 + CH3CH2 CH2=CH2 + CH3CH3

Thermal cracking is an example of a reaction whose energetics are dominated by entropy (S)rather than by enthalpy (H) in the Gibbs Free Energy equation G=H-TS. Although thebond dissociation energy D for a carbon-carbon single bond is relatively high (about 375 kJ/mol)

and cracking is highly endothermic, the large positive entropy change resulting from the

fragmentation of one large molecule into several smaller pieces, together with the extremely hightemperature, makes TS term larger than the H term, thereby favoring the cracking reaction.


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Visbreaking Process

Vis breaking, a mild form of thermal cracking, significantly lowers the viscosity of heavy crude-oil residue without affecting the boiling point range. Residual from the atmospheric distillation

tower is heated (800-950 F) at atmospheric pressure and mildly cracked in a heater (Figure 2).It is then quenched with cool gas oil to control over cracking, and flashed in a distillation tower.

Vis breaking is used to reduce the pour point of waxy residues and reduce the viscosity ofresidues used for blending with lighter fuel oils. Middle distillates may also be produced,

depending on product demand. The thermally cracked residue tar, which accumulates in thebottom of the fractionation tower, is vacuum flashed in a stripper and the distillate recycled.

Coking Processes

Coking is a severe method of thermal cracking used to upgrade heavy residuals into lighter

products or distillates. Coking produces straight-run gasoline (coker naphtha) and variousmiddle-distillate fractions used as catalytic cracking feedstock. The process so completely

reduces hydrogen that the residue is a form of carbon called "coke." The two most commonprocesses are delayed coking and continuous (contact or fluid) coking. Three typical types of

coke are obtained (sponge coke, honeycomb coke, and needle coke) depending upon the reactionmechanism, time, temperature, and the crude feedstock.


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Delayed Coking

Delayed Coking

In delayed coking the heated charge (typically residuum from atmospheric distillation towers) istransferred to large coke drums which provide the long residence time needed to allow thecracking reactions to proceed to completion. Initially the heavy feedstock is fed to a furnace

which heats the residuum to high temperatures (900-950 F) at low pressures (25-30 psi) and isdesigned and controlled to prevent premature coking in the heater tubes (Figure 4). The mixture

is passed from the heater to one or more coker drums where the hot material is heldapproximately 24 hours (delayed) at pressures of 25-75 psi, until it cracks into lighter products.

Vapors from the drums are returned to a fractionator where gas, naphtha, and gas oils areseparated out. The heavier hydrocarbons produced in the fractionator are recycled through the

furnace.

After the coke reaches a predetermined level in one drum, the flow is diverted to another drum tomaintain continuous operation. The full drum is steamed to strip out uncracked hydrocarbons,

cooled by water injection, and decoked by mechanical or hydraulic methods. The coke ismechanically removed by an auger rising from the bottom of the drum. Hydraulic decoking

consists of fracturing the coke bed with high-pressure water ejected from a rotating cutter.

Continuous Coking

Continuous (contact or fluid) coking is a moving-bed process that operates at temperatures

higher than delayed coking. In continuous coking, thermal cracking occurs by using heattransferred from hot, recycled coke particles to feedstock in a radial mixer, called a reactor, at apressure of 50 psi. Gases and vapors are taken from the reactor, quenched to stop any further

reaction, and fractionated. The reacted coke enters a surge drum and is lifted to a feeder andclassifier where the larger coke particles are removed as product. The remaining coke is dropped

into the preheater for recycling with feedstock. Coking occurs both in the reactor and in the surgedrum. The process is automatic in that there is a continuous flow of coke and feedstock.


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B. Catalytic Cracking

Catalytic cracking breaks complex hydrocarbons into simpler molecules in order to increase thequality and quantity of lighter, more desirable products and decrease the amount of residuals.

This process rearranges the molecular structure of hydrocarbon compounds to convert heavy

hydrocarbon feedstock into lighter fractions such as kerosene, gasoline, liquefied petroleum gas(LPG), heating oil, and petrochemical feedstock .

Catalytic cracking is similar to thermal cracking except that catalysts facilitate the conversion ofthe heavier molecules into lighter products. Use of a catalyst in the cracking reaction increases

the yield of improved-quality products under much less severe operating conditions than inthermal cracking. Typical temperatures are from 850-950 F at much lower pressures of 10-20

psi. The catalysts used in refinery cracking units are typically solid materials (zeolite, aluminumhydro-silicate, treated bentonite clay, fuller's earth, bauxite, and silica-alumina) that come in the

form of powders, beads, pellets or shaped materials called extrudites.

There are three basic functions in the catalytic cracking process:

y Reaction: Feedstock reacts with catalyst and cracks into different hydrocarbons;y Regeneration: Catalyst is reactivated by burning off coke; andy Fractionation: Cracked hydrocarbon stream is separated into various products.

The three types of catalytic cracking processes are fluid catalytic cracking (FCC), moving-bed

catalytic cracking, and Thermofor catalytic cracking (TCC). The catalytic cracking process isvery flexible, and operating parameters can be adjusted to meet changing product demand.

Fluid Catalytic Cracking (FCC)

Fluid catalytic cracking is the basic gasoline-making process. Using intense heat (about 1,000

degrees Fahrenheit), low pressure and a powdered catalyst (a substance that accelerates chemicalreactions), the cat cracker can convert most relatively heavy fractions into smaller gasoline

molecules. The fluid cracker consists of a catalyst section and a fractionating section that operatetogether as an integrated processing unit. The catalyst section contains the reactor and

regenerator, which, with the standpipe and riser, forms the catalyst circulation unit. The fluidcatalyst is continuously circulated between the reactor and the regenerator using air, oil vapors,

and steam as the conveying media.


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A typical FCC process involves mixing a preheated hydrocarbon charge with hot, regeneratedcatalyst as it enters the riser leading to the reactor. The charge is combined with a recycle stream

within the riser, vaporized, and raised to reactor temperature (900-1,000 F) by the hot catalyst.As the mixture travels up the riser, the charge is cracked at 10-30 psi. In the more modern FCC

units, all cracking takes place in the riser. The "reactor" no longer functions as a reactor; itmerely serves as a holding vessel for the cyclones. This cracking continues until the oil vapors

are separated from the catalyst in the reactor cyclones. The resultant product stream (crackedproduct) is then charged to a fractionating column where it is separated into fractions, and some

of the heavy oil is recycled to the riser.

Spent catalyst is regenerated to get rid of coke that collects on the catalyst during the process.Spent catalyst flows through the catalyst stripper to the regenerator, where most of the coke

deposits burn off at the bottom where preheated air and spent catalyst are mixed. Fresh catalyst isadded and worn-out catalyst removed to optimize the cracking process.

Moving Bed Catalytic Cracking

The moving-bed catalytic cracking process is similar to the FCC process. The catalyst is in theform of pellets that are moved continuously to the top of the unit by conveyor or pneumatic lift

tubes to a storage hopper, then flow downward by gravity through the reactor, and finally to aregenerator. The regenerator and hopper are isolated from the reactor by steam seals. The

cracked product is separated into recycle gas, oil, clarified oil, distillate, naphtha, and wet gas.

Thermofor Catalytic Cracking

In a typical thermofor catalytic cracking unit, the preheated feedstock flows by gravity throughthe catalytic reactor bed. The vapors are separated from the catalyst and sent to a fractionating


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tower. The spent catalyst is regenerated, cooled, and recycled. The flue gas from regeneration issent to a carbon monoxide boiler for heat recovery.

C. Hydrocracking

Hydrocracking is a two-stage process combining catalytic cracking and hydrogenation, whereinheavier feed stocks are cracked in the presence of hydrogen to produce more desirable products.The process employs high pressure, high temperature, a catalyst, and hydrogen. Hydrocracking is

used for feed stocks that are difficult to process by either catalytic cracking or reforming, sincethese feed stocks are characterized usually by a high polycyclic aromatic content and/or high

concentrations of the two principal catalyst poisons, sulfur and nitrogen compounds.

The hydrocracking process largely depends on the nature of the feedstock and the relative ratesof the two competing reactions, hydrogenation and cracking. Heavy aromatic feedstock is

converted into lighter products under a wide range of very high pressures (1,000-2,000 psi) andfairly high temperatures (750-1,500 F), in the presence of hydrogen and special catalysts.

When the feedstock has a high paraffinic content, the primary function of hydrogen is to preventthe formation of polycyclic aromatic compounds. Another important role of hydrogen in the

hydrocracking process is to reduce tar formation and prevent buildup of coke on the catalyst.Hydrogenation also serves to convert sulfur and nitrogen compounds present in the feedstock to

hydrogen sulfide and ammonia.

Hydrocracking produces relatively large amounts of isobutane for alkylation feedstock.

Hydrocracking also performs isomerization for pour-point control and smoke-point control, bothof which are important in high-quality jet fuel.

Hydrocracking Process

c

Two-Stage Hydrocracking


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In the first stage, preheated feedstock is mixed with recycled hydrogen and sent to the first-stagereactor, where catalysts convert sulfur and nitrogen compounds to hydrogen sulfide and

ammonia. Limited hydrocracking also occurs.

After the hydrocarbon leaves the first stage, it is cooled and liquefied and run through a

hydrocarbon separator. The hydrogen is recycled to the feedstock. The liquid is charged to afractionator. Depending on the products desired (gasoline components, jet fuel, and gas oil), thefractionator is run to cut out some portion of the first stage reactor out-turn. Kerosene-range

material can be taken as a separate side-draw product or included in the fractionator bottomswith the gas oil.

The fractionator bottoms are again mixed with a hydrogen stream and charged to the second

stage. Since this material has already been subjected to some hydrogenation, cracking, andreforming in the first stage, the operations of the second stage are more severe (higher

temperatures and pressures). Like the out turn of the first stage, the second stage product isseparated from the hydrogen and charged to the fractionator.

PARAMETERS AFFECTING CRACKING

a. Temperature- Higher temperature decreases the residence time of charge stock. Smallchange in temperature can change the depth of cracking to a considerable extent, hence, itis necessary to obtain temperature very accurately. Between 450C to 550C there is no

change in product quality at the same depth of cracking.b. Residence Time-It has a tremendous effect on the quality of products as increased time

leads to more severity of cracking.c. Pressure-The effect of pressure is related to only secondary reactions. Lower pressure

favors the production of gas whereas higher pressure favors condensation and tar

production increases.d. Depth of cracking-Depth of cracking depends upon temperature and residence time.

Greater depth of cracking gives higher octane number of gasoline, formation of gas &

carboids. For heavy charge stocks, carboid formation limits the depth of cracking. Forlight charge stocks, ratio of the gas to gasoline is same up to about 60% of the completion

of the process after which the gas formation is more. This critical yield fixes the depth ofcracking for light charge stocks.

e. Chemical Composition of Feed- This parameter affects the process only in thebeginning. The greater the depth of cracking, the less the influence of chemical

composition. But composition of feed affects the yield of products.

CONCLUSION

From this study we see that cracking is undoubtedly an indispensable process when it comes tocrude oil refining. New discoveries are being made and implemented at refinery level which

improves the yield tremendously. This paper reviewed the basic concepts involved in thecracking process.


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References

1. http://www.eoearth.org/upload/thumb/7/75/Cracking2.gif/199px-Cracking2.gif2. http://wapedia.mobi/en/Thermal_cracking3. www.eoearth.org/article/cracking4. Page 162-176;O P Gupta ; Elements of Fuels, Furnaces & Refractories; Khanna

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