chemical processes

CHEMICAL PROCESS INDUSTRIES

CHEMICAL PROCESS INDUSTRIES

CPI 201T-2013

CPI 201T-2013

Bsc Chem Eng, Msc, PhD Ind Chem (GERMANY),PhD Ind Chem (GERMANY),

MBL (UNISA)

Course Contents

Course Contents

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10. Environ

Environmental

mental Issues

Issues and Gr

and Green Che

een Chemistry

mistry

2

Course Contents

Course Contents

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Production of Materials

Production of Materials

3

3

•

• Humans have always exploited theirHumans have always exploited their natural natural environmenenvironment t 

for all their needs including food, clothing and shelter. for all their needs including food, clothing and shelter.

•

• As the cultural development of humans continued, theyAs the cultural development of humans continued, they

looked for a

looked for a greater variety greater variety of materials to cater for theirof materials to cater for their needs.

needs.

•

• The 20th century →The 20th century → explosionexplosion in both thein both the useuse of of 

traditional materials and in the

traditional materials and in the research for developmentresearch for development of a wider range of materials to satisfy technological

of a wider range of materials to satisfy technological developments.

developments.

•

• Major Factor:Major Factor: Reduction in availability Reduction in availability of the traditionalof the traditional

resources to supply the increasing world population. resources to supply the increasing world population.

Production of Materials

• Chemists and chemical engineers continue to play a

 pivotal role in the search for new sources for substitution of traditional materials such as those from the

 petrochemical industry.

•  As the fossil organic reserves dwindle, new sources of the

organic chemicals presently used are sought and 

• New materials (polymers, carbon-based composites) are

developed to replace those have been deemed no longer  satisfactory 

Industrial Chemistry

5

• Industry uses chemical reactions to produce chemicals

 for use by society.

• Many chemicals have been produced to replace naturally 

occurring chemicals that are no longer available or their  sourcing is not economically viable any more.

• Industrial chemical processes cover the full range of 

reactions but concentration on some case studies is

sufficient to illustrate the range of reactions and the role of chemists and chemical engineers involved in these

Industrial Chemistry

 some insight into the qualitative and quantitative aspects of the chemical industry 

 the evaluation of processes suitable and necessary for  efficient and environmentally benign production

• CPI 201T should help increase students’ understanding of 

the history, current condition and future applications of  these industries,

 very important for the economy of any country, which ultimately will rely on beneficiation of local and other   African-mined raw materials

Introduction

• The chemical process industry includes those

manufacturing facilities whose products result from: a) chemical reactions between organic materials, or 

inorganic materials, or both;

a) extraction, separation, or purification of a natural 

 product, with or without the aid of chemical reactions; a) the preparation of specifically formulated mixtures of 

materials, either natural or synthetic.

Introduction

Introduction

•

• Examples of products from the chemical Examples of products from the chemical processprocess

industry are: industry are:





 plastics, resins, dyes,

 plastics, resins, dyes,

 

 pharmaceuticals,

 pharmaceuticals,

 paints,

 paints,

soaps,

soaps,

detergents,

detergents,

 petrochemicals,

 petrochemicals,

 perfumes,

 perfumes,

Introduction

Introduction

•

• Examples of processes from the chemical processExamples of processes from the chemical process

industry are: industry are:

•

• Many of these processes involvMany of these processes involve a number of e a number of unit unit 

operations of chemical engineering depending on

operations of chemical engineering depending on the sizethe size definition of a plant,

definition of a plant,

•

• In addition, In addition, such basic such basic chemical chemical reactions (proreactions (processes) cesses) asas

-- polymerization, polymerization, - oxidation, - oxidation, - reduction, - reduction, - hydrogenation, and

- hydrogenation, and the like.the like.

9

Introduction

Introduction

•

•

What is industrial chemistry (CPI)?

What is industrial chemistry (CPI)?

The development, optimizati

The development, optimization and on and monitoring of monitoring of   fundamental

 fundamental chemical chemical processes processes used in industry used in industry for for  transformi

transforming raw ng raw materials and precursors intomaterials and precursors into useful commercial products for society.

useful commercial products for society.

•

•

Why is it relevant to you?

Why is it relevant to you?

Industrial

Industrial chemistry chemistry plays plays a vita vital role al role as an as an applied applied  science

science in diversin diverse areas e areas influencing influencing human human society society  ranging from

ranging from economic, environmenteconomic, environmental and al and   political s

Goals

• Goals we set to achieve with this course

 Define, describe, and apply basic chemical processes involved in the  production of major commercial products used in society.

 Develop critical skills at analyzing the cost / benefit / impact of  traditional industrial chemical processes on society as a whole.

 Appreciate the role and apply the concepts of green chemistry  for  efficient yet sustainable industrial chemical processes with low  impact on the environment and human health.

Introduction II

• Course strategy: Hints on how to succeed in this course

(CPI)? 

Try to attend every class on time and conscientiously  do assigned reading and problem sets.

This is particularly important as there are no textbooks  for this course

 Actively participate in class/group discussions.

Relate knowledge gained in class which can be applied 

to “real -world” problems.

Creative contributions to group project and   presentations.

Introduction II

• Course strategy: Hints on how to succeed in this course

(CPI)? 

During the course, compile a concise set of notes from lecture and material that includes basic principles and  equations of chemical analysis (useful for final exam).

Questions or doubts about the material being taught can be discussed in class, drop-by for a visit in my office or  send an e-mail message.

Working in groups for support throughout the term is very important.

But most important of all, do not get scared of the

material keep an open mind, relax and try to have fun! 

Introduction III

•

Textbooks

1. Chemical Process Technology ,

 J. Moulijn, M. Makkee, A. v. Diepen (2008) 2. Shreve’s Chemical Process Industries ,

5th Edition, G. T. Austin (1984) 3. Industrial Organic Chemistry,

4rth _{Edition, H.-J. Arbe (2010)} 4. Industrial Inorganic Chemistry,

K.-H. Büchel(2000)

5. Industrial Organic Chemicals,

2nd edition by H.A. Wittcoff, B.G. Reuben,J.S. Plotkin, Wiley-Interscience (2004).

Introduction III

•

Encyclopedias of Industrial Chemistry 

1.

Encyclopedia of Industrial Chemistry

Seventh Edition (2005)

2.

Kirk-Othmer  Encyclopedia of Chemical

Technology Sixth Edition (2006)

3.

Internet  Documents, Wikipedia etc.

Introduction III

Periodicals and Journals

ChemTech

Chemical & Engineering News

Chemical Week 

Industrial & Engineering Chemistry Research

Chemical Engineer 

Manufacturing Chemist 

Chemical Market Reporter 

Chemistry & Industry 

Chemie-Ingenieur-Technik 

17

Largest Chemical Companies In The World

2007

(by Turnover in Billion US$)

Inorganic Bulk Chemicals

•

Sulphuric Acid 

 –

Contact Process

•

Phosphoric Acid 

 –

Lurgi- Fisons Process

 Ammonia

 –

Haber-Bosch Process

•

Nitric Acid 

 –

Ostwald Process

Urea and Fertilizers

•

Sodium Hydroxide

 –

Chloralkali Process

Chlorine

 –

Chloralkali Process

Synthesis Gas Processes

•

Synthesis Gas Production

 –

Coal Gasification

 –

Steam Reforming of Methane

•

Water Gas Shift Reaction

•

Fischer-Tropsch Process (GTL technology)

Methanol Synthesis

•

Methanol Conversion to Chemicals

“The

Petroleum Refining

Petroleum Composition

Fractional Distillation

Hydrotreating

Thermal Cracking

Catalytic Cracking

Catalytic Reforming

Polymerization Processes

•

Fundamentals of Polymers industry

•

Precursors from Petrochemical Industry

Ethylene, propylene, vinyl chloride, styrene, butadiene ethyl terethalate, tetra fluorethylene, urethane

•

Catalytic Polymerisation

•

Addition Polymerisation (Chain Growth)

Radicals-induced Polymerisation

•

Polymer Properties

Transition Temperature, etc.)

Petrochemicals

• Chemical intermediates derived from petroleum

(NB: they can be obtained from other sources as well: coal, natural gas, biomass such as corn, sugar cane)

• Petrochemical classes:

- olefins (ethylene, propylene, butadiene etc.)

- BTX aromatics (benzene, toluene, xylene, styrene, etc.) - alkanes (methane, ethane, propane etc.)

• Industrial products from petroleum:

LPG (propane) for heating, liquid fuels (gasoline, diesel,

kerosene, lubricant, motor oils and greases, wax, sulphur, tar, asphalt, coke, solvents and monomers for polymerisation

Petrochemicals II

Organic Chemical Process Industries

• TNT (Explosives) - Tri nitro toluene • Paints and Varnish

• Phthalic anhydride ( poly ethylene terephthalate

- polyester)

• Soaps and detergents • Printing inks

• Synthetic fibers (polyester)

• Synthetic rubber (butadiene polymers)

• Various plastics (polyethylene, polypropylene, poly 

vinyl chloride, polystyrene, poly butadiene, poly  ethylene terethalate,  poly t etra f luor ethylene (Teflon), poly urethane – varnishes)

Organic Chemical Process Industries

Products derived from propylene

Organic Chemical Process

Industries

Products Derived From Benzene

• Story behind flowsheet 

27

• The operators of coking plants and one processing plant for tar 

benzene became the shareholders of Arsol Aromatics GmbH

• The company produces high quality aromatic raw materials

Story behind flowsheet

chemical raw materials mainly of crude benzene, which is a by product in coking plants of its

shareholders

• The chemical materials, which are manufactured at 

the Arsol Aromatics production plant, are called  aromatics.

• This name is the result of the characteristic aromatic

or perfume-like smell.

• The main parts of this group are benzene, toluene

and xylenes.

Story behind flowsheet

29

• Benzene is a colourless liquid occurring naturally in

 fossil raw materials such as crude oil and hard coal. It is a basic chemical in the manufacturing of a wide range of everyday items.

• Benzene has been attacked in the press as a

hazardous product (human carcinogen!!!)

• Benzene is used within the chemical industry to

 produce other chemicals, which are used to make consumer goods. All handling and application of  benzene must meet strict international standards to  protect the consumer from any risk.

Consumer Products from Benzene

• CD's, CD jewel boxes • toys

• engine oil surfactants

• video and audio cassettes

•  pharmaceuticals, medical devices • latex matresses

• housing insulation •  food packaging • detergents

•  phenolic resins for plywood  • safety helmets • automotive plastics • sports equipment  • tyres •  plastic glasses • computer housings

Many items taken for granted in our modern, everyday lives rely on products made by the aromatics industry.

Whether you are jogging around the block or

competing for the Olympics

 – aromatics are providing

31

•  Toluene is a colourless liquid, also deriving from crude oil or coal tar. Its major 

end-products are polyurethanes; these are very important for the production

of the foam used in furniture, mattresses, car seats, insulation for buildings, coatings for floors and furniture, and refrigerators.

• Furthermore Polyurethanes are also used for artificial sports track, jogging

shoes and in roller blade wheels

Examples:

1.foam for furniture and insulation, matresses, car seats 2.coatings for floors,

3.coatings for furniture and 4.coatings for refrigerators 5.dye carrier 

6. jogging shoes 7.carbonless paper  8.building insulation 9.roller blade wheels

Xylene is a colourless liquid deriving from crude oil or coal tar. There are several

chemical forms of xylene; among these, paraxylene is commercially the most

important. Paraxyleneis used to make polyesters, which have applications in

clothing, packaging and plastic bottles. The most widely-used polyester is

polyethylene terephthalate (PET), used in lightweight, recyclable soft drink bottles, as well as in fibres for clothing, and fillings for anoraks and duvets, in car tyre

cords and conveyor belts. It can also be made into a film used in video and audio tapes, as well as in x-ray films.

 Another chemical form of xylene, orthoxylene, is used to make pipes, coatings

and cables for medical application.

Examples:

1.conveyor belts 2.PET bottles

3.filling of anoraks and duvets 4.fibres for clothing and carpeting 5.video and audiotapes

6.cable coatings 7.x-ray

8.sports equipment 9.plastic pipes

Ce

ment, Glass, Dyes

Manufacturing

• Limestone (CaCO₃) with clay (Al₂O₃) and quarz (SiO₂)

• Heated together at 1450 oC (long Rotary Kiln) • Calcination  Klinker

• Chemically: CO₂ released and quicklime (CaO)

 formed, which reacts to calcium silicates

• Klinker milled very fine and gypsum (CaSO₄.1/2H₂O)

is added (less than 10%)

• Other ingredients are iron and magnesium oxides • Potland cement is obtained

• What is hydraulic cement?

Industrial Chemistry - Fundamentals

Chemical Reactions

Stoichiometry 

Reaction Yields

Thermochemistry 

Equilibrium

Equilibrium Constants

Kinetics

Rate Expressions

Temperature Effect 

Catalysis

Industrial Chemistry

Industrial Considerations

Reaction Evaluation

Selection

Economic Feasibility 

Thermodynamic Feasibility 

Kinetic Feasibility 

•

Chemical Plant Operation

Material Balance

Energy Flow 

Raw Materials

Safety 

37

Thermodynamic Considerations

39

Systems in which chemical 

reactions take place are called 

Reactors

Chemical Reaction Engineering

is the engineering activity

concerned with the application of 

chemical reactions on a

commercial scale

REACTORS occupy a

central role in every

chemical process

It is inside reactors that a

bulk of chemical

transformations take place

41 41





Three crucial questions:

Three crucial questions:

How fast do reactions occur? 

How fast do reactions occur? 

Chemical Kinetics

Chemical Kinetics

Maximum yields achievable

Maximum yields achievable? 

? 

Chemical Thermodynamics

Chemical Thermodynamics

Optimal Scale of op

Optimal Scale of operation?

eration?

Chemical Reactor Engineering

Chemical Reactor Engineering

Reactor Design & Operation

Reactor Design & Operation

•

•

Chemical reaction engineering involves the application of 

Chemical reaction engineering involves the application of 

basic chemical engineering principles to the analysis and 

basic chemical engineering principles to the analysis and 

design of chemical reactors.

design of chemical reactors.

•

•

Many of the operations in a chemical plant 

Many of the operations in a chemical plant 

support the

support the

chemical

chemical reactor

reactor..

•

•

Heat exchange, separations etc. may be

Heat ex

change, separations etc. may be used to pre-treat 

used to pre-treat 

the reactor feed and then to separate the reactor effluent 

the reactor feed and then to separate the reactor effluent 

into constituent parts.

into constituent parts.

•

•

 A complete understanding of reactor analysis require

 A complete unders

tanding of reactor analysis require

knowledge & understanding of all the

knowledge & understanding of all the basic chemical 

basic chemical 

engineering principles

engineering principles

..

Chemical reaction engineering

Chemical reaction engineering

43 43

•

•

In typical chemical processes the capital and operating

In typical chemical processes the capital and operating

costs of the reactor may be only 10 to 25% of the total,

costs of the reactor may be only 10 to 25% of the total,

with separation units dominating the size and cost of 

with separation units dominating the size and cost of 

the process.

the process.

•

•

Yet the performance of the chemical reactor totally 

Yet the performance of the chemical reactor totally 

controls the costs and modes of operation of these

controls the costs and modes of operation of these

expensive separation units, and thus the chemical 

expensive separation units, and thus the chemical 

reactor largely controls the overall economics of most 

reactor largely controls the overall economics of most 

 processes.

 processes.

Chemical reaction engineering

Chemical reaction engineering

•

Improvements in the reactor usually have enormous

impact on upstream and downstream separation

 processes.

•

IN REALITY 

•

We usually encounter an existing reactor that may have

been built decades ago, has been modified repeatedly,

and operates far from the conditions of initial design.

Very rarely we have the opportunity to design a reactor 

 from scratch.

45

CHALLENGES ?

The chemical engineer never encounters a single reaction in

an ideal single phase isothermal reactor.

Real reactors are extremely complex with multiple

reactions, multiple phases, and intricate flow patterns

within the reactor and in inlet and outlet streams.

An engineer needs enough information to understand the

basic concepts of reactions, flow, and heat management and

how these interact.

CHALLENGES II?

The chemical engineer almost never has kinetics for the process she or he is working on. The problem of solving the batch or continuous reactor mass-balance equations with known kinetics is much simpler  than the problems encountered in practice.

Reaction rates in useful situations are seldom known, and even if  these data were available, they frequently would not be particularly  useful.

Many industrial processes are mass-transfer limited so that reaction kinetics are irrelevant or at least thoroughly disguised by the effects of mass and heat transfer.

47

Questions of catalyst poisons and promoters, activation

and deactivation, and heat management dominate most

industrial processes.

Chemical Reactors and their

Reactor Concepts

 –

Fixed bed reactors

 –

Fluidized bed reactors

 –

Stirred tank reactors

 –

Slurry loop reactors

Fixed Bed Reactors

Summary Advantages/Disadvantages

 – High conversion is possible

 – Large temperature gradients may occur

 – Inefficient heat-exchange

Chemical Reactors and their Applications

Fixed Bed Reactors

Concept

 – Collection of fixed solid

particles.

 – The particles may serve as a

catalyst or an adsorbent.

 – Continuous gas flow  – (Trickling liquid)

Applications

 – Synthesis gas production  – Methanol synthesis

 – Ammonia synthesis

 – Fischer-Tropsch synthesis  – Gas cleaning (adsorption)

Fixed Bed Reactors

Challenges/Limitations

 –

Temperature control 

 –

Pressure drop

Chemical Reactors and their Applications

Fixed Bed Reactors

Single-Bed Reactor

 – All the particles are located in a

single vessel

Advantages/Disadvantages

 – Easy to construct  – Inexpensive

 – Applicable when the reactions are

Fixed Bed Reactors

Multi-Bed Reactor

 – Several serial beds with

intermediate cooling/heating stages

Advantages/Disadvantages

 – Applicable for exo-/endothermic

Chemical Reactors and their

Fixed Bed Reactors

SO₃reactor NH₃reactor

Fixed Bed Reactors

Multi-Tube Reactor

 – Several tubes of small diameter

filled with particles.

Advantages/Disadvantages

 – Expensive

 – High surface area for heat exchange

 _{Very good very temperature}

control

 – Applicable for very

Chemical Reactors and their

Fixed Bed Reactors

Steam reformer

Reactor height: 30 m

Number of tubes: 40-10000

Tube length: 6-12 m

Fluidized Bed Reactors

Concept

 – Collection of solid particles dispersed in

a continuous phase.

 – The particles may serve as a catalyst,

adsorbent or a heat carrier.

 – Continuous flow of gas or liquid

Applications

 – Catalytic cracking processes  – Fischer-Tropsch synthesis  – Polymerization

Chemical Reactors and their

HETEROGENEOUS CATALYSIS

WHY IS IT IMPORTANT

•

27 % of GDP and 90 % of chemical

industry involve products made using

catalysts (food, fuels, polymers, textiles,

pharma/agrochemicals,etc)

•

For discovery/use of alternate sources

of energy/fuels/ raw materials for

chemical industry.

•

For Pollution control - Global warming

For preparation of new materials

(organic & inorganic-eg: Carbon

Nanotubes).

Catalysis - Multidisciplinary

•

The catalyst is an inorganic solid;

Catalysis is a surface phenomenon;

•

Solid state and surface structures play important 

roles.

•

 Adsorption , desorption and reaction are subject to

thermodynamic, transport and kinetic controls(chem.

engineering);

•

adsorbate-substrate and adsorbate - adsorbate

interactions are both electrostatic and chemical 

(physical chemistry).

63

Catalysis - Base for

Green Chemistry

•

Pollution control(air and waste streams; stationary 

and mobile)

•

Clean oxidation / halogenation processes using

oxygen, hydrogen peroxide(C 

H

O, C 

H

O, ECH)

•

 Avoiding toxic chemicals in industry ( HF, COCl 

etc.)

Catalysis Basis

of Nanotechnology

•

Methods of catalyst preparation: are most suited for 

the preparation of nanomaterials

•

Nano dimensions of catalysts.

•

Common preparation methods.

•

Common Characterization tools.

•

Catalysis in the preparation of carbon

nanotubes.ollution control(air and waste streams;

stationary and mobile)

65

Catalysis-Milestones in Evolution

• 1814- Kirchhoff : starch to sugar by acid.

• 1817 - Davy : coal gas(Pt,Pd selective but not  Cu,Ag,Au,Fe)

• 1820s - Faraday : H₂ + O₂  H

2O (Pt); C 2H4 and S

• 1836 - Berzelius coins the name: ”Catalysis”;

• 1860- Deacon’s Process 2HCl + 0.5O₂  H

2O + Cl 2

• 1875- Messel : SO₂ + O₂  SO

3(Pt);

• 1880- Mond  CH₄+H₂O  CO+3H

2 (Ni);

• 1902- Ostwald : 2NH₃+2.5O₂  2NO+3H

2O(Pt);

• 1902- Sabatier : C ₂H₄+H₂  C 

2H6 (Ni).

• 1905- Ipatieff: Clays for acid catalysed reactions;

Catalysis-Milestones (con'd)

• 1910-20: NH₃ synthesis ( Haber,Mittasch )

• 1920-30: Methanol synthesis (ZnO-Cr ₂O₃ ) BASF  ;

• 1920-30: Taylor (active sites); BET (surface area)

• 1930: Langmuir-Hinshelwood & Eley -Rideal models ;

• 1930: Fischer - Tropsch synthesis

• 1930-50: Process Engg; FCC / alkylates;acid-base catalysis;Reforming and Platforming.

• 1950-70: Role of diffusion; Zeolites, Shape Selectivity; Bifunctional cata;oxdn cat-HDS; Syngas and H₂ generation.

67

Catalysis-Milestones (con'd)

•

1990 - Today:

•

 Assisted catalyst design using :



surface chem of metals/oxides, coordination

chemistry 



kinetics, catalytic reaction engineering



novel materials (micro/mesoporous materials)

Catalysis in the Chemical Industry

•

Hydrogen Industry (coal, NH

 , methanol, FT,

hydrogenations / HDT, fuel cell).

•

Natural gas processing (SR, ATR, WGS, POX)

Petroleum refining (FCC, HDW, HDT, HCr, REF)

Petrochemicals(monomers,bulk chemicals).

•

Fine Chemicals (pharma, agrochem, fragrance,

textile,coating,surfactants,laundry etc)

69

Definition of a Catalyst

•

Catalyst is a substance that increases the rate of the

reaction at which a chemical system approaches

equilibrium , without being substantially consumed in

the process

•

 A Catalyst affects only the rate of the reaction,



i.e.

the Kinetics.

•

It changes neither the thermodynamics of the reaction

nor the equilibrium composition

Definition of a Catalyst

•

It changes neither the thermodynamics of the reaction

nor the equilibrium composition

•

Thermodynamics says NOTHING about the rate of a

reaction.

•

Thermodynamics : Will a reaction occur ? 

•

Kinetics

: If so, how fast ? 

 A reaction may have a large, negative

occurring.

G

 , but the

rate may be so slow that there is no evidence of it 

71

Definition of a Catalyst

• Example

Conversion of graphite to diamonds is a thermodynamically favored process (

C

C

Kinetics makes this reaction nearly impossible

(Requires a very high pressure and temperature over  long time)

• Conclusion:

 A reaction may have a large, negative D_G

rxn , but the rate

may be so slow that there is no evidence of it occurring. G negative).

Example of a Catalytic Reaction

Conversion hydrogen and oxygen to water 

•

H

+0.5O

H

O;

G

= -58 Kcal/mol;

In the gas phase:

Dissociation energies

•

D(H-H) = 103 Kcal/mol ; D(O-O)=117 Kcal/mol;

E

~ 10 Kcal/mol for H+O

or H

+O

HO

or

H

O.

Hence,

kinetically

gas-phase reaction improbable.

Catalytic reaction

Pt forms Pt-H and Pt-O bonds with E

~ 0;Moreover,

Pt-H + Pt-O

Pt-OH

Pt -OH

has E

~ 0 .

73

Reaction path for conversion of A + B into AB

Activation Energy

Activation Energy : The energy required to overcome the reaction

barrier. Usually given a symbol E _aor ∆G≠

The Activation Energy (Ea) determines how fast a reaction occurs, the higher   Activation barrier, the slower the reaction rate. The lower the Activation

75

Catalyst lowers the activation energy for both forward and reverse reactions.

Activation Energy

This means , the catalyst changes the reaction path

by lowering its activation energy and consequently

the

catalyst increases the rate of reaction.

77

How a Heterogeneous Catalyst works ?

Absorption and Adsorption

79

Adsorption

In physisorption

1. The bond is a van der Waals interaction 2. adsorption energy is typically 5-10

kJ/mol. ( much weaker than a typical chemical bond )

3. many layers of adsorbed molecules may  be formed.

Adsorption

For Chemisorption

1. The adsorption energy is

comparable to the energy of a chemical bond.

2. The molecule may chemisorp intact (left) or it may dissociate (right). 3. The chemisorption energy is 30-70

kJ/mol for molecules and 100-400 kJ/mol for atoms.

81

Characteristics of Chemi- and Physisorptions

d E(d) physisorption atomic chemisorption  physisorption/ desorption chemisorption CO Physisorption Chemisorption small minima large minima

weak Van der Waal formation of surface attraction forces chemical bonds

Adsorption and Catalysis

Adsorbent: surface onto which adsorption can occur.

example: catalyst surface, activated carbon, alumina

Adsorbate: molecules or atoms that adsorb onto the substrate.

example: nitrogen, hydrogen, carbon monoxide, water 

Adsorption: the process by which a molecule or atom adsorb onto a surface of  substrate.

Coverage: a measure of the extent of adsorption of a specie onto a surface

H H H H H H H H H adsorbate

adsorbent

H H H H H

83

Adsorption Mechanisms

Langmuir-Hinshelwood mechanisms:

1. Adsorption from the gas-phase 2. Desorption to the gas-phase

3. Dissociation of molecules at the surface 4. Reactions between adsorbed molecules

Two Questions:

• Is the reaction has a Langmuir-Hinshelwood mechanism? • What is the precise nature of the reaction steps?

Cannot be solved

Example

The Reaction A₂ + 2B = 2AB

may have the following mechanism A₂ + * = A₂* A₂* + * = 2A* B + * = B* A* + B* = AB* + * AB* = AB + *

Langmuir-Hinshelwood mechanisms

85

Adsorption Mechanisms

Eley-Rideal mechanism:

1. Adsorption from the gas-phase 2. Desorption to the gas-phase

3. Dissociation of molecules at the surface 4. Reactions between adsorbed molecules

5. Reactions between gas and adsorbed molecules

The last step cannot occur in a Langmuir-Hinshelwood mechanism

Example

The reaction A₂ + 2B = 2AB

may have the following Eley-Rideal mechanism A₂ + * = A₂*

A₂* + * = 2A* A* + B = AB + *

where the last step is the direct reaction between the adsorbed molecule A* and the gas-molecule B.

87

Eley-Rideal or Langmuir-Hinshelwood?

For the Eley-Rideal mechanism:

the rate will increase with increasing coverage until the surface is completely covered by A*.

For the Langmuir-Hinshelwood mechanism:

the rate will go through a maximum and end up at zero, when the surface is completely covered  by A*.

This happens because the step B + * = B* cannot proceed when A* blocks all sites. The trick is that the step B + * = B*

Catalyst Preparation

Usually very active catalyst that do not require high surface area

e.g., Iron catalyst for ammonia production (Haber process)

(2) Supported Catalyst

requires a high surface area support to disperse the primary catalyst

the support may also act as a co-catalyst (bi-functional)

or secondary catalyst for the reaction (promoter)

89

Supported Catalyst

 Nickel clusters

SiO₂

Molecules in Zeolite Cages and Frameworks

+ p-xylene

ZSM-5

Y-zeolite Paraffins

91

What is ZSM-5 Catalyst ?



It is an abbreviation for (Zeolite Scony Mobile Number 5 )

First synthesized by Mobil Company in 1972



It replaces many Homogeneous Catalysts were used in

many petrochemical processes



ZSM-5 has two diameters for its pores : d

= 5.6 Å , d

= 5.4 Å

Where as, Zeolite Y has a diameter = 7.4 Å

Different Zeolite Catalysts



ZSM-5 has two diameters for its pores : d

= 5.6 Å , d

= 5.4 Å

Where as, Zeolite Y has a diameter = 7.4 Å