topic 1.1 - catalysis and catalytic reactors

CHE625 Advanced Chemical Reaction Engineering

Lecturer: Pn. Sharmeela bt Matali Office : PA-11-8C (level 11, Tower 2) Contact: 03-5543 6328 (O) 013-299 0609 [email protected] Credit to: Cik Nor Faeqah Pn Norhasyimi

Course Learning Outcome:

By the end of this topic, you should be able to:

Explain the steps in catalytic reactions; write the mechanism and

chemical kinetics.

Able to describe rate limiting step.

Able to describe Langmuir-Hinshelwood Kinetic Mechanism.

Ref : The Eng. Of Chemical Reactions ( Schmidt) Chapter 7 (pg 269)

FACULTY OF CHEMICAL ENGINEERING

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IDEAL REACTORS - REVISION

INTRODUCTION


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Design Equation for Ideal Homogeneous Reactors

Ideal Batch Reactor Mass Balance Constant density system


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Design Equation for Ideal Homogeneous Reactors

Ideal Plug Flow Tubular Reactor (PFTR/PFR)

Ideal Constant Stirred Tank Reactor (CSTR)


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TO TEST your BASICS of CRE The elementary of liquid phase reaction A ->B + 2C is carried out isothermally in a flow reactor with no pressure drop. The rate constant at 50oC is 10-4min-1 and the activation energy is 85 kJ/mol. Pure A enters at 10 atm and 127oC, and molar flow rate is 2.5 mol/min. Calculate the space time and reactor volume for 90% conversion in CSTR and PFR.

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CATALYTIC REACTORS AND MASS TRANSFER

CHAPTER 1


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A catalyst is a substance that affects the rate of a reaction but does not enter into the stoichiometry of the reaction. Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Catalysis is the occurrence, study, and use of catalysts and catalytic processes. Catalysts that speed the reaction are called positive catalysts WHILE slow the reaction are called inhibitors (or negative catalysts). Substances that increase the activity of catalysts are called promoters, and substances that deactivate catalysts are called catalytic poisons.

What is Catalyst and Catalysis? 1.1 Catalytic Reactions


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Lets watch a video on catalysis:

http://www.youtube.com/watch?v=A_PhvIktMOw&feature=endscreen


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Example of Catalysis

Gaseous H2 and O2 are inert at room temperature. They react rapidly when exposed to Pt catalyst.


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Different shapes and sizes of catalyst.

Catalysts and Catalysis


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Catalytic packed-bed reactor, schematic.

Catalysts and Catalysis


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Theory of Catalysis

Activated complex formation theory: Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process regenerating the catalyst. The following is a typical reaction scheme, where C represents the catalyst, X and Y are reactants, and Z is the product of the reaction of X and Y:

X + C XC (1) Y + XC XYC (2) XYC CZ (3) CZ C + Z (4)

X + Y Z FACULTY OF CHEMICAL ENGINEERING

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Theory of Catalysis

Adsorption theory (Old): The reactants in gaseous state or in solu0ons, are adsorbed on the surface of the solid catalyst. The increase in concentra0on of the reactants on the surface increases the rate of reac0on. Adsorp0on being an exothermic process, the heat of adsorp0on is u0lised in enhancing the rate of the reac0on.

Adsorption theory (Modern): The modern adsorp0on theory is the combina0on of intermediate compound forma0on theory and the old adsorp0on theory. The mechanism involves ve steps: (1) Diusion of reactants to the surface of the catalyst. (2) Adsorp0on of reactant molecules on the surface of the catalyst. (3) Occurrence of chemical reac0on on the catalysts surface through forma0on of an intermediate (Figure depicted below). (4) Desorp0on of reac0on products from the catalyst surface, and thereby, making the surface available again for more reac0on to occur. (5) Diusion of reac0on products away from the catalysts surface.


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Theory of Catalysis

H H

Pt Pt Pt

H H

OO

O O

Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt

H O H

Adsorption theory


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A Mechanism for the oxidation of carbon monoxide

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What is adsorp1on?

Adsorption is a physical or chemical phenomenon by which the molecules present in a liquid or a gas attach to the surface of a solid.

The substance on which surface adsorp1on occurs is termed as the adsorbent, and the substance which adsorbed from the bulk phase is known as the adsorbate.

Depending on the force of aBrac1on, adsorp1on is mainly two types: (1) Physical adsorp1on (Physisorp1on) and (2) Chemical adsorp1on (Chemisorp1on).

Surface means both external and internal surface.


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Physical Adsorp1on

Physical adsorp1on (Physisorp1on) Physical adsorp1on is a phenomenon which takes place purely due to the van der Waals forces of aBrac1on.

- It can be compared with the condensa1on of vapour of liquids.

- It is a reversible phenomenon. - Because of very week force of aBrac1on, the physical adsorp1on can not bring to any change of chemical structure of the adsorbent and adsorbate.


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Chemical Adsorp1on

Chemical adsorp1on (Chemisorp1on) Chemical adsorp1on is adsorp1on which results from chemical bond forma1on (strong interac1on) between the adsorbent and the adsorbate in a monolayer on the surface.

Example: Organic compound get adsorbed on the solid surface with chemical bond forma1on.


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Why does solid substance adsorb?

The adsorption phenomenon comes from the existence of non-compensated forces of a physical nature on the surface of the solid. All the bonding requirements of the constituent atoms of the material are filled by other atoms in the material.

However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates.

Adsorbate Adsorbent

Adsorbate


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Heterogeneous Reactor and Voidage,

Reactor volume, V = Volume of fluid plus volume of catalyst

V = V fluid + V catalyst

Void fraction or the fraction of the reactor volume occupied by fluid


VV fluid==

reactor of Volumefluid of Volume

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Different types of Rates in Heterogeneous Catalytic Reactions

Homogeneous reactions:

Heterogeneous reactions:


timevolumemoles

=r

timeareamoles''

=r

timemassmoles'

=r

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Pseudo-homogeneous rate

r x (volume ) = r x (area of catalyst)

Surface area of catalyst


catalyst of masscatalyst of area surface

=gs

")1( rsr cg =

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Space time,

Homogeneous Reactors:

Heterogeneous Reactors V fluid = V


reactor he through tpasses rate flow volumetricreactor ain fluid of volume

=

rate flow metricinlet volureactor of volume

0==

V

0

V

=

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What are the criteria of the catalyst?

4. No catalyst can change an equilibrium state of a reac1on.

The following criteria dene a good quality catalyst for a reac1on:

1. Only small quan1ty is needed for a reac1on. 2. They are specic. One catalyst is needed for specic reac1on only.

3. Physical proper1es may change during a reac1on but it does not take part in the reac1on.


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What are the proper1es of the catalysts? Porous catalyst catalyst that has a large area Molecular sieves small pore that will admit small molecule

Monolithic catalyst can be either porous or non-porous

Supported catalyst - consist of par1cles of an ac1ve material dispersed over a less ac1ve substance.

Unsupported catalyst Promoters small amount of ac1ve ingredients

Deac1va1on of catalyst - Aging - Poisoning - Fouling or coking


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Catalyst Properties Porous catalyst catalyst that has a large area Molecular sieves small pore that will admit small

molecule Monolithic catalyst can be either porous or non-

porous Supported catalyst - consist of particles of an

active material dispersed over a less active substance.

Unsupported catalyst Promoters small amount of active ingredients


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Catalyst Properties Deactivation of catalyst Aging Poisoning Fouling or coking

Catalytic reaction involved: Physical adsorption Chemical adsorption


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Porous catalyst Why we need porous catalyst??? A few major catalyst: Amorphous silica High area aluminas Zeolites/aluminosilicates Carbon Supported noble metal catalysts (Pt, Pd, Ag,

Rh, Ni etc)


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Explanation of Catalytic Cracking through Zeolites- video

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Catalytic Reactors

Packed Bed Reactor Slurry reactor Fluidized bed reactor Riser reactor

Catalytic Reactors Packed bed Reactor Assume no mixing Mass balance:


rdzdC

u jj =

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Catalytic Reactors Slurry and Fludized Bed Reactor Assume well mixed Mass Balance:


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The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again.

The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again.

Example and Exercise in Class QUESTION 1 Tutorial 1 Example 7-1 (Schmidt) page 277 Problem 7-2 pg 318

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topic 1.1 - catalysis and catalytic reactors

Documents

chemical kinetics

reaction of x

catalytic reactors

activity of catalysts

negative catalysts

positive catalysts

flow reactor

theory of catalysis