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Transcript of 2014 CRE II
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CRE II: Heterogeneous Catalytic
Reaction
L1/2
Prof. K.K.Pant
Department of Chemical EngineeringIIT Delhi.
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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
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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