Advanced Chemical Reaction
Engineering
Lecture 1
Lecturer : 郭修伯
Syllabus
• Fundamentals of CRE
– Ideal reactor types and
design equations
• Interpretation of rate
data
• Non-elementary
homogeneous reactions
• Non-isothermal reactors
• Multiple reactions
• Non-ideal reactors
• Catalysis and catalytic
reactors
• External diffusion effects
on heterogeneous
reactions
• Diffusion and reaction in
porous catalysts
• Residence time
distributions
What is Chemical Reaction Engineering (CRE) ?
Understanding how chemical reactors work lies at the heart of almost
every chemical processing operation.
Raw
material
Separation
Process
Chemical
process
Separation
Process
Products
By products
Design of the reactor is no routine matter, and many alternatives can be
proposed for a process. Reactor design uses information, knowledge and
experience from a variety of areas - thermodynamics, chemical kinetics,
fluid mechanics, heat and mass transfer, and economics.
CRE is the synthesis of all these factors with the aim of properly designing
and understanding the chemical reactor.
J. Wood at Bham Univ.
Text book and Recommended Books
• Elements of Reaction Engineering, 2nd Edition.
H.Scott Fogler, Prentice Hall.
• Chemical Reaction Engineering, 2nd or 3rd Edition.
Octave Levenspiel, John Wiley and Sons.
• Reactor Design for Chemical Engineers. J.M.
Winterbottom and M.B. King
Fundamentals
• Ideal Reactors :
–
–
–
–
Perfectly mixed batch reactor (Batch)
Continuous stirred tank reactor (CSTR) or Backmix reactor
Plug flow reactor (PFR)
Packed bed reactor (PBR)
• Chemical kinetics
– All reactions are presented as being homogeneous reactions.
• Multiple reactors
• Isothermal ideal Batch, CSTR, and PFR
Ideal Reactor Types
• It has neither inflow nor outflow of reactants or
products which the reaction is being carried out.
• Perfectly mixed
• No variation in the rate of reaction throughout
the reactor volume
BATCH
Batch Reactor
• All reactants are supplied to the reactor at the outset. The reactor
is sealed and the reaction is performed. No addition of reactants
or removal of products during the reaction.
• Vessel is kept perfectly mixed. This means that there will be
uniform concentrations. Composition changes with time.
• The temperature will also be uniform throughout the reactor however, it may change with time.
• Generally used for small scale processes, e.g. Fine chemical and
pharmaceutical manufacturing.
• Low capital cost. But high labour costs.
• Multipurpose, therefore allowing variable product specification.
Example of a liquid phase batch reaction.
Sodium hydroxide + ethyl acetate = sodium acetate + ethanol
NaOH
CH3COOC2H5
C2H5OH
CH3COONa
and
Unreacted NaOH
CH3COOC2H5
Typical Laboratory Glass
Batch Reactor
Typical Laboratory High
Pressure Batch Reactor
(Autoclave)
Typical Commercial Batch Reactor
Ideal Reactor Types
• Normally run at steady state.
• Quite well mixed
• Generally modelled as having no spatial
variations in cencentration, temperature, or
reaction rate throughout the vessel
CONTINUOUS STIRRED TANK REACTOR (CSTR)
BACKMIX REACTOR
Backmixed, Well mixed or CSTR
FA0
(CA0)
•Usually employed for
liquid phase reactions.
CA
CA
FA
(CA)
Vr, g Vr, l
•Use for gas phase usually
in laboratory for kinetic
studies.
CA
Assumption: Perfect mixing occurs.
Schematic representation of a CSTR
?
Characteristics
• Perfect mixing: the properties of the reaction mixture are
uniform in all parts of the vessel and identical to the properties
of the reaction mixture in the exit stream (i.e. CA, outlet = CA, tank)
• The inlet stream instantaneously mixes with the bulk of the
reactor volume.
• A CSTR reactor is assumed to reach steady state. Therefore
reaction rate is the same at every point, and time independent.
• What reactor volume, Vr , do we take?
– Vr refers to the volume of reactor contents.
– Gas phase: Vr = reactor volume = volume contents
– Liquid phase: Vr = volume contents
Cutaway view of a Pfaudler
CSTR/ Batch Reactor
Ideal Reactor Types
•
•
•
•
Normally operated at steady state
No radial variation in concentration
Referred to as a plug-flow reactor
The reactants are continuously consumed as
they flow down the length of the reactor.
PLUG FLOW REACTOR (PFR), TUBULAR REACTOR
PFR, Tubular reactor
• There is a steady movement of materials along the length
of the reactor. No attempt to induce mixing of fluid
element, hence at steady state:
– At a given position, for any cross-section there is no pressure,
temperature or composition change in the radial direction.
– No diffusion from one fluid element to another.
– All fluid element have same residence time.
Used for either gas phase or liquid phase reactions.
The plug flow assumptions tend to hold when there is good radial mixing
(achieved at high flow rates Re >104) and when axial mixing may be
neglected (when the length divided by the diameter of the reactor > 50
(approx.))
N.B.
In the case of a gas phase reaction, the pressure history of the reaction
must be noted in case the number of moles change during the reaction.
e.g. A  B + C
As the reaction progresses the number of moles increases. Therefore at
constant pressure, fluid velocity must increase as conversion increases.
Rate law for rj
• rA = the rate of formation of species A per unit volume
[e.g., mol/dm3-s]
• -rA = the rate of a disappearance of species A per unit volume
• rj is a function of concentration, temperature, pressure, and
the type of catalyst (if any)
• rj is independent of the type of reaction system (batch, plug
flow, etc.)
• rj is an algebraic equation, not a differential equation
Design equations for the ideal reactors:
based on material balance
Conversion
• Conversion is defined to answer the questions:
– How can we quantify how far a reaction has progressed?
– How many moles of product C are formed for every mole
reactant A consumed?
• The conversion XA is the number of moles of A that
have reacted per mole of A fed to the system:
moles of A reacted
XA 
moles of A fed
Material Balance for Any Simple Ideal
Reactor - Isothermal
Rate of accumulation of reactant = Rate of reactant flow – Rate of reactant flow –
in element of volume
INTO
OUT OF
element of volume
element of volume
(1)
(2)
Rate of reactant LOSS due to
Chemical Reaction
within the element of volume
(3)
(4)
Element of reactor volume
Reactants enter
Reactant accumulates
within the element
Reactants leave
Reactant disappears due to
reaction within the element
Mole Balance - Batch Reactor
• No material enters or leaves the reactor.
• If composition in uniform (i.e. perfect mixing) material balance can be written over whole reactor.
• No flow in or out of reactor. Terms (2) and (3) = 0.
Rate of accumulation of
reactant A in reactor
(1)
=
-
Rate of reactant A loss
by reaction in reactor
(4)
Rate of accumulation of A,
[moles/time]
dN A
dt
Where NA = moles of A in system
Where NA0 is the initial moles
NA = NA0 (1-XA)
dNA = -NA0 dXA
XA = fractional conversion of A = (NA0-NA)/NA0
NA = moles of A at conversion XA
dN A
dX A
  N A0
dt
dt
Rate of disappearance of A,
[moles/time]
Rate of disappearance = (-rA) Vr
 N A0
dX A
 (rA )Vr
dt
If system is constant volume, then
where (-rA) = moles A reacting / (unit volume) (time)
Vr = Reactor volume, but really refers to the
volume of fluid in reactor.
 N A0
N A0
 C A0
Vr
dX A
 (rA )Vr
dt
Where CA0 is the initial concentration of A (mol/m3)
Integrating the equation gives the design equation for the batch reactor
t  C A0
XA

0
dX A
(rA )
Mole Balance - CSTR
CSTR (at steady state) - NO ACCUMULATION.
Accumulation =
0
=
FA = FA0 (1-XA)
FA0 = FA + (-rA)Vr
Input FA in -
Output
FA out
-
Disappearance by reaction
(-rA)Vr
 FA0 XA = (-rA) Vr
Vr
XA

FA0 (rA )
Mole Balance - PFR
In a plug flow reactor the composition of the fluid varies from point to point along a flow path;
consequently, the material balance for a reaction component must be made for a differential element
of volume dVr .
dVr
CA0
FA0
XA0= 0
FA
XA
INPUT
Input of A, moles/time = FA
Conversion of A = XA
FA+dFA
XA+dXA
CAf
FAf
XAf
OUTPUT
Output of A, moles/time = FA + dFA
Conversion of A = XA + dXA
Disappearance of A by reaction, moles/time = (-rA) dVr
Accumulation =
0
=
Input - Output - Disappearance by reaction
FA
- (FA+dFA) (-rA)dVr
PFR (at steady state) - NO ACCUMULATION.
- dFA = (-rA)dVr
dFA = -FA0 (dXA)
dVr dX A

FA0 (rA )
Vr

FA0
XA

0
dX A
(rA )
The heart of the design of an ideal reactor:
(-rA) as a function of conversion (concentration, partial pressure etc.)
We will discuss this issue in the next course.
Factors Involved in Reactor Design
• Feedstock composition
– Single feedstock
– Reactant in a solvent
– Multi-component feedstock
• Scale of process
– output of product
• Process kinetics
– Effect of composition
(concentration)
– Effect of temperature
– Catalyst
– Thermodynamics
• Reactor type
– Batch / continuous
– Semi batch / Semi continuous
– Isothermal, non-isothermal,
adiabatic
– Single pass / recycle
– Multiple reactors
• Others
– Materials of construction
– instrumentation
– safety
Example Reactor Types
• Noncatalytic homogeneous gas
reactor
• Homogeneous liquid reactor
• Liquid-liquid reactor
• Gas-liquid reactor
• Non-catalytic gas-solid reactor
– Fixed bed
– Fluidised bed
• Fixed bed catalytic reactor
• Fluid bed catalytic reactor
• Gas-liquid-solid reactor
• Ethylene polymerisation
(high pressure)
• Mass polymerisation of styrene
• Saponification of fats
• Nitric acid production
•
•
•
•
•
Iron production
Chlorination of metals
Ammonia synthesis
Catalytic cracking (petroleum)
Hydrodesulphurisation of oils
Selection of Reactors
• Batch
•
•
•
•
small scale
production of expensive products (e.g. pharmacy)
high labor costs per batch
difficult for large-scale production
• CSTR : most homogeneous liquid-phase flow reactors
• when intense agitation is required
• relatively easy to maintain good temperature control
• the conversion of reactant per volume of reactor is the smallest of the flow
reactors - very large reactors are necessary to obtain high conversions
• PFR : most homogeneous gas-phase flow reactors
• relatively easy to maintain
• usually produces the highest conversion per reactor volumn (weight of
catalyst if it is a packed-bed catalyze gas reaction) of any of the flow reactors
• difficult to control temperature within the reactor
• hot spots can occur
• Fluidised bed reactor (circulating fluidised bed CFB)
Mole balances on 4 common reactors
Reactor
Mole Balance
Comment
Batch
dN j
No spatial variation
CSTR
V 
PFR
PBR
 r jV
dt
dF
F j0  F j
 rj
j
dV
dF
dW
j
 rj
 r j
No spatial variation,
steady state
Steady state
Steady state