KRD CHAPTER 1
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Chemical Engineering Department CBB REACTION ENGINEERING CHAPTER 1: MOLE BALANCES (part 1) “what you focus on will grow – so focus on doing it, understanding it” 1
Transcript of KRD CHAPTER 1
Chemical Engineering DepartmentCBB REACTION ENGINEERING
CHAPTER 1: MOLE BALANCES(part 1)
“what you focus on will grow – so focus on doing it, understanding it”
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Basic knowledge
Application
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1. Define Reaction Rate
2. Define General mole balance equation in relation to
reactor design
3. Understanding on various industrial reactors
LEARNING OBJECTIVES OF CHAPTER 1
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1. Define Reaction Rate
2. Define General mole balance equation in relation to
reactor design
3. Understanding on various industrial reactors
LEARNING OBJECTIVES OF CHAPTER 1
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Indicates how fast a number of moles of one chemical species being transform to another chemical species
Rate of reaction
CONVERSION RATECHEMICAL SPECIES A
CHEMICAL SPECIES B
What is Chemical Species?
• Any chemical component with a given identity
• Determined by the kind, number and configuration of the species’ atom
CC
H H
CH3 CH3
Cis-2-butene
CC
H
HCH3
CH3
Trans-2-butene
Considered as 2 different species due to the different configuration even when the numbers of atoms of elements are the same
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When has a chemical reaction takes place?
REACTION OCCURRED
Changed in number of
atoms
Changed in structure
Changed in atom
configuration
Decomposition
22233 CHCHHCHCH
Isomerisation
CombinationNO2ON 22
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RATE OF REACTION
Rate of reactant disappearance
Rate of product formation
-rA
rB
UNIT:
Rate of chemical species react (or formed) per unit volume (mol/dm3.s)
Rate of chemical species react (or formed) per unit catalyst weight (mol/gcat.s)
• Rate equation is an algebraic equation
• Rate equation is an intensive properties – depends on
concentration, temperature, pressure, or type of catalyst,
if any, present in a system
• Rate equation is not influence by type of reactor used!!
NOTE: dCA/dt is not the definition for
reaction rate
Reaction rate, -rA
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• Example: Is NaOH reacting? CSTR - operated at steady state;
inlet flow rate = outlet flow rate Perfectly well mixed system;
concentration of samples taken
at 10 a.m is the same as
concentration taken at 5 p.m
Therefore: dCA/dt = 0
Does this mean that -rA = 0; i.e.
no reaction occurs?
Reaction rate, -rA
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• Example: Is NaOH reacting? CSTR - operated at steady state;
inlet flow rate = outlet flow rate Perfectly well mixed system;
concentration of samples taken
at 10 a.m is the same as
concentration taken at 5 p.m
Therefore: dCA/dt = 0
Does this mean that -rA = 0; i.e.
no reaction occurs?
Reaction rate, -rA
dtdCr A
A
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1. Define Reaction Rate
2. Define General mole balance equation in relation to
reactor design
3. Understanding on various industrial reactors
LEARNING OBJECTIVES OF CHAPTER 1
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General Mole Balance Equation
General Mole Balance Equation
IN – OUT + GENERATION = ACCUMULATION
UNIT: moles/time
FAO FA GA dNA/dt
dtdNGFF A
AAA 0
VrG AA .
Only valid if ALL SYSTEM VARIABLE IS
A CONSTANT!!
e.g. SMALL VOLUME
BAD NEWS:
Reactor system is RARELY SMALL such that you can consider all the system variable are SPATIALLY UNIFORM!!e.g. temperature, concentration & catalytic activity
HOW DO WE DEAL WITH NON-UNIFORMITY?
General Mole Balance Equation
dtdNdVrFF A
V
AAA 0
rA,1
rA,2
V1
V2
11,1, VrG AA
M
iiiA
M
iiAA VrGG
1,
1,
Taking limits: Let M and V 0
GENERAL MOLE BALANCEEQUATION FOR
REACTOR SYSTEM
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Mole Balance For Different Reactor Type
Batch reactordtdNdVrFF A
V
AAA 0
dtdNVr A
A .
IN – OUT + GENERATION = ACCUMULATION
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Mole Balance For Different Reactor Type
• Continuous stirred tank reactor (CSTR)
dtdNdVrFF A
V
AAA 0
A
AA
rFFV
0
00 AAA VrFF
IN – OUT + GENERATION = ACCUMULATION
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Mole Balance For Different Reactor Type
Plug Flow Reactor (PFR)
dtdNdVrFF A
V
AAA 0
IN – OUT + GENERATION = ACCUMULATION
Mole Balance For Different Reactor Type
• Packed Bed Reactor (PBR) -rA’ = mol A reacted
time. mass of catalyst
SUMMARY
Reactor Mole Balance RemarksBatch Well mix, no
spatial variation, unsteady state
CSTR Well mix, no spatial variation, steady state
PFR Steady state
PBR Steady state
dtdN
Vr AA
Types of Reactor:
1. Batch reactor
2. Continuous-Stirred Tank Reactor (CSTR)
3. Plug Flow Reactor (PFR) or Tubular
Reactor
4. Packed Bed Reactor (PBR)
Industrial reactors
Types of reaction
Liquid phase reaction Gas phase reaction
Batch / Semi batch reactor
CSTR
PFR
Tubular reactor
Packed bed reactor
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Example 1-1: How large is it?
Consider the reaction: AB (first order reaction)
The reaction is carried out in a tubular reactor with constant volumetric flow rate.
1. Derive the equation to relate the reactor volume to the entering and exiting concentration of A, the rate constant, k, and the volumetric flow rate, v.
2. Determine the reactor volume required to reduce the exit concentration to 10% of the entering concentration. Data: volumetric flow rate = 10 dm3/min, k = 0.23 min-1
EXAMPLE
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Use Example 1-1. • Calculate the volume of CSTR for the conditions
used in Example 1-1. • Which gives the larger volume?• What contributes to these differences?
EXAMPLE
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FORMATIVE ASSESSMENT
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1. What assumptions were made in the derivation of the design equation for
• the batch reactor?• the plug-flow reactor?• the CSTR?• the packed bed reactor?
2. Is reaction rate an extensive quantity? Explain.
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END OF LECTURE
