chemical processes

CHEMICAL PROCESS INDUSTRIES CPI 201T-2013 Lecture 2 By

Dr Alex Sofianos Bsc Chem Eng, Msc, PhD Ind Chem (GERMANY), MBL (UNISA)

Course Contents 1. Introduction 2. Catalysis 3. In Inor orga gani nicc Bul Bulk k Comm Commod odit ity y Chem Chemic ical alss 4. Synt ynthesi hesiss Gas Gas Pr Proc oce esses ses 5. Petroleum Re Refining 6. Po Poly lyme meri risa sati tion on an and d Petr Petroc oche hemi mica cals ls 7. Org Organ anic ic Ch Chem emic ical al Proc Proces esss Ind Indus ustr trie iess 8. Ceme Cement nt,, Glas Glass, s, Dyes Dyes Manu Manuffac actu turi ring ng 9. Hydr Hydrom ome etallu allurrgic gical Proc Proces esse sess 10. Environmental Environmental Issues and Green Green Chemistry Chemistry

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Course Contents 1. Introduction 2. Catalysis 3. In Inor orga gani nicc Bul Bulk k Comm Commod odit ity y Chem Chemic ical alss 4. Synt ynthesi hesiss Gas Gas Pr Proc oce esses ses 5. Petroleum Re Refining 6. Po Poly lyme meri risa sati tion on an and d Petr Petroc oche hemi mica cals ls 7. Org Organ anic ic Ch Chem emic ical al Proc Proces esss Ind Indus ustr trie iess 8. Ceme Cement nt,, Glas Glass, s, Dyes Dyes Manu Manuffac actu turi ring ng 9. Hydr Hydrom ome etallu allurrgic gical Proc Proces esse sess 10. Environmental Environmental Issues and Green Green Chemistry Chemistry

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Production of Materials •

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environment  Humans have always exploited their natural environment  for all their needs including food, clothing and shelter. As the cultural development of humans continued, they looked for a greater variety of materials to cater for their needs. The 20th century → explosion in both the use of 

traditional materials and in the research for development of a wider range of materials to satisfy technological developments. Major Factor: Reduction in availability of the traditional resources to supply the increasing world population. 3

Production of Materials •

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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 

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Industrial Chemistry •

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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  processes. 5

Industrial Chemistry •

This study of some case studies would allow:

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 •

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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. 7

Introduction •

Examples of products from the chemical process industry are:  plastics, resins, dyes,  pharmaceuticals,  paints, soaps, detergents,  petrochemicals,  perfumes, Inorganics (fertilizers)

synthetic organic synthetic

materials. 8

Introduction •

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Examples of processes from the chemical process industry are: Many of these processes involve involve a number of unit  operations of chemical engineering depending on the size definition of a plant, In addition, such basic chemical reactions (processes) (processes) as - polymerization, - oxidation, - reduction, - hydrogenation, and the like.

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Introduction •

What is industrial chemistry (CPI)? The development, optimization optimization and monitoring of   fundamental chemical processes used in industry for  transforming transforming raw materials and precursors into useful commercial products for society.

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Why is it relevant to you? Industrial chemistry plays a vital vital role as an applied  science in diverse diverse areas influencing human society  ranging from economic, environmental environmental and   political stability stability through job creation 10

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. 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.

 Appreciate

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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. 12

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!  13

Introduction III •

Textbooks 1. Chemical Process Technology ,

2. 3. 4. 5.

 J. Moulijn, M. Makkee, A. v. Diepen (2008) Shreve’s Chemical Process Industries , 5th Edition, G. T. Austin (1984) Industrial Organic Chemistry, 4rth Edition, H.-J. Arbe (2010) Industrial Inorganic Chemistry, K.-H. Büchel(2000) Industrial Organic Chemicals, 2nd edition by H.A. Wittcoff, B.G. Reuben,J.S. Plotkin, Wiley-Interscience (2004). 14

Introduction III •

Encyclopedias of Industrial Chemistry 

1.

Ullmann’s Encyclopedia of Industrial Chemistry Seventh Edition (2005)

2.

Kirk-Othmer  Encyclopedia of Chemical Technology Sixth Edition (2006)

3.

Internet D   ocuments, Wikipedia etc.

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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   Applied Catalysis 16

Largest Chemical Companies In The World 2007 (by Turnover in Billion US$)

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Inorganic Bulk Chemicals •

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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 Soda Ash – Solvay Process 18

Synthesis Gas Processes Synthesis Gas Production

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Coal Gasification

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Steam Reforming of Methane

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Water Gas Shift Reaction

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Fischer-Tropsch Process (GTL technology)

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Methanol Synthesis

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Methanol Conversion to Chemicals “The Methanol Economy”  19

Petroleum Refining •

Petroleum Composition

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Fractional Distillation

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Hydrotreating

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Thermal Cracking

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Catalytic Cracking

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Catalytic Reforming

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Down Stream Operations 20

Polymerization Processes •

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Fundamentals of Polymers industry Precursors from Petrochemical Industry Ethylene, propylene, vinyl chloride, styrene, butadiene ethyl terethalate, tetra fluorethylene, urethane

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Catalytic Polymerisation Addition Polymerisation (Chain Growth) Radicals-induced Polymerisation Polymer Properties (MW, Crystallinity, Glass Transition Temperature, etc.) 21

Petrochemicals •

Chemical intermediates derived from petroleum

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

coal,

Petrochemical classes: - olefins (ethylene, propylene, butadiene etc.) - BTX aromatics (benzene, toluene, xylene, styrene, etc.) - alkanes (methane, ethane, propane etc.)

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Industrial products from petroleum:

LPG (propane) for heating, liquid fuels (gasoline, diesel, kerosene, lubricant, motor oils and greases, wax, sulphur, asphalt, coke, solvents and monomers for polymerisation

tar,

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Petrochemicals II

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Organic Chemical Process Industries •

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 Adipic acid ( precursor of Nylon) 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) 24

Organic Chemical Process Industries Products derived from propylene

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Organic Chemical Process Industries Products derived from benzene

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Products Derived From Benzene •

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Story behind flowsheet 

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 such as benzene, toluene, xylenes and arsol ( a solvent) 27

Story behind flowsheet •

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The Arsol Aromatics GmbH is manufacturing 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. They are used as raw material for many different  goods of consumption. 28

Story behind flowsheet •

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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. 29

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 hosiery 

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 you with state-of-the art 30 equipment

Consumer Products from Toluene •

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 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 31

Consumer Products from Xylene 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. Paraxylene is 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 10.cables 32

Cement, Glass, Dyes Manufacturing •

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

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Heated together at 1450 oC (long Rotary Kiln)

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Calcination  Klinker •

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Chemically: CO2 released and quicklime (CaO)  formed, which reacts to calcium silicates

Klinker milled very fine and gypsum (CaSO 4.1/2H2O) is added (less than 10%)

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Other ingredients are iron and magnesium oxides

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Potland cement is obtained

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What is hydraulic cement? 33

Flow Sheet of a Process

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Industrial Chemistry - Fundamentals •

Chemical Reactions

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Stoichiometry 

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Reaction Yields

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Thermochemistry 

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Equilibrium Equilibrium Constants LeChatelier’s Principle Kinetics Rate Expressions Temperature Effect  Catalysis 35

Industrial Chemistry Industrial Considerations •

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Reaction Evaluation Selection Economic Feasibility  Thermodynamic Feasibility  Kinetic Feasibility 

Chemical Plant Operation Material Balance Energy Flow  Raw Materials Safety  Pollution 36

Thermodynamic Considerations

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Thermodynamic Considerations

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Reactor Choice Considerations 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

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Reactor Choice Considerations

REACTORS occupy a central role in every chemical process It is inside reactors that a bulk of chemical transformations take place 40

Reactor Design & Operation 

Three crucial questions: How fast do reactions occur? 

Chemical Kinetics Maximum yields achievable?  achievable? 

Chemical Thermodynamics Optimal Scale of operation? op eration?

Chemical Reactor Engineering 41

Chemical reaction engineering •

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Chemical reaction engineering involves the application of  basic chemical engineering principles to the analysis and  design of chemical reactors. Many of the operations in a chemical plant  – support the chemical reactor. reactor. Heat exchange, exchange, separations etc. may be used to pre-treat  the reactor feed and then to separate the reactor effluent  into constituent parts.  A complete understanding understanding of reactor analysis require –  knowledge & understanding of all the basic chemical  engineering principles . 42

Chemical reaction engineering •

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In typical chemical processes the capital and operating costs of the reactor may be only 10 to 25% of the total, with separation units dominating the size and cost of  the process.

Yet the performance of the chemical reactor totally  controls the costs and modes of operation of these expensive separation units, and thus the chemical  reactor largely controls the overall economics of most   processes. 43

Chemical reaction engineering •

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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. 44

Chemical reaction engineering 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. 45

Chemical reaction engineering 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. 46

Chemical reaction engineering

Questions of catalyst poisons and promoters, activation and deactivation, and heat management dominate most industrial processes.

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Chemical Reactors and their Applications

Reactor Concepts  –

Fixed bed reactors

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Fluidized bed reactors

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Stirred tank reactors

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Slurry loop reactors

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Bubble columns

Chemical Reactors and their

Fixed Bed Reactors Summary Advantages/Disadvantages  –

High conversion is possible

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Large temperature gradients may occur

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Inefficient heat-exchange

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Suitable for slow- or non-deactivating processes

Chemical Reactors and their

Fixed Bed Reactors Concept Collection of fixed solid particles. The particles may serve as a catalyst or an adsorbent. Continuous gas flow (Trickling liquid)

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Applications  –

Synthesis gas production

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Methanol synthesis

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Ammonia synthesis

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Fischer-Tropsch synthesis

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Gas cleaning (adsorption) Chemical Reactors and their Applications

Fixed Bed Reactors Challenges/Limitations  –

Temperature control 

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Pressure drop

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Catalyst deactivation

Chemical Reactors and their

Fixed Bed Reactors Single-Bed Reactor  –

All the particles are located in a single vessel

Advantages/Disadvantages  –

Easy to construct

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Inexpensive

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Applicable when the reactions are not very exo-/endothermic

Chemical Reactors and their Applications

Fixed Bed Reactors Multi-Bed Reactor  –

Several serial beds with intermediate cooling/heating stages

Advantages/Disadvantages  –

Applicable for exo-/endothermic reactions

Chemical Reactors and their Applications

Fixed Bed Reactors NH3 reactor SO3 reactor

Chemical Reactors and their

Fixed Bed Reactors Multi-Tube Reactor  –

Several tubes of small diameter filled with particles.

Advantages/Disadvantages  –

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Expensive High surface area for heat exchange  Very good very temperature control Applicable for very exo/endothermic reactions Chemical Reactors and their Applications

Fixed Bed Reactors Steam reformer

Chemical Reactors and their

Reactor height:

30 m

Number of tubes:

40-10000

Tube length:

6-12 m

Tube diameter:

70-160 mm

Fluidized Bed Reactors Concept  –

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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

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Fischer-Tropsch synthesis

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Polymerization

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Waste combustion

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Drying Chemical Reactors and their Applications

Fluidized Bed Reactors

Chemical Reactors and their

HETEROGENEOUS CATALYSIS

AN INTRODUCTION

WHY IS IT IMPORTANT •

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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 • •

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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). The chemical reaction is organic chemistry  62

Catalysis - Base for Green Chemistry •

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Pollution control(air and waste streams; stationary  and mobile) Clean oxidation / halogenation processes using oxygen, hydrogen peroxide(C 2H4O, C 3H6O, ECH)  Avoiding toxic chemicals in industry ( HF, COCl 2 etc.)

Fuel cells( H2 generation)

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Catalysis Basis of Nanotechnology •

Methods of catalyst preparation : are most suited for  the preparation of  nanomaterials

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Nano dimensions of catalysts.

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Common preparation methods.

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Common Characterization tools.

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Catalysis in the preparation of carbon nanotubes.ollution control(air and waste streams; stationary and mobile) 64

Catalysis-Milestones in Evolution • 1814- Kirchhoff : starch to sugar by acid. • 1817 - Davy : •

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coal gas(Pt,Pd selective but not  Cu,Ag,Au,Fe) 1820s - Faraday : H2 + O2  H2O (Pt); C 2H4 and S 1836 - Berzelius coins the name: ”Catalysis”; 1860- Deacon’s Process 2HCl + 0.5O2   H2O + Cl 2 1875- Messel : SO2 + O2   SO3 (Pt); 1880- Mond  CH4+H2O   CO+3H2 (Ni); 1902- Ostwald : 2NH3+2.5O2  2NO+3H2O(Pt); 1902- Sabatier : C 2H4+H2   C 2H6 (Ni). 1905- Ipatieff: Clays for acid catalysed reactions; isomerisation, alkylation, polymerisation. 65

Catalysis-Milestones (con'd) • 1910-20: • 1920-30:

• 1920-30: • 1930: • 1930: • 1930-50: • 1950-70:

• 1970:

NH3 synthesis ( Haber,Mittasch ) Methanol synthesis (ZnO-Cr 2O3 ) BASF  ; Taylor (active sites); BET (surface area) Langmuir-Hinshelwood & Eley -Rideal models ; Fischer - Tropsch synthesis Process Engg; FCC / alkylates;acid-base catalysis;Reforming and Platforming. Role of diffusion; Zeolites, Shape Selectivity; Bifunctional cata;oxdn cat-HDS; Syngas and H2 generation. Surface Science approach to catalysis ( Ertl   )

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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) new processes (Green Chemistry)

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Catalysis in the Chemical Industry •

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Hydrogen Industry  (coal, NH3 , 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) Environmental Catalysis (autoexhaust, deNOx, DOC)

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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 •

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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 Grxn , but the rate may be so slow that there is no evidence of it  occurring. 70

Definition of a Catalyst • Example

Conversion of graphite to diamonds is a thermodynamically favored process ( G negative).

C (graphite)

C (diamond)

Kinetics makes this reaction nearly impossible (Requires a very high pressure and temperature over  long time) Conclusion:  A reaction may have a large, negative DGrxn , but the rate may be so slow that there is no evidence of it occurring. •

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Example of a Catalytic Reaction Conversion hydrogen and oxygen to water 

H2+0.5O2 H2O; In the gas phase: •

G 0298 = -58 Kcal/mol;

Dissociation energies

D(H-H) = 103 Kcal/mol ; D(O-O)=117 Kcal/mol; E# ~ 10 Kcal/mol for H+O 2 or H2+O HO2 or H2O. Hence, kinetically gas-phase reaction improbable. Catalytic reaction •

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Pt forms Pt-H and Pt-O bonds with E# ~ 0;Moreover, Pt-H + Pt-O Pt-OH Pt -OH2 has E# ~ 0 . 72

Kinetic Vs. Thermodynamic

Reaction path for conversion of A + B into AB 73

Activation Energy Activation Energy : The energy required to overcome the reaction barrier. Usually given a symbol E a or ∆G≠

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

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Activation Energy

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

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Activation Energy

This means , the catalyst changes the reaction path by lowering its activation energy and consequently the catalyst increases the rate of reaction. 76

How a Heterogeneous Catalyst works ?

Substrate has to be adsorbed on the active sites of the catalyst

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Absorption and Adsorption H H H H

H H H H

H

H

H H H

H H H H

H HH H H H H H H H H

H2 adsorption on  palladium

Surface process

H H

H H H

H H

H

H2 absorption   palladium hydride

 bulk process

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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.

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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.

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Characteristics of Chemi- and Physisorptions E(ads)

<

Physisorption

E(d)

small minima weak Van der Waal attraction forces

E(ads) Chemisorption large minima formation of surface chemical bonds

CO  physisorption/ desorption chemisorption

physisorption

atomic chemisorption

d

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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

coverage q = fraction of surface sites occupied H

H

H

H

H

adsorbent

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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

without experimental or computational studies

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Langmuir-Hinshelwood mechanisms Example

The Reaction

A2 + 2B = 2AB

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

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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

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Eley-Rideal 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.

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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*

requires a free site. 87

Catalyst Preparation (1) Unsupported Catalyst 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)

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Supported Catalyst Highly dispersed metal on metal oxide  Nickel clusters

SiO2 89

Molecules in Zeolite Cages and Frameworks

+ p-xylene

ZSM-5

Paraffins Y-zeolite 90

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 : d1= 5.6 Å , d2= 5.4 Å

Where

as, Zeolite Y has a diameter = 7.4 Å

91

Different Zeolite Catalysts ZSM-5

has two diameters for its pores : d1= 5.6 Å , d2= 5.4 Å

Where

as, Zeolite Y has a diameter = 7.4 Å

92