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3rd Edition
CHE594
Chemical Reaction Engineering
TOPIC 1 INTRODUCTION
CHE594: CHEMICAL REACTION ENGINEERING
CHE594: CHEMICAL REACTION ENGINEERING
CHEMICAL REACTION??
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
Reaction occurred. How??
Color change
Formation of a solid (precipitate) within a clear solution
Evidence of a gas
Evolution or absorption of heat
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
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.
CHE594: CHEMICAL REACTION ENGINEERING
In industrial chemical process, the typical situation of chemical process is shown
below:
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
CHE594: CHEMICAL REACTION ENGINEERING
CONCENTRATION OVER TIME
RATE LAW/ RATE OF EQUATION
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
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.
CHE594: CHEMICAL REACTION ENGINEERING
CHE594: CHEMICAL REACTION ENGINEERING
HOMOGENEOUS HETEROGENEOUS
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
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…..
CHE594: CHEMICAL REACTION ENGINEERING
CHE594: CHEMICAL REACTION ENGINEERING
ELEMENTARY NON-
ELEMENTARY
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
Elementary reactions are often represented by an equation showing both the molecularity and the rate constant
For example:
The rate of equation is:
CHE594: CHEMICAL REACTION ENGINEERING
Consider this reaction
Rate of equation that refers to B
Rate of equation that refers to D
Rate of equation that refers to T
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
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.
CHE594: CHEMICAL REACTION ENGINEERING
Insert EQ. 1 and 2
Change temp. in Kelvin
Consistent unit!!
CHE594: CHEMICAL REACTION ENGINEERING
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
CHE594: CHEMICAL REACTION ENGINEERING
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: CHEMICAL REACTION ENGINEERING