Part III_CRE II Lectures

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

Ea

Ea′

ΔG = ΔG A+B

A+B+ catalyst

ΔG C

uncatalyzed

ΔG C + catalyst

catalyzed 2

Role of Catalysis in a National Economy 

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

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> 90 % of petro refining & petrochemicals processes use catalysts

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

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> 95% of pollution control technologies

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

Three Scales of Knowledge Application

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

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Heterogeneous Catalytic Reactors • Packed Bed (single or multi-tube)

Fluidized Bed

Slurry Reactor

Design goals

– rapid and intimate contact be tween catalyst and reactants – ease of separation of product s from catalyst

Catalyst Recycle Reactor

Rates of Catalytic Reactions 

Pseudo-homogeneous reaction rate 

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Mass-based rate  

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r′ = moles / masscat · time r′ = r / ρcat

Heterogeneous reactions happen at surfaces Area-based rate  

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r = moles / volume · time

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′′ / ρsite

TOF (s−1)

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   

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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 

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Traditionally reported as pressures (torr, atm, bar)

Surface concentrations 

“Coverage” per unit area 

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Maximum coverage called monolayer 

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nj = molesj / area

Metal particle surface

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

Fractional coverage  

θj = nj / nj,max 0 ≤ θj ≤ 1

θj = 1/6

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

Flowsheet Synthesis

2D & 3D C AD Solids Micro-scale Modeling Design Modules Multiscale Tran sport

Process Engineering Control Systems

Flow Patterns

Simulation & Optimization

Tools, Fabrication & Assembly Micro-process Components

Materials of Construction Micro Systems Engineering

Component Integr ation Multi-scale Transp ort

Micro Process Plant

Integrated Sensors

Raw Materials & Feedstocks Chemistry & Catalysis Reaction Kinetics

Catalyst Characterization Reaction Pathways & Mechanisms

Sampling Sensors

Micro Analyzers (GC, LC, MS, TOF) Micro Process Analytical

Data handling & Micro PAT Systems Chemometrics Integration

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

To get the overall rate expression, write the individual steps on the same basis In terms of Volume (Or) 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, 18

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

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.

Representation of a single cylindrical 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 (-Πr2 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. 28

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 (Ɛ) 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 For small mL ( 4), Ɛ ~ 1/mL,

the conc. Of the reactant drops rapidly within the pore → pore diffusion

Strongly influences the reaction rate. (Strong pore resistance)

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 32

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 33

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

MT > 4 or M W> 4

Reactant fully penetrates the particle and reaches all its surface. Then the particle is in the diffusion free regime. Center of the particle is starved for reactant and is unused. Then the particle is in strong pore resistance regime. 36

Shows the limits for negligible and for strong pore diffusion resistances 7. Particles of different sizes Comparing the behavior of two particle sizes R1 and R2, we find, Diffusion free regime Strong diffusion resistance 37

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. 38

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 40

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) 42

For First-order catalytic reactions, Plug flow reactor Mixed flow reactor

CAin = CAo and ƐA ≠ 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. 2. 3. 4.

Differential flow reactor Integral (plug flow) reactor Mixed flow reactor 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. 45

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

4. Batch reactor

In this system, we follow the changing composition with time & Interpret the results with batch reactor performance. When the recycle is large enough mixed flow is approximated A recycle reactor without through Flow becomes a batch reactor. 48

The Packed Bed Catalytic Reactors CATALYST DEACTIVATION DIAGRAM Pd Sites Vent Flow Controller Pre-heater

Al2O3

Reactor Gas Chromatograp h

A Fresh Catalyst (high dispersion; high surface area)

Pore cintering Cintered Pd

Integrator He O2

He

CH4

Al2O3

R.P. T

T

T

Temperature Controller

EXPERIMENT SETUP

γ-Al2O3

B Old Catalyst Low dispersion (low activity)

C Old catalyst Low surface area (low activity)

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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 50

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 51

Merits and demerits of fixed bed and fluidized bed reactors

Characteristic Feature 1. Gas Flow

Fixed-Bed Reactor

Fluidized-Bed Reactor

Plug Flow (√)

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

Efficient contact (√)

Fixed bed favored (√) 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 53

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. 54

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. 56

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. 58

Staged Mixed Flow Reactors

Staged Packed bed with recycle

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

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