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June 5, 2008 FILS – Chemical ReactorsPreparatory Notions Revisited

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Chemical ReactorsChemical Reactors

Vasile Lavric, ProfessorVasile Lavric, Professor

Chemical Engineering Department


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BIBLIOGRAPHYBIBLIOGRAPHY

1. Octave Levenspiel, Chemical Reaction Engineering, Third Edition, John Wiley & Sons, New York, 1999

2. Ronald W. Missen, Charles A. Mims, Bradley A. Saville, Introduction to Chemical Reaction Engineering and Kinetics, John Wiley & Sons, New York, 1999

3. E. Bruce Nauman, Chemical Reactor Design, Optimization, and Scale-up, McGraw-Hill, New York, 2002

4. Lanny D. Schmidt, The Engineering of Chemical Reactions, Oxford University Press, New York, 1998



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BIBLIOGRAPHY (Cont.)BIBLIOGRAPHY (Cont.)

5. Holland, C.D., Anthony, R.G., Fundamentals of Reaction Engineering, Prentice-Hall Inc., Englewood Cliffs, 1979.

6. Froment, G.F., Bischoff, K.B., Chemical Reactor Analysis and Design, John Wiley & Sons, New York, 1979.

7. Lee, H.H., Heterogeneous Reactor Design, Butterworth Publishers, Boston, 1985.

8. Levenspiel, O., The Chemical Reactor Omnibook+, OSU Book Stores Inc., Corvallis, 1984.

9. Danckwerts, P.V., Gas-Liquid Reactions, McGraw-Hill, New York, 1970.10.Denbigh, K.G., Turner, J.R., The Chemical Reactor Theory - an Introduction,

2nd edition, Cambridge Univ. Press, 1971.11.Hill, C.G. jr., An Introduction to Chemical Kinetics and Reactor Design, John

Wiley & Sons, New York, 1977.12.Tarhan, M.O., Catalytic Reactor Design, McGraw-Hill, New York, 198313.Lavric, V., Chemical Reactors, 1996 (UPB lithography)



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ExaminationExaminationWritten test – multiple choices, five variants to select from; free access to all

documentation

The rules of grading:A) no answer checked, Zero points;B) all answers checked, Zero points;C) for the rest, the following formula applies:

where g stands for the number of good answers checked, pg stands for the weight fraction of the answer g and b stands for bad answers checked.

Examples:

C.1 in all, three good answers (⅓ weight each), all checked: since no

bad answer is checked;

C.2 two good answers and one bad:

C.3 no good answer and one bad answer: Zero


1gg p

Pb

13 3 10 1

P

12 131 1 3

P


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Outline of the Course:Outline of the Course:

1. Introductory notions revisited

2. Flow through chemical reactors – characterization and

influence

3. Homogeneous isothermal reactors – ideal and real

flow

4. Thermal behavior of chemical reactors



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Definition:

The Chemical Reactor is the confined space where a chemical process develops, resulting in transformation of some species (reactantsreactants) in others (productsproducts), under strict observation of the mass conservation laws.


Every industrial chemical process aims towards economicallyeconomicallyfabricating a desired productdesired product from a variety of starting materials through a succession of treatment steps, which can be abstracted in a general processing flowsheet.

The Hybrid Chemical Reactor, ensures in situ separation of the valuable product or intermediate, not only lowering the overall production costs, but increasing the process rate due to higher chemical driven forces.


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The flowsheet of a typical chemical process


Products

Row materials

Pre-chemical processing physi-cal treatment steps

Chemical processing stages

Post-chemical processing physical treat-ment steps


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Design of the chemical reactor - many alternatives can be proposed for a process.

Optimum solution – concerns not just the chemical reactor.

One design may have low chemical reactor cost, but the materials leaving the unit may be such that their treatment requires much higher costs.

The economics of the over-all process must be considered


Predictions not suffi-ciently close to reality

Abstraction

RealityThe process to be known

and studied

Mathematical ModelThe “reality” as we are able to predict it

Physical ModelThe “reality” as we

are able to grasp

Simplifying Assumptions

Boundary Conditions

Improvement


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Information used in chemical reactor modeling


Automation & Control

Thermo-dynamics

Stoichiometry & Ki-

netics

Balance equations

Chemical Reactor

Modeling

Momentum transport

Heat transport

Mass transport

Phases flow


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Classification of chemical reactions useful in reactor modeling


Oxidation of SO2 to SO3Reduction of iron ore toiron and steel

Cracking of crude oilGas-liquid absorption with

reaction

Attack of solids by acids

Oxidation of ammonia to produce nitric acid

Roasting of ores

Ammonia synthesisBurning of coal

Hetero-geneous

Enzyme and microbial reactions

Reactions in colloidal systemsFast reactions such as burning of a flame

Most liquid-phase reactionsMost gas-phase reactions

Homo-geneous

CatalyticNon-catalytic


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Classification of chemical reactions by stoichiometry

• single

• multiple

• series

• parallel

• competitive

• side – by – side

• mixed

• polymerization


1 2k kA R S

1

2

k

k

A R

A S

1

2

k

k

A R

B S

1

2

k

k

A B R

R B S


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Elementary reactionsElementary reactions - individual reaction steps that are

caused by collisions of molecules

• mono, bi and tri-molecular processes

• the rate equation corresponds to a stoichiometric equation

• when no correspondence between stoichiometry and rate exists, the reaction is sad to be non-elementary.

Elementary vs. NonElementary vs. Non--elementary elementary Reactions Reactions


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• chemical species are denoted by Aj, where j=1,,S with S being the number of all chemical species involved in the chemical process;

• stoichiometric coefficients are denoted by j, for a single reactions and ij for multiple reactions, with i=1,,R, where R stands for the total number of chemical reactions between the S chemical species;

• the stoichiometric coefficients for products are positive, while for reactants are negative, to show the progression of the chemical process

Elements of systematic Elements of systematic stoichiometrystoichiometry

Single reactions 1

0S

j jj

A

Multiple reactions 1 1

0R S

i j ji j

A

0A rank


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Measuring the progress of a chemical process - quantifying the degree of participation of a molecular species in this chemical process

• intensive units - normalized value, independent of the starting point• extensive units/variables – their values will depend upon the initial quantity

of each and every species participating at the chemical process

As intensive variable, the conversion is the widest used;As extensive variable, the degree of advancement or the reaction extent are the

widest used.

The rule of thumb advocates that, for a single reaction chemical process, conversion is the appropriate measure of transformation, while the degree of advancement should be used for all other cases.

Measuring the progress of the Measuring the progress of the chemical processchemical process


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The conversionSingle reaction chemical process


1

0S

j jj

A

d

r

k

ka A b B qQ s S

The degree of transformation for each reactant species is given bythe fractional conversions

0 0

0 0

;A A B BA B

A B

N N N NX X

N N

0

0

AB A

B

NbX X

a N

0 1A A AN N X

0B A BA A

bN N M X

a

0Q A QA A

qN N M X

a

0S A SA A

sN N M X

a

0

0

jA

jAA

NM

N

Limiting reactant


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The degree of advancement / The extentSingle reaction chemical process


1

0S

j jj

A

0 ; 1, 2, ,j jA A

j

N Nj S

0j jA A jN N

Multiple reaction chemical process

0

; 1, 2, ,j jA A

ii

ij

N Ni R

0

1j j

R

A A ij ii

N N

01 1

S R

T ij ij i

N N

1 1

0R S

i j ji j

A


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Molar concentration

Single reaction chemical process – constant volume

Composition of the chemical mixtureComposition of the chemical mixture

Single reaction chemical process – variable volume

j

j

A

A

NC

V

0j

jA A jA AC C M X

a

0

00 1 1j

j jA jA A jA A

A AA A A A

N M X M Xa aC CV x X

0 1

00 1

0 1 0

1

for gas phase reactions

for liquid phase reactions

A A

AA A

A A A

A

X X

XX XA

X X X

X

N N

NV V

V


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Molar fraction

Single reaction chemical process

Composition of the chemical mixtureComposition of the chemical mixturej

j

A

AT

Ny

N

0

01 1

j

j jA jA A jA A

A S Sj j

A jA A jA Aj j

N M X M Xa ay

N M X M Xa a

Multiple reaction chemical process

0 01 1

01 1 1 1

1

j j

j

R R

A ij i A ij M ii i

A S R S R

ij i ij M ij i j i

N yy

N


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ThermodynamicsThermodynamics• the heat liberated or absorbed during the chemical process

• the maximum possible extent of reaction

ThermodynamicsThermodynamics

0 0

1

lnj

S

j Aj

G G RT K

,1

j

S

p j p Aj

C C


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Chemical kineticsChemical kinetics• concerned with the rates and mechanisms of chemical reactions;

• search for the factors that influence the rate of reaction;

• tool for: gaining insight into the reacting systems nature, understanding how

chemical bonds are made/broken, and estimating their energies and stability;

• the mode of reaction of compounds provides clues to their structure;

• the basis for important theories in combustion and dissolution;

• permit satisfactory design of equipments to effect the reactions on a technical

scale

• in polymerization processes: the stoichiometry and kinetics are complex. A

polymer is always a mixture of macromolecules having different chain lengths -

statistics can be used, to simplify the mathematical treatment, lumping the infinite

number of conservation equations (polymers and/or free radicals) into an easier

mathematical model, focused on several characteristics of the molecular mass

distribution, like mean, dispersion and symmetry;

Chemical kineticsChemical kinetics


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Definition of reaction rateDefinition of reaction rate


moles of which modify during chemical process 1

volume of fluid timej

j

A jA

dN Ar

V dt

V can be replaced by A (aria), W (mass of catalyst), Vw (volume of catalyst), Vr (volume of \reactor)

volume mass of surface volume volume

of fluid solid of solid of solid of reactorj j j j jA A A A Ar r r r r

For complex reactions - the equivalent reaction rate for every reaction

1 1 1, 1,2, ,jA i

iij i

dN dr i R

V dt V dt


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The change in moles of any species is, for a single reaction


j

j

A

j A

dNV r V r

dt

The change in moles of any species is, for multiple reactions

1 1

j j

j

R RA A

ij i Ai i

i

dN dNV r V r

dt dt


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Equilibrium of Elementary Reversible Reactions


,d

r

k

CkA B Q S K

forward d A Br k C C reverse r Q Sr k C C

, , 0S forward S reverser r Equilibrium

d R SC

r A B

k C CK

k C C

Molecularity and Order of Reaction Homework

Temperature dependent term of a rate equation Homework


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Chemical Processes KineticsChemical Processes Kinetics

Searching for a mechanism (Lavric, 1996)

homework

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