Cre chapter 1 overview of chemical reaction engineering

Chemical
Reaction
Engineering

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Chapter 1

Overview of
Chemical Reaction
Engineering

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Introduction

 Reactor design uses information, knowledge, and
experience:
Thermodynamics:
Chemical kinetics
Fluid mechanics
Heat transfer
Mass transfer
Economics
 Chemical reaction engineering is the synthesis

of all these factors with the aim of properly designing
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a chemical reactor.


Performance equation
To find what a reactor is able to do we need to know the kinetics, the
contacting attern and the performance equation.
Performance equation
relates input to output

=f [input, kinetics, contacting]
Contacting pattern or how materials
flow through and contact each other
in the reactor, how early or late they
mix, their clumpiness or state of
aggregation. By their very nature
some materials are very clumpy-for
instance, solids and noncoalescing
liquid droplets.

Kinetics or how fast things
happen If very fast, then
equilibrium tells what will leave
the reactor. If not so fast, then
the rate of chemical reaction,
and maybe heat and mass
transfer too, will determine what
will happen.

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Classification of
Reactions
Homogeneous: takes place in one phase
Heterogeneous: it requires the presence
of at least two phases to proceed at the rate
that it does.
More complicated:
+ enzyme-substrate reactions
+ very rapid chemical reactions

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Classification of
Reactions
Noncatalytic

Homoge Most gas-phase reactions
neou
s
Fast reactions such as
burning of a flame

Heterog
eneo
us

Burning of coal
Roasting of ores
Attack of solids by acids
Gas-liquid absorption with
reaction
Reduction of iron ore to iron
and steel

Catalytic
Most liquid-phase
reactions
Reactions in colloidal
systems
Enzyme and microbial
reactions
Ammonia synthesis
Oxidation of ammonia to
produce nitric acid
Cracking of crude oil
Oxidation of SO2 to SO3


Variables Affecting the Rate of
Reaction
In homogeneous systems:
temperature

pressure
composition
....
In heterogeneous systems
mass transfer
heat transfer

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Definition of Reaction Rate
The rate of change in number of moles of this
component due to reaction is dNi /dt
Based on unit volume of reacting fluid:

Based on unit mass of solid in fluid-solid systems

Based on unit interfacial surface in two-fluid systems
or based on unit surface of solid in gas-solid systems
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Definition of Reaction Rate
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

Speed of Chemical Reactions
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Reactor Classifying
To carry out chemical reactions discontinuously operated reactors or
continuously operated reactors can be used.
• Discontinuously: more frequently applied to produce fine chemicals
• Continuously: more advantageous for the production of larger
amounts of bulk chemicals.
To study the different behavior of these types of reactors another important
criterion serves to distinguish two limiting cases: mixed flow and plug flow
behavior
For theoretical studies and to compare the different reactors, four different
ideal reactors can be defined using the above classification:


a) Batch Reactor (BR, perfectly mixed, discontinuous
operation):
Features:
• All components are in the reactor before the reaction
starts
• Composition changes with time
• Composition throughout the reactor is uniform
Adv.:
• Simple, flexible, high conversion…

Disadv.:
• Dead times for charging, discharging, cleaning,…
• Difficult to control and automate
•…
BR are applied in particular for:
• Relatively slow reactions
• Slightly exothermic reactions
Areas of application for BR are:
• Reactions in pharmaceutical industry
• Polymerisation reactions
• Dye production
• Speciality chemicals


b) Semi-batch Reactor (SBR): perfectly
mixed, semi continuous operation
Features:
• One reactant is introduced first and then
the second is dosed in a controlled
manner.
• Composition changes with time
• Composition throughout the reactor is
uniform
Adv.:
• Controlled reaction rate and heat generation
• ...
Disadv.:
• Same as BR
• More complicated than BR
•…

c) Continuously Stirred Tank Reactor (CSTR):
perfectly mixed, continuous operation
A,B

A,B,products

Features:
• Reactants are continuously introduced,
products (+ unconverted reactants) are
continuously withdrawn
• Composition does not change with time
• Composition throughout the reactor is uniform
Adv.:
• Controlled heat generation
• Easy to control and automate
• No dead times
• Constant product quality,...
Disadv.:
• Complicated
• Can become unstable
• Large investmnent cost,...


d) Plug Flow Tubular Reactor (PFTR): no mixing,
continuous operation
A, B

tubular reactor

A, B,
products

Features:
• Composition varies from point to point along a flow
path
Adv.:
•High conversion
•Easy to automate
•No dead times
•Better to cool (compare to stirred tanks)
•…
Disadv.:
•Complicated
•Danger of “hot spot”
•…


To be
reviewed by
students
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1. Chemical thermodynamics
Chemical thermodynamics deal with equilibrium states of reaction system.
This Section will concentrate on the following two essential areas:

a) The calculation of enthalpy changes connected with chemical reactions,
and
b) The calculation of equilibrium compositions of reacting systems.

Enthalpy of reaction
The change of enthalpy caused by a reaction is called reaction enthalpy ∆HR.
This quantity can be calculated according to the following equation:
N

H R  i H Fi
i 1

∆HFi is the enthalpy of formation of component i
∆HR < 0, the reaction is exothermic
∆HR > 0, the reaction is endothermic


It is simple to calculate the reaction enthalpy at a certain standard state
∆HR0 from the corresponding standard enthalpies of formation ∆HFi0. The
standard enthalpies of formation are available from databases for P = P0
= 1 bar and T = T0 = 298 K.
For pure elements like C, H2, O2,...: ∆HFi0 = 0.
The reaction enthalpy is a state variable. Thus, a change depends only
on the Initial and the end state of the reaction and does not dependent
on the reaction parthway.
Temperature and pressure dependence of reaction enthalpy

 H R 
 H R 
d H R  

 dP  
 dT
 P  T
 T  P
The pressure dependence is usually very small. For ideal gas
behaviour, the reaction enthalpy does not depend on pressure.


The correlation of reaction enthalpy and temperature is related to
the isobaric heat capacities of all species involved in the considered
reaction, cPi.
N

T

i 1

Pi
T 298 K

H R T  H R0   i

c T dT

Assuming that the reactants and the products have different but
temperature independent heat capacities, the temperarue
dependence of the reaction enthalpy can be estimated as follows:

H R T  H R0  T  T0 cP ,products  cP ,reactants 

2. Chemical
equilibrium
•

Chemical reactions approach to an equilibrium, when the product and
reactant concentrations do not change anymore.

• A reacting system is in chemical equilibrium if the reaction rates of the
forward and backward reactions are equal.
• The basic quantity required to indentify the equilibrium state is the
Gibbs free enthalpy of reaction GR.
• The change of this quantity becomes zero when the equilibrium is
reached (i.e. dGR = 0)
For constant pressure and temperature, the change of free Gibbs
enthalpy of reaction can be described as follows:
N

dGR  i i d
i 1

or

N
 dGR 

  i i
 d  T , P i 1

Thus, for the chemical equilibrium:

 dGR 

 0
 d  T , P

Free Gibbs enthalpy

The equilibrium is reached when the free
Gibbs enthalpy of reaction is minimum.

Or dGR=0 (or in an integrated form: ∆GR = 0)
Thus, the equilibrium is characterized by:
N

 
i 1

i

i

0

 GR 

  0
   T , P

 GR 

  0



T ,P

 GR 

 0
   T , P


Changing of free Gibbs enthalpy
for a chemical reaction