CHE594_C1W01

SITI WAHIDAH BINTI PUASA PHONE NO: 03-55436327

011-32338927 Reference: Fogler 4th Edition, Levenspeil

3rd Edition

CHE594

Chemical Reaction Engineering

TOPIC 1 INTRODUCTION

CHE594: CHEMICAL REACTION ENGINEERING

CHEMICAL REACTION??

A process in which at least one new substance is produced as a result of chemical change

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

Reaction occurred. How??

Color change

Formation of a solid (precipitate) within a clear solution

Evidence of a gas

Evolution or absorption of heat

Reaction type??

Combination reaction A + B → AB

Decomposition reactions AB → A + B

Displacement reactions AB + C → AC + B

Metathesis reactions (double-replacement reactions) AB + CD → AD + CB

Combustion reactions reaction with oxygen

A component/species can lose its chemical identity. HOW??

Decomposition Molecule is broken down into smaller molecules, atoms, fragment

Combination Molecule/atom to combine with other molecule/atom Benzene react with propylene to form cumene

Isomerization Molecules change in configuration i.e.

In industrial chemical process, the typical situation of chemical process is shown

below:

Based on unit volume of reacting fluid

Based on unit mass of solid in fluid-solid system

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

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

CONCENTRATION OVER TIME

RATE LAW/ RATE OF EQUATION

Reaction rate is defined as changes in concentration over time

rate dCi

Unit SI is mol L-1s-1

Reaction rate can be quantified in terms of disappearing reactant or appearing product

For relative rate of reactions, various species that involved in reaction can be obtained from stoichiometric coefficient:

aA +bB → cC + dD

Consider this reaction……..

A+ 2B → C + D

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

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.

HOMOGENEOUS HETEROGENEOUS

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

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

Variable factors that affect rate of reaction…..

ELEMENTARY NON-

ELEMENTARY

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

H2+I22HI

-rH2=k[H2][I2]

Consider a single reaction with stoichiometric equation:

A+ B → R

The rate of disappearance of A is given by

-rA = kCA CB

Such reaction is called ELEMENTARY REACTION

Elementary reactions are often represented by an equation showing both the molecularity and the rate constant

For example:

The rate of equation is:

Consider this reaction

Rate of equation that refers to B

Rate of equation that refers to D

Rate of equation that refers to T

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

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,

Rate of expression

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

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

This is practically well presented by Arrhenius’ Law

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

k0: frequency or pre-exponential factor E: activation energy of the reaction T: temperature

The temperature dependency of reactions (‘k’) is determined by the activation energy and temperature level of the reaction

Example problem:

Milk is pasteurized if it is heated to 63°C 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.

Insert EQ. 1 and 2

Change temp. in Kelvin

Consistent unit!!

dtVrdN

VdtrdN

Same result = same final concentration

For same final concentration

lnlnln

CHE594_C1W01

combination reaction

reaction conditions

product che594

result of chemical change

industrial chemical

introduction che594

oxygen che594

unit volume of fluid

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