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CHE594 Chemical Reaction Engineering

Dr. SALAM A. MOHD Phone Number:

Reference Fogler 5th edition, Levenspiel 3rd edition

• Classification of reactions

• Definition of reaction rate Elementary and non-elementary reaction

Molecularity and order of reaction

Kinetic models for non-elementary reaction

• Temperature dependent term of rate equation

TOPIC 1 Introduction to Chemical Reaction Engineering

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• What is a chemical reaction?

The change of a substance into a new substance which has new/different chemical identity

i.e acid + metal a type of salt

2HCl + 2Mg 2MgCl + H2

2HCl + 2Na 2NaCl +H2

Mg(OH)2 + 2HCl MgCl2 + 2H2O

changes in physical effect i.e. emission of heat, formation of precipitation, colour change, etc.

Raw material Product Chemical reaction

• In industrial chemical process, the typical situation of chemical process is shown below:

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• Generally, chemical reactions can be classified into homogeneous and heterogeneous reactions.

• Homogeneous reaction is a reaction that take place in one phase alone. i.e. reaction between 2 gases, 2 liquids, 2 solids

• Heterogeneous reaction is a reaction that that requires the presence of at least 2 phases (or more) to proceed at certain reaction rate. i.e. reaction between gas and liquid, gas and solid, liquid and solid

Classification of Reactions

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• Variables that affect rate of reaction

In homogeneous systems the temperature, pressure, and composition are obvious variables.

In heterogeneous, the problem becomes more complex. Material may have to move from phase to phase during reaction; hence, the rate of mass transfer can become important.

IDEAL REACTORS

Batch Mixed Flow

Reactor (MFR)

Plug Flow Reactor (PFR)

8

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• Simply put, reaction rates can be defined as speed of reactions.

• Some reactions can be very, very slow i.e. Sewage treatment plants

• Some reactions can be very, very fast i.e. Reactions in rocket engines

• The rate of a reaction can be expressed as the rate of disappearance of a reactant or as the rate of appearance of a product

Definition of Reaction Rate

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• Consider this reaction A → B

-rA = the rate of a disappearance of species A

rB = the rate of formation of species B

• By definition, the reaction rate is the increase in molar concentration of a product of a reaction per unit time.

(EQ 2)

(EQ 3)

• Consider this reaction

4 moles of A reacted with 8 moles of B to produce 4 moles of C and 4 moles of D

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• If the rate of change is in number of moles of component i due to reaction, , the rate of reaction in various forms can be defined: based on unit volume of reacting fluid

based on unit mass of solid in fluid-solid systems

(EQ 4)

(EQ 5)

based on unit interfacial surface in two-fluid systems or based on unit surface of solid in gas-solid systems

based on unit volume of solid in gas-solid systems

based on unit volume of reactor, if different from the rate based on unit volume of fluid

(EQ 6)

(EQ 7)

(EQ 8)

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In homogeneous systems the volume of fluid in the reactor is often identical to the volume of reactor. In such a case V and Vr are identical and Eqs. 4 and 8 are used interchangeably.

In heterogeneous systems all the above definitions of reaction rate are encountered, the definition used in any particular situation often being a matter of convenience.

From Eqs. 4 to 8 these intensive definitions of reaction rate are related by:

(EQ 9)

Consider a single reaction with stoichiometric equation

The rate of disappearance of A is given by

Such reaction is called elementary reaction

Elementary reactions: the rate of equation corresponds to a stoichiometric equations

H2+I22HI -rH2=k[H2][I2]

Elementary reaction

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When there is no direct correspondence between stoichiometry and rate, then we have non-elementary reactions. The classical example of a non-elementary reaction is that between hydrogen and bromine,

which has a rate expression

Non-elementary reactions: no direct correspondence between stoichiometry and rate

Non-elementary reaction

• The molecularity of an elementary reaction is the number of molecules involved in the reaction, and this has been found to have the values of one, two, or occasionally three.

• Note that the molecularity refers only to an elementary reaction.

• Let us say, materials A, B, . . . , D, can be approximated by an expression of the following type:

aA+bB+cC+dD product

The molecularity shows the power or the order of the reaction

Molecularity and Order of Reaction

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Reaction Order • You can tell the overall reaction order by the units of k

C A -r A

Reaction Order Rate Law k

(mol/dm 3 ) (mol/dm

3 *s) zero -r A = k (mol /dm3*s)

1st -r A = kC A s

-1

2nd -r A = kC A 2 (dm3 /mol*s)

Time

tion)(Concentra =k ofunit The

n-1

k is the specific reaction rate (constant)

• Elementary reactions are often represented by an equation showing both the molecularity and the rate constant. For example

The rate of equation is:

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• Consider this reaction

• Rate of equation that refers to B

• Rate of equation that refers to D

• Rate of equation that refers to T

• But from stoichiometry point of view, the equation will be

• Hence, A non-elementary reaction is one whose stoichiometry does not match its kinetics. For example,

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• Non-elementary reaction always involve intermediate and multiple reactions

• However, it is difficult to quantify the concentration of intermediate since it exists only for few minutes.

• Types of intermediate can be grouped into free radicals, ions and polar substances, molecules, transition complexes, non-chain reactions and chain reactions

• Single reaction – a single stoichiometric equation and single rate equation represent the progress of reaction

• Multiple reaction – more than one stoichiometric equation represent the observed changes and more than one kinetic expression needed to follow the changing composition of all reaction components

– Series reactions

– Parallel reactions

A R S

A R

S

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Single and Multiple Reaction

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• Testing of kinetic models

What is the possible reaction mechanism?

Kinetics Model for Non-elementary Reaction

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Since stoichiometry of A and B are symmetrical therefore just

interchange A and B in the model and set k2 = 0 and we will get:

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• Temperature dependency on Arrhenius Law For many reactions, and particularly elementary

reactions, the rate expression can be written as a product of a temperature-dependent term and a composition dependent term, or

ri = ƒ1 (temperaure) . ƒ2 (composition)

= k. ƒ2 (composition)

This is practically well presented by Arrhenius’ Law

Temperature Dependent Term of a Rate of Equation

• The rate of equation/ the rate law is an algebraic equation that depends on reacting materials and reaction conditions. It is independent of the type of reactor (batch or continuous).

• k is rate constant which is temperature dependent

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At the same concentration, but at two different temperatures, Arrhenius' law indicates that

Activation Energy • The activation energy can be thought of as a barrier to the

reaction.

• One way to view the barrier to a reaction is through the reaction coordinates. These coordinates denote the energy of the system as a function of progress along the reaction path.

•For the reaction

the reaction coordinate is

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For the reaction to occur, the reactants must

overcome an energy barrier EB. The energy barrier

is related to the activation energy, E. The barrier

height is a result of

(1) the molecules needing energy to distort or

stretch their bonds in order to break them and to

thus form new bonds,

(2) the reaction molecules come close together

they must overcome both steric and electron

repulsion forces, and

(3) the quantum effects that can in some cases

produce a barrier.

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• It can be concluded that 1. From Arrhenius' law a plot of ln k vs 1/T gives a straight line,

with large slope for large E and small slope for small E (slope = E/R).

2. Reactions with high activation energies are very temperature-sensitive; reactions with low activation energies are relatively temperature-insensitive.

3. k0 does not affect the temperature sensitivity.

• Activation Energy and Temperature Dependency

The temperature dependency of reactions is determined by the activation energy and temperature level of the reaction, as illustrated

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• Milk is pasteurized if it is heated to 63oC for 30 min,

but if it is heated to 74°C it only needs 15 s for the

same result. Find the activation energy of this

sterilization process.

EXERCISE

The rate is inversely proportional to the reaction time,

meaning that the rate is so:

Given,

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1. In the homogeneous gas-phase reaction, state the relationship between rCH4 and rO2.

2. For the reaction of , ,determine the rate law.

39

OHHCOOHOCH 2242

3

3222

3

2

1NHHN

EXERCISE