Catalysis and Catalytic reactors RE10

transcript

CH10: Catalysis and Catalyst

Chemical Engineering Guy

www. Chemical Engineering Guy .com

Chemical Reaction Engineering Methodology

CH3: Elements of Chemical Reaction EngineeringH. Scott Fogler (4th Edition)

Chemical Reaction Engineering Methodology

CH3: Elements of Chemical Reaction EngineeringH. Scott Fogler (4th Edition)

Content

• Section 1: Catalysts

– Definitions

– Hetero-Homogeneous Catalysts

– Catalyst Properties

– Classification

• Section 2: Catalytic Reactions

– Common Industrial Applications

• Section 3: Steps In Catalytic Reactions

– Theory and Steps

– Example of Cumene Degradation

Section 1

Catalysis Basic Concepts

Catalysis Use and Importance

• Wine, cheese and bread Previous batch was needed for the next one

• Major users– Petroleum refining– Chemical Processes– Automotive

• 1/3 of chemical processes will need eventually the use of a catalyst

• The global demand on catalysts in 2010 was estimated at approximately 29.5 billions USD.

• Automotive and Chemical industry overall– the global catalyst market is expected to experience fast growth

in the next years.

Catalysis Use and Importance

Definition of Catalyst

• A substance that affects the rate of a reaction but emerges from the process unchanged

• A catalyst usually changes a reaction rate by promoting a different molecular path ("mechanism") for the reaction

Definition of Catalyst

• A catalyst changes only the rate of a reaction; it does nor affect the equilibrium.

– That is, no higher conversion is achieved

Catalysts

• Usually for “Faster” reaction design

• Less Activation Energy/Less free energy is required to reach the transition state

• but the total free energy from reactants to products does not change– That is, the change in enthalpy or enthalpy of reaction

Catalysts Free Energy Diagram

Catalysts

• As a catalyst is regenerated in a reaction, often only small amounts are needed to increase the rate of the reaction.

• In practice, however, catalysts are sometimes consumed in secondary processes.

• There are many type of catalyst recovery due to the high prices of catalysts

Inhibitor

• The opposite of a catalyst, a substance that reduces the rate of a reaction, is an inhibitor.

• This is typical for enzymes as well

Typical Mechanism

• Typical mechanismX + C → XC (1)

Y + XC → XYC (2)

XYC → CZ (3)

CZ → C + Z (4)

• Overall reaction:

X + Y → Z

Typical Mechanism

Types of Catalysts

• Homogeneous

• Heterogeneous

• Enzymes and biocatalysts

Homogeneous Reaction

• The processes use the catalyst is in solution

• Types

– Acid Catalysis

– Oraganometallic Catalysis

– Enzymatic reactions

Enzymes

• Enzymes possess properties of both:– Homogeneous

– heterogeneous catalysts.

• As such, they are usually regarded as a third, separate category of catalyst.

• Typical for Biotechnological Processes

Heterogeneous Reaction

• Heterogeneous: involves more than one phase: usually the catalyst is a solid and the reactants and products are in liquid or gaseous form

• A heterogeneous catalytic reaction occurs at or very near the fluid-solid interface

• Reactions between gases-Iiquids are usually mass-transfer limited

Heterogenous Reaction Examples

• From mighty Wikipedia

Catalyst Properties

• A large interaction area is almost always essential in attaining a significant reaction rate

• This is provided by an inner porous structure

– i.e., i solid contains many tine pores, and the surface of these pores supports the a needed for the high rate of reaction

• These Cat. Are called poro-catalyst

Porous Catalysts

Chemisorption

• Chemisorption results in the sharing of electrons between the adsorbate and the adsorbent.

Chemisorption

• Two step process:

1. Molecular adsorption, where the adsorbateremains intact.

• Example is alkene binding by platinum.

• In dissociation adsorption: one or more bonds break concomitantly with adsorption.

2. The barrier to dissociation affects the rate of adsorption.

• An example of this the binding of H2, where the H-H bond is broken upon adsorption.

Surface Reactions

• Langmuir-Hinshelwood mechanism.

• Rideal-Eley mechanism.

• Precursor mechanism.

Langmuir-Hinshelwood mechanism.

• The two molecules A and B both adsorb to the surface.

• While adsorbed to the surface, the A and B "meet,”and bond

• The new molecule A-B desorbs.

Rideal-Eley mechanism.

• One of the two molecules, A, adsorbs to the surface.

• The second molecule, B, meets A on the surface, having never adsorbed to the surface, and they react and bind.

• Then the newly formed A-B desorbs.

Langmui vs. Rideal Models

Catalyst Examples

• Naturals:

– Clays

– Zeolite

• Synthetics

– Crystalline aluminosillicates

Clays and Zeolites

Zeolite use in Para-Xylene

• Benzene and Toluene enter the zeolite

• They both react to form a mix of ortho, parametha xylene

• The size of the mouth only accepts p-xylene going out

Zeolite use in Para-Xylene

• Many interior sites isomerize ortho and methaxylene to para-xylene

• High selectivity of para-xylene

Synthetic Catalysts

• Aluminosillicates

Supported catalysts

• Finely, minute, pulverized catalyst (active material)

• It is dispersed on a less reactive material

– Support

• Promoters small amounts of material that increases the activity of the catalyst

Carbon-Supported Pt Catalyst

Catalysis+Support and Selectivity

Preparation of Catalyst+Support

Supported Catalysts: Examples

• Packed-Bed Catalytic converter of the auto

• Platinum-alumina for petroleum reformation

• Vanadium Pentoxide on silica for sulfuric acid production

Deactivation

• Decline on catalyst’s activity with time

– Aging phenomena• gradual change in structure

– Poisoning• irreversible deposition of

substances on the active sites

– Fouling/coking• carbonous deposition on all

the entire surface

Deactivation by Sintering (Aging)

• Loss of catalytic activity due to a loss of active surface area (due to high gas-phase temperatures)

– Crystal agglomeration and growth of the metals deposited on the support

– Loss of activity by narrowing or closing of the pores inside the catalyst pellet.

Deactivation by Sintering (Aging)

• A change in the surface structure– Recrystallization

– Formation or elimination of surface defects

• Sintering is usually negligible at temperatures below 40% of the melting temperature of the solid

Deactivation by Coking or Fouling

• This mechanism of decay is common to reactions involving hydrocarbon.

• It results from a carbonaceous (coke) material being deposited on the surface of a catalyst.

Deactivation by Coking or Fouling

• When the catalyst is already Fouled or coked, the material is normally called “spent catalyst”

Deactivation by Poisoning

• Poisoning molecules become irreversibly chemisorbed to active sites

• This reduce the number of sites available for the main reaction.

• Normally is done by impurities

• Petroleum feed stocks contain trace impurities such as:

– sulfur, lead, and other components which are too costly to remove.

Section 2

Catalytic Reactions

Basic Industrial Classification

• Alkylation and Dealkylation Reactions

• Isomerization Reactions

• Hydrogenation and Dehydrogenation Reactions

• Oxidation Reactions

• Hydration and Dehydration Reactions.

• Halogenation and DehaIogenation Reactions.

Alkylation and Dealkylation Reactions

• Alkylation addition of an alkyl group to an organic compound

• Common catalyst: Friedel-Crafts AlCl3 + HCl

Alkylation and Dealkylation Reactions

• Dealkylation Cracking of petrochemicals

• Common catalyst: silica-alumina; silica-magnesia and even clays (montmorilonite)

Cracking Units

Isomerization Reactions

• Change of Structure

• Hydrocarbon molecules are rearranged into a more useful isomer

• The process is particularly useful in enhancing the octane rating of petrol, as branched alkanes burn more efficiently in a car engine than straight-chain alkanes.

Isomerization Units

Hydrogenation and Dehydrogenation Reactions

• Some metals used in Hydrogenation are:

– Co, Ni, Rh, Ru, Os, Pd, Ir, and Pt.

• Non-used metals:

– V, Cr, Nb, Mo, Ta, and W

– each of which has a large number of vacant d-orbitals

– These are inactive as a result of the strong adsorption for the reactants or the products or both

• Hydrogenation reactions are favored at lower temperatures (<200ºC)

• Dehydrogenation reactions are favored at high temperatures (at least 200ºC)

• Example:

– Industrial butadiene (synthetic rubber production) can be obtained by the dehydrogenation of butenes

Oxidation Reactions

• The transition group elements (group VIII) and subgroup are used extensively in oxidation reactions:

– Ag, Cu, Pt, Fe, Ni

• In addition, V2O5 and MnO2 are frequently used for oxidation reactions

Oxidation Reactions Types

• Oxygen Addition

• Oxygenolysis of carbon-hydrogen bonds

Oxidation Reactions Types

• Oxygenation of nitrogen-hydrogen bonds:

• Complete combustion

Burner/Furnace Units

Thermal Oxidizers

Incineration Unit

Hydration and Dehydration Reactions.

• Used to get rid of H2O molecules

• Hydration and dehydration catalysts have a strong affinity for water

• One such catalyst is AI2O3, which is used in the dehydration of alcohols to form olefins

Hydration and Dehydration Reactions.

• Other examples:– Clays

– Phosphoric acid

– Phosphoric acid salts on inert carriers

Dehydration Units

HaIogenation and DehaIogenationReactions.

• Addition of Halogens Elements (group 7)

• Cl, F, Br, I, etc…

• Typical catalysts: CuCI2, AgCI. Pd, AlBr3, CCl4

HaIogenation and DehaIogenationReactions.

Catalyst Classification Summary

Section 3

Steps in a Catalytic Reaction

List of Steps in a Typical Heterogeneous Catalytic Reaction

1. Mass transfer (diffusion) of the reactant(s) from the bulk fluid to the external surface of the catalyst Pellet

2. Diffusion of the reactant from the pore mouth through the catalyst pores to the immediate vicinity of the internal catalytic surface

3. Adsorption of reactant A onto the catalyst surface4. Reaction on the surface of the catalyst AB5. Desorption of the products from the surface6. Diffusion of the products from the interior of the pellet to

the pore mouth at the external surface7. Mass Transfer of the products from the external pellet

surface to the bulk fluid

List of Steps in a Typical Heterogeneous Catalytic Reaction

1. External diffusion of reactant

2. Internal Diffusion of reactant

3. Adsorption of reactant A

4. Reaction on the surface of the catalyst AB

5. Desorption of the products from the surface

6. Internal diffusion of products

7. External diffusion of products

Visual SummaryAB

Visual Summary

Overall Rate of Reaction• Typically, is related to the rate of the slowest step in the

mechanism

• Classification of steps– Mass Transfer related steps (1,2,6 and 7)

– Reaction Kinetic related steps (3,4 and 5)

Overall Rate of Reaction

• Then there are two cases

– Mass Transfer limitations

– Reaction Kinetic/Chemisorption limitations

• When the diffusion steps:

• If (1,2,6 and 7) are very fast vs. with the steps (3, 4 and 5)

– Transport or diffusion steps do not affect the overall rate of the reaction.

Overall Rate of Reaction

Focus on Reactor Engineering

• We will focus in the actual Reaction

– Steps 3,4 and 5

– Adsorption, Surface Reaction, Desorption

• Mass Transfer phenomena limitations are more commonly studied in other courses

Visual Aid

Step 1: External Diffusion

• The reactant will diffuse to the “bulk” material

• The surface of the boundary layer is the one with most resistance

• Lets call CAb to the concentration of reactant A in the bulk

Step 1: External Diffusion

• Let Kc be the mass transfer coefficient

• Kc is function of Diffusion of A in B and the film length

• If diffusion of A in B is low and distance is large… you have a slow coefficient and therefore a slow reaction

• At fast velocities low length

• At low velocities high length

Visual Aid

Step 2: Internal diffusion

• Once the particle is “inside”, it must achieve the activation site

• Suppose it diffuses to a Concentration of CAs

• Kr is dependent only of the particle diameter

Kr = 1/ Dp

Step 2: Internal diffusion

• The bigger the particle, the larger the path needed!

Visual Aid

Step 3: Adsorption

• S Active Site

• Two models of Adsorption

– Molecular/non-dissociated adsorption

– Dissociative adsorption

Step 3: Adsorption

• Rate of Attachment vs. Detachment

• Combining Both Rates:

Step 3: Adsorption

• If we apply the KA ratio (adsorption equilibrium)

• We will get:

Step 3: Adsorption

• Applying an Active site Balance:

• In equilibrium, the rate should be equal to 0

• Solving for CCO·S

• And rearranging…

Step 3: Adsorption

• This expression is generally called Langmuir Isotherm

Visual Aid

Step 4: Surface Reaction

• Recall that:

• After the reactant is absorbed, it may react in the next ways:– Single Site

– Dual Site

– Eley-Rideal

• Single Site

• Each step is elementary reaction

• The reaction occurs directly on-site

• The model is left as:

• Where Ks is the surface reaction constant

Ks = ks/k-s

• Dual-Site

• A reacts with B in the adjacent site

• This type of reactions are the so called Langmuir-Hinshelwood kinetic model

• Eley-Rideal Mechanism Reaction

• Similar to Langmuir but only requires 1 site

Visual Aid

Step 5: Desorption

• Let C be the product and S the Active Site

• Desorption to the gas phase… The rate of reaction can be modeled with

• Let KDC be the equilibrium constant

Step 5: Desorption

• It is just the opposite (negative sign)

• For the Equilibrium Constant then:

• Therefore:

Visual Aid

Step 6: Internal Diffusion of Products

• Similar to step 2

• The length of the particle is still a factor

Visual Aid

Step 7: External Diffusion of Products

• Similar to step 1

• The diffusion factor of C in B is now the factor

– That is, now we are concerned with the product rather than with the reactant

Summary of Rates

• Step 1

• Step 2

• Step 3

• Step 4

• Step 5

• Step 6

• Step 7

Mass Transfer RatesMass Transfer PhenomenaAdsorption Rates Chemisorption relevantRate of Reaction kinetic relevant

Application to Cummene Decomposition

• Not diffusion-limited

• Product: Benzene and Propylene

• Catalyst: Platinum bed

• Application of Langmuir Mechanism

Application to Cummene Decomposition

• Each step is treated as an elementary reaction

• Due to gas-phase

– We will use Partial Pressures

– Remember Concentration may be related to Partial pressure: Pc = = Cc RT

Application to CummeneDecomposition

Application to Cummene Decomposition:Adsorption

• Adsorption of cummene in the Pt-bed

Visual Aid

Application to Cummene Decomposition:Surface-Reaction

• The rate Law for the surface reaction step producing adsorbed benzene and propylene in the gas phase.

• Using the surface criterion equilibrium

• Propylene is not adsorbed on the surface. Consequently, its concentration on the surface is zero being

Application to Cummene Decomposition:Surface-Reaction

Visual Aid

Application to Cummene Decomposition:Desorption

Visual Aid

Application to Cummene Decomposition:Rate-Limiting Step

• Typically, you would search for the rate-limiting step:

– Rate of Absorption

– Rate of Surface-Reaction

– Rate of Desorption

End of Block RE10

• By now you should know:

– Definition of a catalysis and a catalyst

– Importance of the Catalyst Industry

– What is an inhibitor

– Type of Catalytic Reactions (homo and heterogeneous)

– The importance of chemisorption

– Basic Reaction Mechanisms such as: Langmuir Models and Eley-Rideal Models

End of Block RE10

• You now know:– The Importance of the Supported Catalysts

– Why deactivation occurs and its types (aging, coking, poisoning)

– Common Industrial Processes and the type of catalysts they use

– The basic steps of the Catalytic Reaction (7)

– What a limiting step is

– How to model a basic catalytic reaction mechanism

Questions and Problems

• I included some extra problems and exercises

• All problems are solved in the next webpage

– www.ChemicalEngineeringGuy.com

• Courses

–Reactor Engineering

»Solved Problems Section

• CH10 – Catalysis and Catalytic Reactors

More Information…

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Text Book & Reference

Essentials of Chemical Reaction EngineeringH. Scott Fogler (1st Edition)

Chemical Reactor Analysis and Design FundamentalsJ.B. Rawlings and J.G.

Ekerdt (1st Edition)

Elements of Chemical Reaction EngineeringH. Scott Fogler (4th Edition)

Bibliography

Elements of Chemical Reaction EngineeringH. Scott Fogler (4th Edition)

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