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Introduction to Catalysis

Catalyst A substance that alters the reaction rate of a particular chemical reaction is called a catalyst.

Chemically unchanged at the end of the reaction.

Positive Catalyst (catalyst): Increases the rate of reaction

Negative Catalyst (Inhibitors): Decreases the rate of reaction

How does a catalyst change rate of reaction???

By providing alternative pathway or mechanism to lower/higher activation energy

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A + B

C

G

Ea

uncatalyzed

A + B + catalyst

C + catalyst

G

Ea

catalyzed

k(T) = k0e-Ea/RT Ea < Ea k0 > k0 k > k

G = G

Role of Catalysis in a National Economy

24% of GDP from Products made using catalysts (Food, Fuels, Clothes, Polymers, Drug, Agro-chemicals)

> 90 % of petro refining & petrochemicals processes use catalysts

90 % of processes & 60 % of products in the chemical industry

> 95% of pollution control technologies

Catalysis in the production/use of alternate fuels (NG,DME, H2, Fuel Cells, biofuels)

Three Scales of Knowledge Application

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Types of Catalysts (1) Homogeneous Catalysts

(2) Heterogeneous Catalysts

(3) Auto-Catalysts

(1) Homogeneous Catalysts:

Catalyst with the same phase as reactants.

Usually in aqueous phase or gaseous phase.

Ex: Oxidation of I- with S2O82- with Fe3+ ion as a catalyst

2I- + S2O82- ==> I2 + 2SO42- -----------------------------------------

-- 2I- + 2Fe3+ ==> 2Fe2+ + I2

2Fe2+ + S2O82- ==> 2Fe3+ + 2SO42-

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(2) Heterogeneous Catalysts

Catalyst with different phase as reactants.

Usually Catalyst in solid form and reactants in aqueous or gaseous form.

Ex: SRM, POX , CDM, Hydrogenation of ethane (Ni as catalyst), CNT

CNT

Hydrogenation of ethane

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(3) Auto-Catalysts

The product in the reaction acts as a catalyst of the reaction.

This product is called auto-catalyst.

Ex: 2MnO4- + 16H+ + 5C2O42-==> 2Mn2+ + 8H2O + 10CO2

Applications of catalysts:

(1) Chemical Industries

(2) Catalytic converters in automobile exhaust

(3) Biological catalysts as enzymes (fermentation, baking)

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(2) Catalytic converters in automobile exhaust

Heterogeneous Catalytic Reactors

Design goals rapid and intimate contact be

tween catalyst and reactants ease of separation of product

s from catalyst

Packed Bed (single or multi-tube)

Slurry Reactor

Fluidized Bed

Catalyst Recycle Reactor

Rates of Catalytic Reactions

Pseudo-homogeneous reaction rate r = moles / volume time

Mass-based rate

r = moles / masscat time r = r / cat

Heterogeneous reactions happen at surfaces Area-based rate

r = moles / areacat time r = r / SA, SA = area / mass

Heterogeneous reactions happen at active sites Active site-based rate

Turn-over frequency TOF = moles / site time TOF = r / site

TOF (s1) Hetero. cats. ~101

Enzymes ~106

Adsorption and Reaction at Solid Surfaces

Physisorption: weak van der Waals attraction of a fluid (like N2 gas) for any surface Eads ~10 40 kJ/mol Low temperature phenomenon Exploited in measuring gross surface area

Chemisorption: chemical bond formation between a fluid molecule (like CO or ethylene)

and a surface site Eads ~ 100 500 kJ/mol Essential element of catalytic activity Exploited in measuring catalytically active sites

Measuring Concentrations in Heterogeneous Reactions Kinetics

Fluid concentrations Traditionally reported as pressures (torr, atm, bar)

Surface concentrations Coverage per unit area

nj = molesj / area Maximum coverage called monolayer

1 ML: nj,max = ~ 1015 molecules / cm2

Fractional coverage j = nj / nj,max 0 j 1

j = 1/6

Metal particle surface

Catalysts Characterization

Characteristics Methods

Surface area, pore volume & size

N2 Adsorption-Desorption Surface area analyzer (BET and Langmuir)

Pore size distribution BJH (Barret, Joyner and Halenda)

Elemental composition of catalysts

Metal Trace Analyzer / Atomic Absorption Spectroscopy

Phases present & Crystallinity X-ray Powder Diffraction TG-DTA (for precursors)

Morphology Scanning Electron Microscopy

Catalyst reducibility Temperature Programmed Reduction

Dispersion, SA and particle size of active metal

CO Chemisorption, TEM

Acidic/Basic site strength NH3-TPD, CO2 TPD

Surface & Bulk Composition XPS

Coke measurement Thermo Gravimetric Analysis, TPO

Catalyst Activity Testing

Activity to be expressed as:

- Rate constants from kinetics - Rates/weight - Rates/volume - Conversions at constant P,T and SV. - Temp required for a given conversion at constant partial & total pressures - Space velocity required for a given conversion at constant pressure and temp

Micro Process Plant

Chemistry & Catalysis

Raw Materials & Feedstocks

Catalyst Characterization

Reaction Pathways & Mechanisms

Reaction Kinetics

Micro Systems Engineering

Tools, Fabrication & Assembly

Micro-process Components

Materials of Construction

Component Integration

Multi-scale Transport

Micro Process Analytical

Integrated Sensors

Sampling Sensors

Data handling & Chemometrics

Micro PAT Systems Integration

Micro Analyzers (GC, LC, MS, TOF)

Process Engineering

Flowsheet Synthesis

Control Systems

2D & 3D CAD Solids Modeling

Micro-scale Design Modules

Flow Patterns

Simulation & Optimization

Multiscale Transport

Heterogeneous Reactions

The complications of rate equations:

More than one phase is present: Movement of material from phase to phase

For rate expression: Apart form chemical kinetics term mass transfer is also incorporated

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To get the overall rate expression, write the individual steps on the same basis

(Or)

In terms of Volume

In terms of Weight

(Or)

In terms of Surface

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Rearrange the mass transfer and reaction steps into same rate form

(Or)

If the steps are in series,

If the steps are in parallel,

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Complications :

Consider reaction steps in series:

If all the steps are linear in concentration then it is easy to combine them.

If any of the steps is non-linear in concentration then it will be difficult to get a overall rate expression.

In such cases, approximate the rate equation (vs.) concentration curve by a first-order expression.

It is hard to know the concentration of materials at intermediate steps.

So, these concentrations are eliminated during combining the rates.

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Overall reaction rate for a linear process:

Dilute A diffuses through a stagnant liquid film onto a plane surface consisting of B, reacts to produce R which diffuses back into the mainstream. Develop the overall rate expression for this first order L/S reaction.

A (l) + B(s) R(l)

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By diffusion, the flux of A to the surface is,

(1)

Since this reaction is first-order w.r.t A, based on unit surface,

(2)

At steady state, the flow rate to the surface is equal to the reaction at surface (steps in series)

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from which the intermediate (CAs )can be determined as,

(3)

Replacing eq. (3) into either eq. (1) or eq. (2), gives

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Contacting patterns for two-phase systems

Ideal contacting patterns for two flowing fluids

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Pore diffusion resistance combined with surface kinetics

Representation of a single cylindrical pore

Consider a single cylindrical pore of length (L), with reactant A diffusing into the pore, and reacting on the surface by a 1st order reaction.

The reaction is taking place at the walls of the pore and the product is diffusing out of the pore.

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Now, check with flow of materials into and out of any section of the pore can be shown as:

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At study state a material balance for reactant A for this elementary section :

Output Input + Disappearance by reaction = 0

By Substituting the output, input and disappearance by reaction terms we get:

For our convenience divide the above equation by (-r2 D (x))

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Now apply limit as x approaches Zero the obtained equation changes to:

The 1st order chemical reaction is expressed in terms of unit surface area of the wall of the catalyst pore

Therefore, K will have the unit of length per time

In general the interrelation b/w rate constants on different basis is given by:

Hence for cylindrical catalyst pore:

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Thus in terms of volumetric units the final equation takes the form:

The above eq. is a linear differential eq. whose general solution is:

Where,

M1 & M2 are constants and we need two boundary conditions to evaluate them.

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First boundary condition (x=0), (Pore entrance)

Second boundary condition (x=L), (Pore exit)

According to the given model, there is no pore exit and there is no flux or movement of the material through the interior end of pore.

With the appropriate mathematical manipulations of CA and boundary Conditions:

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Hence the concentration of reactant (CA/CAs) with in the pore is :

There is a progressive drop in concentration on moving into the pore.

And this dependent on the dimensionless quantity mL (or) MT called as Thiele modulus.

Effectiveness factor () is introduced to measure how much the reaction rate is decreased because of the resistance to pore diffusion.

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Distribution and Avg. value of reactant concentration within a catalyst pore as a function of the parameter mL = L (k/D)

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The effectiveness factor (Vs)Thiele Modulus

This graph can easily show the effectiveness Pore diffusion on modification of the rate of reaction and it depends on whether mL is large or small

For small mL ( 4), ~ 1/mL, the conc. Of the reactant drops rapidly within the pore pore diffusion Strongly influences the reaction rate. (Strong pore resistance)

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Porous Catalyst Particles Main steps involved in heterogeneous catalytic reactions

1. Transport of the reactants from the bulk of a mixture to a catalyst particle

2. Transport of the reactants in the pores of the catalyst particles to an active site

3. Adsorption of the reactants to the active site

4. Reaction of reactants to form an adsorbed product 5. Desorption of the product from the active site 6. Transport of the products in the pores of the catalytic particle out of the particle

7. Transport of the products from the particle to the bulk of the mixture

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The results of a single pore can approximate the behavior of particles of various shapes (spheres, cylinders, flat plates etc.) for these systems,

1. Use of proper diffusion coefficient Replace the molecular diffusion coefficient D by the effective diffusion coefficient of the fluid in the porous structure.

2. Proper measure of particle size To find the effective distance penetrated by the gas to get all the interior surfaces we should define a characteristic size of particle.

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3. Measures of reaction rates The rate of reaction can be expressed in many equivalent ways.

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4. Finding pore resistance effects from experiment

Define a modulus which only includes observable and measurable quantities. This is known as Wagner-Weisz-Wheeler Modulus (Wagner Modulus).

5. Pore resistance limits

MT < 0.4 or MW < 1.15 Reactant fully penetrates the particle and reaches all its surface. Then the particle is in the diffusion free regime.

MT > 4 or M W> 4 Center of the particle is starved for reactant and

is unused. Then the particle is in strong pore resistance regime.

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7. Particles of different sizes

Shows the limits for negligible and for strong pore diffusion resistances

Comparing the behavior of two particle sizes R1 and R2, we find,

Diffusion free regime Strong diffusion resistance

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Heat effects during the reaction

Non-Isothermal Effects

When the reaction is so fast that the heat released (or absorbed) in the pellet cannot be removed rapidly to keep the pellet close to the temperature of the fluid, then the non-isothermal effects intrude.

Two different kinds of temperature effects may be encountered:

1. Within- particle T 2. Film T

Temperature variation within the pellet

The pellet may be hotter (or colder) than the surrounding fluid.

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Exothermic reactions

Heat is released and particles are hotter than the surrounding fluid.

Therefore the non-isothermal rate > isothermal rate as measured by bulk conditions.

Endothermic reactions

Heat is absorbed and particles are colder than the surrounding fluid. non-isothermal rate < isothermal rate

If the harmful effects of thermal shock, or sintering of the catalyst Particles, or drop in selectivity do not occur than one can encourage exothermic reaction.

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Non-isothermal effectiveness factor curve for temp. variation with in the particle

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The differential form of Eq. (1) is,

(2)

Integrating over the whole reactor gives,

(3)

Weight-time and volume-time terms,

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Performance equations for reactors containing Porous catalyst particles

For Plug Flow

Elementary slice of solid catalyzed plug flow reactor

At steady state a material balance for reactant A gives,

Input = output + accumulation

(1)

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For First-order catalytic reactions,

Plug flow reactor

Mixed flow reactor

CAin = CAo and A 0 ( first order reactions)

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Experimental Methods for finding rates

The experimental strategy in studying catalytic kinetics usually measuring the extent of conversion of gas passing in steady flow through a batch of Solids.

Any flow pattern can be used, as long as the pattern selected is known. If it is not known then the kinetics cannot be found.

We will discuss on the following experimental devices.

1. Differential flow reactor 2. Integral (plug flow) reactor 3. Mixed flow reactor 4. Batch reactor for both gas and solid

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1. Differential (flow) reactor

If we choose to consider the reaction rate to be constant at all points within the reactor then we can have a differential reactor.

Since reaction rates are concentration-dependent this assumption is usually reasonable for small conversions or for small reactors.

For each run in a differential reactor the plug flow performance equation becomes:

Thus each run gives directly a value for rate at avg. conc., a series of runs gives a set of rate-conc. Data.

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2. Integral (plug flow) reactor

If the variations in the reaction rate within a reactor is so large then to account such variations in the method of analysis, then we have an integral Reactor.

Since the reaction rates are conc. dependent, such large variations in rate may be expected to occur when the composition of the reactant fluid changes significantly in passing through the reactor.

We may follow two procedures in searching for a rate equation.

Integral analysis Differential analysis

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3. Mixed Flow Reactor

A mixed flow reactor requires a uniform composition of fluid through out.

For a mixed flow reactor the performance Equation is given by:

Carberry basket-type experimental mixed flow reactor

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Recycle reactor

When the recycle is large enough mixed flow is approximated

4. Batch reactor

In this system, we follow the changing composition with time & Interpret the results with batch reactor performance.

A recycle reactor without through Flow becomes a batch reactor.

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The Packed Bed Catalytic Reactors

FlowController

GasChromatograp

h

Reactor

Pre-heater

Integrator

TemperatureController

He

He CH4

O2

R.P.

T T T

Vent

EXPERIMENT SETUP

AFresh Catalyst (high dispersion; high surface area)

Pd Sites

Al2O3

Al2O3 -Al2O3

Cintered PdPore cintering

BOld Catalyst

Low dispersion (low activity)

COld catalyst

Low surface area (low activity)

CATALYST DEACTIVATION DIAGRAM

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The reactant gas can be made to contact solid catalyst in many ways, and each has its specific advantages and disadvantages.

Reactors cab be divided into two broad types. 1. Fixed Bed Reactor 2. Fluidized Bed Reactor

Fixed Bed Reactors

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Fluidized Bed Reactors

Moving-Bed Reactor is an intermediate case which incorporates some of the advantages and disadvantages of fixed-bed and fluidized-bed reactors

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Merits and demerits of fixed bed and fluidized bed reactors

Characteristic Feature

Fixed-Bed Reactor Fluidized-Bed Reactor

1. Gas Flow Plug Flow () Efficient contact () Fixed bed favored ()

Complex Flow & by passing (X) High Catalyst content (X)

2. Temperature control

Large Fixed beds (X) (low cond.) Exothermic Rxn. (X) (Hot Spot)

Good Control of Temp. Explosive nature of Rxn. can also performed

3. Particle Size (small)

Plugging & High-Pressure drop (X) Effective use of catalyst Pore and diffusion rxn. high

4. Catalyst Regeneration

Regeneration is difficult(X) Liquid-like fluidized state Can be pumped easily

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The two main difficulties in catalytic reactor design

1. How to overcome non-isothermal behavior in packed beds. 2. How to overcome non-ideal flow of gas in fluidized beds.

Moving-Bed Reactor

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The temperature field in a packed bed reactor for an exothermic reaction creates a radial movement of heat and matter

The stage adiabatic packed bed reactor presents different situation. (Since no heat transfer in the zone of reaction. The temperature and conversions are related in much simple way.

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Staged Adiabatic Packed Bed Reactors

With proper interchange of heat and proper gas flow, staged adiabatic packed bed reactors became versatile system

Staged Packed Beds (Plug flow) with intercooling

Staged Mixed Flow Reactors

Cold Shot Cooling

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Staged Packed Beds (Plug flow) with intercooling

Sketch showing how staged packed beds can closely approach the optimal temperature

Optimization of operations reduces to minimize the total amount of catalyst needed to achieve a given conversion.

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Reversible Exothermic Reactions

Three variables to optimize the amount of catalyst 1. Incoming Temp. (Ta) 2. Amount of catalyst used in 1st stage ( b along with the adiabatic) 3. Amount of intercooling (c along the bc line)

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How to find exact (Ta) 1.Guess (Ta)

2. Move along the adiabatic line until the following condition is satisfied.

This gives point b (amount of catalyst needed and outlet temperature from that stage

3. Cool to point c which has same rate as b: Rxn rate leaving the reactor = Rxn rate entering next reactor (or stage) 4. Moving along the adiabatic from point c to d until point 2 is satisfied (d).

5. If point d is the desired final conversion then our Guess is correct.

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Staged Mixed Flow Reactors

Choose the distribution of the catalyst So as to maximize the KLMN area which Then Minimizes the shaded area

Staged Packed bed with recycle

Introduction to CatalysisSlide Number 2Role of Catalysis in a National EconomySlide Number 4Slide Number 5Slide Number 6Slide Number 7Slide Number 8Heterogeneous Catalytic ReactorsRates of Catalytic ReactionsAdsorption and Reaction at Solid SurfacesMeasuring Concentrations in Heterogeneous Reactions KineticsSlide Number 13Catalyst Activity Testing Slide Number 15Heterogeneous Reactions Slide Number 17Slide Number 18Slide Number 19Slide Number 20Slide Number 21Slide Number 22Slide Number 23Slide Number 24Slide Number 25Slide Number 26Slide Number 27Slide Number 28Slide Number 29Slide Number 30Slide Number 31Slide Number 32Slide Number 33Slide Number 34Slide Number 35Slide Number 36Slide Number 37Slide Number 38Slide Number 39Slide Number 40Slide Number 41Slide Number 42Slide Number 43Slide Number 44Slide Number 45Slide Number 46Slide Number 47Slide Number 48Slide Number 49Slide Number 50Slide Number 51Slide Number 52Slide Number 53Slide Number 54Slide Number 55Slide Number 56Slide Number 57Slide Number 58Slide Number 59