Guide to Lectures

8/2/2019 Guide to Lectures

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CATALYSIS


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What is a catalyst?

A material that is capable of enhancing therate and selectivity of a chemical reaction and

cyclically regenerated in the process


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Terminologies

Homogenous catalyst:

A catalyst in the same phase with the reactants

+ HNO3

NO2

H2SO4, 500

C

Nitration of benzene

Heterogeneous catalyst

Catalyst and reactant are in different phases


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Porous catalyst:Catalyst with large areas resulting from pores

Molecular sieves:Materials with pores that are selective based on molecule sizes.

Catalyst Promoters: Substances the increase the activity of a

catalyst are called promoters

Catalyst support: materials used to enhance catalystefficiency by providing a large surface area.

Terminologies (contd.)


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Active phase : Material responsible for the catalytic reaction

Support: Inert materials that are used to give used to disperse the active

phase yielding high degree of dispersion, mechanical and thermalstabilities.

Catalyst promoters: Substances that changes the activity of a catalyst byelectronically or physically changing the active phase without being acatalyst itself

Textural promoters: these type of promoters facilitate the achievementof well dispersed catalytic phases and ensure that this dispersion is

maintained at reaction conditions. Examples include alumina

Chemical promoters: these materials enhance the activity or selectivityof catalysts. Most of which are alkaline or alkaline earth metals.

Terminologies (contd.)


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Catalyst properties

Turnover frequency: This is the number of times an overall catalytic reactiontakes place per catalytic site, per unit time for a fixed set of reaction conditions.Catalyst activity

=

Turnover number: This is the number of times an overall catalyticreaction takes place per catalytic site for a fixed set of reactionconditions. Catalyst productivity

Deactivation: Decline in catalyst activity with time

Poisoning: The deposition of foreign materials on active catalyst surface


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X

catalyst

Y

catalyst

X Y

catalyst

P

PSeparation

Bonding Reaction

Catalysis Cycle


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Mechanism of Benzene Nitration1. Electrophile formation

HNO3 +2H2SO4 NO2+ + 2HSO4- + H3O

+

2. Electrophilic substitution

3. Catalyst Regeneration and product formation

NO2+ NO2

+H

NO2+H

HSO4-

H2

SO4

NO2


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Arrhenius equationX Y+

kX Y

= []

= []

=

X+Y

XY

r - rate (mol dm-3 s-1)k- rate constant[X][Y] concentration in mol dm-3

Ea- Activation energyR- Gas constantT- temperature in K

A- pre exponential factor

Ea


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Non catalyticreaction

Energybarrier

adsorption desorption

Energy

X

Y

X Y

P

P

Energy profile of a catalytic reaction


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K=

Arrhenius Equation

Catalyst effect on reaction rate has to do with providing a route with a

lower activation energy.QAssume an uncatalysed reaction with an activation energy of

45KJ mol-1 at a temperature of 250C. A catalysts for the samereaction under same conditions provide a route with activationenergy of 20KJ, show the difference K assuming A is constant. R=

8.31 JK-1 mol-1

A- Frequency of collisions and orientation

counts the fraction of molecules possessing energies equal

or more than activation energy at a particular temperature

Suppose Ea = 100JK mol at T = 500KThen at Ea= 50 KJ mol T= 250K!!!!


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STEPS IN A CATALYTIC REACTION

X+Y P

PX+Y

1 7

2 6

X+Y P

4

X+Y P

1

2

3

5

6

7External diffusion

Internal diffusion


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1. Mass transfer from bulk fluid to catalyst surface2. Reactant diffusion from pore mouth to internal catalytic

surface3. Adsorption of reactants onto catalytic surface4. Reaction on catalyst surface5. Products desorption6. Diffusion from interior of the pore surface to catalyst mouth7. Mass transfer from catalyst surface to bulk fluid.

STEPS IN A CATALYTIC REACTION


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Adsorption isotherms

S- Active siteA.S A adsorbed on SC

t

total molar concentration of active sites per unit mass of catalyst (mol/g.cat)Cv- Molar concentration of vacant sitesPi- Partial pressure of i in gas phase (atm)Ci.s- Surface concentration of sites occupied by i (g mol/ g cat)

Assumption : no catalyst deactivation

A+S A.S

A B

= + . + .Site balance

Isotherms show the amount of gas adsorbed by a solid at differentpressures but same temperature


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Molecular / Non dissociated adsorption

+ = .

Rate of attachment of AB to surface is proportional to

Number of molecules collision with surface per second and the numberof vacant sites, while collision rate is proportional to the partial pressureof AB PAB.

=

Rate of detachment of AB molecules to the surface isproportional to concentration of sites occupied by themolecule CAB.S

= .

= .

kA

and k-A

areproportionalityconstants


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=

= .

= + ..

At equilibrium, rate of adsorption is 0

= .

Substitute Cvfrom site balance equation

. = . = .

. =

1 +

LangmuirIsotherm


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.

=1

+


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Langmuir isotherm for adsorption of molecular CO

CCO.S

Mol/gcatalyst

PCO, kPa


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Dissociative adsorption

AB+2S A.S + B.S

.

,

. (. )


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= ..

=

..

=

, = 0

= .. . = .

= .

.

= .


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= + . + .= + 2.sitebalance

= 2.

.

= .

.

2. = .

.

= . + 2. .

. =.

1 + 2 .

.=

1

+

2


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.versus


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PETROLEUM REFININGTECHNOLOGY


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CRUDE OIL

Composition/OrigingMixture of various hydrocarbons

From decaying matterVaries in composition based on originOther compounds in crude oil include Inorganic salts, suspendedsolids, water soluble metals, water

Non organic theories1.

2. Nebular condensation theory (from extra terrestrial origin)

3. Volcanic activities

CaC2 + 2H2O C2H2 + Ca(OH)2


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PRE TREATMENT OF CRUDE OIL

DUE TO IMPURITIES FROM CRUDE OIL ORIGIN AND THOSEGOTTEN DURING TRANSPORTATION, THERE IS NEED TO PRETREAT CRUDE OIL BEFORE IT IS PROCESSED

IMPURITIES CAN CAUSE

CORROSSIONFOULINGCATALYST POISONING


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DESALTINGDESA

CRUDE OIL IS USUALLY PRE TREATED TO REMOVE SALTSBEFORE IT IS SENT TO THE DIFFERENT UNITS OF

A REFINERY FOR PROCESSING

AT ABOUTS 250F WATER IS INJECTED INTO THE CRUDE

OIL TO DISSOLVE THE SALTS; THIS IS LATER PASSED TO ADESALTER WHERE THE SALT WATER IS SEPERATED FROMTHE CRUDE.


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CRUDE REFINING

THE PROCESSING OF CRUDE OIL YIELDS A WIDE RANGE OF

USEFUL PRODUCTS.

1. Liquefied petroleum gas2. Gasoline (Petrol)3. Naphtha4. Kerosene/ paraffin (Jet Air Fuel)5. Diesel6. Fuel oils7. Lubricating oils8. Paraffin wax9. Asphalt10. Petroleum coke


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CRUDE OIL REFINING

PROCESSES:

1.DESALTING2. ATMOSPHERIC DISTILLATION

3. VACUUM DISTILLATION4. NAPTHA HYDROTREATER5. CATALYTIC REFORMER6.FLUID CATALYTIC CRACKER7. ISOMERIZATION UNITS

8. COKING UNITS9. MEROX10. STEAM REFORMING


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ATMOSPHERIC DISTILLATION COLUMN

The desalted crude oil is fed into a distillation column

that operates at near atmospheric pressure. Hot and

partially vaporized crude is flashed onto a tray where

the vapors moving up are condensed by cooled refluxstreams; this results in a separation with various

distillates collected as side streams and the vapors not

condensed in the tower are taken off as the over head

stream. The resid from the atmospheric column is now

further processed in a different column.


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VACUUM DISTILLATION UNIT

The residue from the atmospheric unit is distilled under

sub atmospheric conditions in the crude vacuum

distillation unit.This unit operates similar to the atmospheric unit but in

this case, the tower operates under reduced

pressure(vacuum) conditions


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CATALYTIC REFORMING

The catalytic reforming unit is used to upgrade low octane naphtha tohigh octane material. This is achieved by reforming some hydrocarbonsin the feed to those of higher octane value

Straight run naphtha (80-200C) desulphurization

Catalytic reformer blending

n-C6H14 CH3CH(CH3)CH2CH2CH3 H=-5.9kJmol-1

n-C6H14+ 4H2

H=+266.5kJmol-1

CH3

+ 3H2

H=+250.6kJmol-1


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Catalytic Cracking

Catalysts: Faujasite, Erionite, ModerniteFeed: Heavier boiling fraction

Mw>200, boil at 340C at atmospheric pressure

Process: Feedstock preheated at about 400C fed into

catalyst riser Feed vapourises and cracked to smaller

molecules with the aid of catalysts Hydrocarbons fluidizes the catalyst and at

about 500C and 1.7bars enters the reactor Cracking takes place in riser within 2-4 seconds In the reactor

Vapours seperated from spent catalyst withthe aid of a cyclone

Spent catalyst sent to stripping section Catalyst regenerator Riser reactor


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HYDROTREATING UNIT

In the hydro treatment unit, hydrogen is used to get rid

of sulfur impurities, the hydrogen reacts with the sulfur

compounds and hydrogen sulfide is formed which is

then removed as a gas.


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MEROX UNIT

This unit is used to convert mercaptans to organic

disulfides


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QUALITY PARAMETERS

OCTANE NUMBER

The octane number of a fuel is measured in a test engine, and is

defined by comparison with the mixture of 2,2,4-trimethylpentane

(iso-octane) and heptane which would have the same anti- knockingcapacity as the fuel under test: the percentage, by volume, of 2,2,4-

trimethylpentane in that mixture is the octane number of the fuel.

CETANE NUMBER

The performance rating of a diesel fuel, expressed as thepercentage of cetane in a mixture with 1-methylnaphthalene that

shows the same ignition properties. The higher the cetane number,

the better the performance.


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QUALITY PARAMETERS

DENSITY

Mass per unit volume

CLOUD POINT

The temperature at which dissolve solids begin to

precipitate

FLASH POINT

This is the lowest temperature at which a fuel can vapourise to form ignitable

mixture in air

FIRE POINT

The fire point of a fuel is the temperature at which it will continue

to burn for at least 5 seconds after ignition by an open flame

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