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

dt

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

rA

arB

brC

crD

d


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!!


tCCr

tV

NNr

VtrNN

dtVrdN

VdtrdN

dt

dN

Vr

AAA

AA

A

AAA

t

A

N

N

A

AA

AA

A

A

1

1

1

0

0

0

00

1

2

11

1

0

0

2

tCC

tCC

r

r

AA

AA

A

A

Same result = same final concentration

2

1

1

2

t

t

r

r

A

A

1

2

11

1

0

0

2

tCC

tCC

r

r

AA

AA

A

A


RTE

AA

ekk

kCr

0

For same final concentration

A

A

A

A

Ck

Ck

r

r

1

2

1

2

1

2

1

2

k

k

r

r

A

A

eRT

ERT

Er

r

e

e

e

r

r

ek

ek

k

k

r

r

A

A

RTE

RTE

RTE

RTE

A

A

RTE

RTE

A

A

lnln

lnlnln

21

0

0

1

2

1

2

12

1

2

1

2

1

2

1

2

21

11ln

1

2

TTR

E

r

r

A

A


CHE594_C1W01

Documents

combination reaction

reaction conditions

product che594

result of chemical change

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introduction che594

oxygen che594

unit volume of fluid