2014 cre ii

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CRE II: Heterogeneous Catalytic

Reaction

L1/2

Prof. K.K.Pant

Department of Chemical EngineeringIIT Delhi.

[email protected]

mailto:[email protected]

mailto:[email protected]

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CRE-II 3 – 1 – 0Prerequisite CHL-122

Course Outline

•Introduction to Catalysis, Classification, Catalyst preparation,

evaluation of Catalyst properties. 5

•Physical and Chemical adsorption, Different types of

adsorption isotherms, determination of surface area and porevolume of the Catalyst 6

•Kinetics of solid catalyzed gas phase reaction, Reaction

mechanism, Laboratory reactors for catalytic gas solidreactions. 8-9

•Mass transfer, Diffusion and Chemical reactions in catalysts.

Effects of external mass transfer and heat transfer,

Effectiveness factors. 8-9

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•Course outline , contd..

•Fixed bed catalytic reactors, reactor models, concept of

heterogeneous models. 4

•Non-catalytic gas-solid reactions, different model for gas-solid

reaction, 4

•Gas liquid reactions, film and penetration theories, enhance

factor in gas-liquid reactions, Concept of Reactor Design 4

•Reactor systems for gas-liquid reactions. Laboratory / design

activity (time permitting). 2-3

Grading: QUIZ : 10 (ONE ONLY), TERM Paper 4, Assignments/

Tut. : 2+4, Minors 2( 40), Major (40)

NO MAKE UP EXAMS, One grade will be lowered for attendance

below 65%. Pass grade 30 Marks, Relative grading.Use of mobile in L/T class is Strictly Prohibited

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Books:

•Fogler H.S, „Elements of chemical reaction engineering‟ IV th Ed .

( Text Book: MUST be BRING during TUTORIAL SESSIONs)

•Smith J.M., „Chemical‟ engineering kinetics‟, Mcgraw Hill,

•Chemical Rexn Engg :beyond fundamentals by Doraiswamy

•Carberry, J „Chemical and catalytic reaction engineering‟ Mcgraw

Hill

•Froment G.F & Bischoff K.B., „Chemical reactor analysis and

design‟ John Wiley

•C.G. Hill, „An Introduction to Chemical Engineering Kinetics and

Reactor Design‟

•Levensipiel : Chemical Rexn Engg.

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What is a catalyst ??

• Alters the rate of reaction with high selectivity

• Does it participate in the reaction ?• How does it change the rate ? – Offers an

alternate path with low E.

• Does it affect HR, GR, and Eq. constant ?

• Does it affect yield & selectivity ?

• Does it initiate a reaction ?

https://groups.google.com/forum/#!forum/chl221-sem1-2014-15

https://groups.google.com/forum/







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-What is a Catalyst ?

• Allow reactions to occur under a milder

conditions, e.g. at lower temperatures for those

heat sensitive materials

• It is important to remember that the use of

catalyst DOES NOT vary Δ G & K eq values of

the reaction concerned, it merely change the

PACE of the process.

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• Whether a reaction can proceed or not and to what

extent a reaction can proceed is solely determined

by the reaction thermodynamics, which is

governed by the values of ΔG & K eq, NOT by the

presence of catalysts.

• The reaction thermodynamics provide the driving

force for a Rxn; the presence of catalysts changesthe way how driving force acts on that process.

• Criterion for Chemical reaction equilibrium for species i,

• Σ ʋkiµi = 0, k= 1,2,3...r (reactions) Gibb’s free energy

change for each rexn (Δ G)= Σ ʋiµi =0

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e.g CH4(g) + CO2(g) = 2CO(g) + 2H2(g)

Δ G°373=151 kJ/mol (100°C) & Δ G°973

= -16 kJ/mol (700°C)

=>At 100°C, Δ G°373=151 kJ/mol > 0.

=> There is no thermodynamic driving force,

and the reaction won‟t proceed with or without

a catalyst

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At 700°C, Δ G°973= -16 kJ/mol < 0. The

thermodynamic driving force is there. However,

simply putting CH4 and CO2 together in a reactor

does not mean they will react.

Without a proper catalyst heating the mixture in reactor

results no conversion of CH4 and CO2 at all. When

Pt/ZrO2 or Ni/Al2O3 is present in the reactor at the

same temperature, equilibrium conversion can be

achieved (<100%).9

/

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Assignment /Tut. 1 Due date: July 29th

1. Give the Thermodynamic criteria for a Chemical Reaction.

2. Write down an Algorithm for calculation of product

composition in a multiple reaction from Thermodynamic

analysis (case study: Steam reforming of Ethanol

(PRODUCTS: CH4, CO, CO2, and H2, T= 500-800 C). State

other conditions used.

3. Different types of Reactors used in Chemical Process

Industries.

4. Reactor design equations for Batch , CSTR and plug flow

reactor and their applications in process industries.10

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1. Low cost and scalable

2. Renewable starting materials

3. Environmentally‐friendly operation

4. Specialty Chemicals

5. Non‐toxic by

‐products

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GREEN CATALYSIS

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Basic concept of green catalysis

1. Indicators to measure the efficiency and

environmental impact of a reaction.

Atom Efficiency: is the molecular weight of the

desired product divided by the total molecular weight

of All Products.

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Concept of Atom efficiency & E -Factor

Atom efficiency =

The molecular weight of thedesired product

The total weight of all

products.

E- Factor : Another useful indicator of

environmental acceptability is the E factor-

the weight of waste or undesirable by

product by the Weight of the desired product.

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• Mass balances of alternative routes in chemicalprocessing can be compared using measures E

factor and mass Index .

• The E factor :Ratio of Waste [kg ] to

Product[kg]), is an output orientated indicator,

• Mass index (Ratio of all Raw materials [kg] to

the Product [kg]) is an input oriented indicator.

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E factor:

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For example the conventional oxidation of

a secondary alcohol

3C6H5 –CHOH –CH3 + 2Cr 2O3 + 3H2SO4

3C6H5 –CO –CH3 + Cr 2(SO4)3 + 6H2O

396 360 Atom efficiency of 360/864 =~ 42%.

C 6 H 5 –CHOH –CH 3 + 1/2O2

C 6 H 5 –

CO –

CH 3 + H 2 O

Atom efficiency of 120/138 = 87%, with water as the

only by product in alternate process.

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“A catalyst accelerates a chemical reaction

without affecting the position of the equilibrium.”

Returning to its original form

• After reaction cycles a catalyst with exactly the

same nature is „reborn‟.

• In practice a catalyst has its lifespan it

deactivates gradually during use.

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Action of Catalysts

• Catalysis action - Reaction kinetics and mechanism

Catalyst action leads to the rate of a reaction to

change. This is realised by changing the course ofreaction (compared to non-catalytic reaction)

• Forming complex with reactants/ products,

controlling the rate of elementary steps in the

process.19

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Schematic representation of the energetic in a catalytic cycle. The uncatalyzed

reaction(a)has a higher Gibbs energy of activation ΔG than any step in the

catalyzed reaction (b)The Gibbs energy of reaction ΔrGØ for the overall

reaction is unchanged from (a) to (b).

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• The reaction activation energy is altered

• The intermediates formed are different

from those formed in non-catalytic

reaction• The rates of reactions are altered (both

desired and undesired ones)

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Action of Catalysts

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-Reactions proceed under less demanding conditions

• Allow reactions occur under a milder conditions,

e.g. at lower temperatures for those heat

sensitive materials

• It is important to remember that the use of

catalyst DOES NOT vary Δ G & K eq values of the

reaction concerned, it merely change the PACEof the process.

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Comparison of Catalysts:

For comparative measurements, such as catalyst

screening, determination of process para-meters,optimization of catalyst production conditions, and

deactivation studies, the following activity measures

can be used:

– Conversion under constant reaction

conditions

– Space velocity for a given, constantconversion

– Space –time yield

– Temperature required for a given

conversion 23

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Comparison of catalyst activities

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Turnover frequencies, Rates and

numbers

CATALYSIS IS A KINETIC PHENOMENON

Sequence of elementary steps at steady state:

diffusion (bulk, film, surface) - adsorption-reaction-desorption- diffusion

TOF= number of product molecules formed per unit

area per sec(molecules.cm-2.sec-1)

TOF= number of product molecules formed per active

site per sec(molecules.sec-1) only if active site is

known.

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TOT= 1/TOF = turnover time, time necessary

to form a product molecule(sec);TOR = Turnover rate = TOF X Surface area

TON= TOF X total reaction time;

TON must be >100 to be industrially useful.

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The suitability of a catalyst for an industrialprocess depends mainly on the following

three properties:

– Activity – Selectivity

– Stability (deactivation behavior)

The question which of these functions is the

most important is generally difficult toanswer because the demands made on the

catalyst are different for each process.

Mode of Action of Catalysts

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Activity

Activity is a measure of how fast one or morereactions proceed in the presence of the catalyst.

Activity can be defined in terms of kinetics.

In a kinetic treatment, reaction rates are measured

in the temperature and concentration ranges thatwill be present in the reactor.

Rate = k(T) f(C)

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Every catalytic reaction is a sequence of

elementary steps, in which reactant moleculesbind to the catalyst, where they react, after which

the product detaches from the catalyst, liberating

the latter for the next cycle.

Steps of Catalytic Reactions

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Potential energy diagram of a heterogeneous catalyticreaction

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Conversions, Rates and Rate

constants

• Conversion = % Reactant converted;

• Reaction rate = kp X f(Pi) or kc X f(Ci)

• k = A exp(-

E

#

/RT); A is temp independent.• TOFs between 0.0001 and 100 in industry; Temp

adjusted to get the desired rates.

• E# ~ 35-45 Kcal/mol for isom, cyclisation,

cracking, dehydo / hydrogenolysis; HighT needed.

E# ~ 6-12 Kcal/mol for hydrogenation;

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Chemical Kinetics

Collision Theory

– Collisions between reacting molecules are

necessary before a reaction can occur.

– Only those collisions having sufficient energy are

effective in bringing about a reaction activation

energy.

– Colliding molecules must be properly oriented

with respect to one another for the reaction to take

place.

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Selectivity

• The selectivity (Sp) of a reaction is the

fraction of the starting material that is

converted to the desired product P.

It is expressed by the ratio of the amount

of desired product to the reacted quantity

of a reaction A . In addition to the desiredreaction, parallel and sequential reactions

can also occur.

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np

np vAvpSp= = mol/mol or %nA,o-nA nA,o - nA vp

vA

Selectivity

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Stability

•The chemical, thermal, and mechanical stability

of a catalyst determines its lifetime in industrial

reactors.

•Catalyst stability is influenced by decomposition,coking, and poisoning. Catalyst deactivation can

be followed by measuring activity or selectivity as

a function of time.

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•Presently the efficient use of raw materials and

energy is of major importance, and it is prefer-

able to optimize existing processes than to

develop new ones.

• For various reasons, the target quantities

should be given the following order of priority:

Selectivity >Stability> Activity

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Catalysis in the Chemical Industry

• Hydrogen Industry (coal, NH3, methanol, FT,

hydrogenations / HDT, fuel cell).

• Natural gas processing (SR,ATR,WGS,POX)

• Petroleum refining (FCC, Hydrotreating,

Hydrocracking, Reforming, Alkylation etc.

etc.)

• Petrochemicals(monomers, bulk chemicals).

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• Fine Chem.(pharma, agrochem, fragrance,

textile, coating, surfactants,laundry etc)

• Environmental Catalysis (auto exhaust,

deNOx, )

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Types of Catalysts & Catalytic

Reactions

• The types of catalysts

– Classification based on the its physical

state, a catalyst can be

• gas

• liquid

• solid

– Classification based on the substances

from which a catalyst is made

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• Inorganic (gases, metals, metal oxides,

inorganic acids, bases etc.)

• Organic (organic acids, enzymes etc.)

• Types of catalysts

– Classification based on the ways catalysts work

• Homogeneous - both catalyst and allreactants/products are in the same phase (gas

or liq)

• Heterogeneous - reaction system involvesmulti-phase (catalysts + reactants/products)

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Classification of Catalysts

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Comparison between Homogeneous

and heterogeneous catalysts

2014 cre ii

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