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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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Dissociative adsorption
= ..
=
..
=
, = 0
= .. . = .
= .
.
= .
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Dissociative adsorption
= + . + .= + 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