1
The term chemical species
p
refers to any
y chemical
component or element with a given identity.
The identity of a chemical species is determined
by thee kind,
i d, number
u be a
andd co
configuration
figu atio o
of that
species' atoms
Even though two chemical compounds have exactly
the same number of atoms of each element, they
could still be different species
p
because of different
configurations
A chemical reaction has taken p
place when a
detectable number of molecules of one or more
species have lost their identity and assumed a new
form by a change in the kind or number of atoms
in the compound and/or by a change in structure
or configuration of these atoms.
There are three basic ways a species may lose its
chemical identity:
• Decomposition CH 3 CH 3  H 2  CH 2  CH 2
• Combination
C bi i
N 2  O 2  2NO
• Isomerization
Systems
y
in which chemical
reactions take place are called
reactors
Chemical Reaction Engineering
is the engineering activity
concerned with exploitation of
chemical reactions on a
commercial scale
Design of reactors involves:
Choosing the best type of
reactor for a given reaction
Choosing the optimum
operating conditions
Determining
D
t
i i th
the Si
Size off th
the
reactor
6
Physical t t
treatment steps
t t
Chemical treatment steps
Physical treatment steps
CRE deals with Chemical treatment steps
p
Choice of the reactor dictates:
• Pre and post treatment steps
Chemical reactor is the place in the process where the
most value is added: lower
lower-value
value feeds are converted
into higher-value products.
7
Information needed to predict what a reactor can do
Performance equation
relates input
p to output
p
Input
Contacting pattern
Output
Reactor
Kinetics
Output = f [input, kinetics, contacting]
This is called the performance equation.
Homogeneous
g
Elementary
Single
Classification
of Reactions
Heterogeneous
g
Non-elementary
Multiple
Chemical
Reversible
Exothermic
Bio-chemical
Irreversible
Endothermic
Constant density Variable density
Catalytic
Non-catalytic
Non catalytic
Reactor design require
(almost all core areas of chemical engineering)
Thermodynamics
Chemical Kinetics
Fluid Mechanics
Heat & Mass transfer
Mathematics
Economics
Thermodynamics
• Feasibility of a reaction
• Heat of reaction, effect of temperature
• Equilibrium
Eq ilibri m yields,
ields constant
constant, composition
Chemical Kinetics
• Quantitative studies of the rates at which
chemical processes occur
• Factors on which these rates depend
• Reaction mechanism
A description of a reaction in terms of its
constituent molecular acts is known as the
mechanism of the reaction.
reaction
Chemical Kinetics & Thermodynamics
•
Time is a variable in kinetics but not in
thermodynamics; TD does not deal with respect to
time; equilibrium is a time-independent state.
•
Information about the mechanism of chemical change
can be obtained from kinetics but not from
thermodynamics.
•
The rate of chemical change is dependent on the path
off reaction;
ti
th
thermodynamics
d
i
i
is
concerned
d with
ith
“state” and change of state of a system.
•
Chemical kinetics is concerned with the rate of
reaction and factors affecting the rate, and chemical
thermodynamics is concerned with the position of
equilibrium and factors affecting equilibrium.
•Chemical kinetics is the studyy of chemical
reaction rates and reaction mechanisms.
•The studyy of chemical reaction engineering
g
g
(CRE) combines the study of chemical
kinetics with the reactors in which the
reactions occur.
•Chemical
Chemical kinetics and reactor design are at
the heart of producing almost all industrial
chemicals
The Ch
Th
Chemical
i lR
Reaction
ti E
Engineering
i
i (CRE)
principles learned here can also he applied in
areas such as:
• Waste treatment
• Microelectronics
• Nanoparticles
• Living
Li i systems
t
•Traditional areas of the manufacture of
chemicals and phamaceutica1s.
The Chemical Reaction Engineering (CRE)
principles learned here can also he applied in
areas such as:
• Waste treatment
• Microelectronics
Mi
l t i
• Nanoparticles
• Living systems
d o
areas
e s oof thee manufacture
u c u e oof
• Traditional
chemicals and phamaceutica1s.
Some of the examples that illustrate the wide
application of CRE principles are shown in
Fi
Figure
1-2
12
Batch/Continuous
Isothermal/Nonisothermal
Classification
of reactors
Ideal/Non-ideal
Homogenous/Heterogeneous
Types of reactors
Homogeneous
g
Heterogeneous
g
Batch
Packed bed
Plugg flow
Moving bed
CSTR
Fluidized bed
Laminar flow
Recycle
The rate of reaction tells us
how fast a number of moles of
oonee chemical
c e ca spec
species
es are
a e being
be g
consumed to form another
chemical species
p
How can reaction rate be expressed?
• Select one reaction component for
consideration and define the rate in terms
of this component, i.
• If the rate of change in number of moles
of this component due to reaction is
dNi/dt, then the rate of reaction in its
various forms is defined as follows:
Definition of reaction rate
Based on unit volume of reacting fluid
1 dN i
moles i formed
ri 
V dt (mass of solid)(time)
Ni : moles of i
V : volume of fluid
Based on unit mass of solid in fluid-solid systems
1 dN i
ri 
W dt
moles i formed
((mass of solid)(time)
)(
)
W = Mass of solid
Based on unit interfacial surface area in two-fluid system or
based on per unit surface area of solids in gas-solid systems
1 dNi
ri
S dt
molesi formed
(surface)(time)
S = interfacial area
Based on unit volume of solid in gas-solid systems
1 dN i
ri
Vs dt
m oles i form ed
( volum e of solid )( tim e)
Vs : volume of solid
Based on unit volume of reactor
ri 
1 dN i
Vr dt
moles i formed
(volume of reactor)(time)
Vr = reactor volume
Vri  Wri  Sri  Vs ri Vr ri
EXAMPLE 1.1 THE ROCKET ENGINE
A rocket engine, Fig. El.l, burns a stoichiometric mixture of fuel (liquid hydrogen)
in oxidant (liquid oxygen). The combustion chamber is cylindrical, 75 cm long and
60 cm in diameter, and the combustion process produces 108 kg/s of exhaust
gases. If combustion is complete, find the rate of reaction of hydrogen and of
oxygen.
We want to evaluate:
Next, let us look at the reaction occurring
EXAMPLE 1.2 THE LIVING PERSON
A human being (75 kg) consumes about 6000 kJ of food per day. Assume that the
food is all glucose and that the overall reaction is all glucose and that the overall
reaction is:
Find man's metabolic rate (the rate of living, loving, and laughing) in terms of
moles of oxygen used per m3 of person per second.
We want to find:
Let us evaluate the two terms in this equation. First of all, from our life
experience we estimate the density of man to be:
Therefore, for the person in question
Next, noting that each mole of glucose consumed uses 6 moles of oxygen
and releases 2816 kJ of energy, we see that we need