1. Advanced Chemical Reaction Engineering
Lecture 1
Lecturer : 郭修伯

2. Syllabus Fundamentals of CRE Interpretation of rate data
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

3. What is Chemical Reaction Engineering (CRE) ?
Understanding how chemical reactors work lies at the heart of almost every chemical processing operation.
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.
Products
Raw
Separation
Chemical
Separation
material
Process
process
Process
By products
J. Wood at Bham Univ.

4. 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

5. Fundamentals Ideal Reactors : Chemical kinetics Multiple 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

6. 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

7. 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.

8. Example of a liquid phase batch reaction.
NaOH
CH3COOC2H5
Sodium hydroxide + ethyl acetate = sodium acetate + ethanol
C2H5OH
CH3COONa
and
Unreacted NaOH

9. Typical Laboratory Glass Batch Reactor

10. Typical Laboratory High Pressure Batch Reactor (Autoclave)

11. Typical Commercial Batch Reactor

12. CONTINUOUS STIRRED TANK REACTOR (CSTR)
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

13. Backmixed, Well mixed or CSTR
FA0
(CA0)
Usually employed for liquid phase reactions.
Use for gas phase usually in laboratory for kinetic studies. FA
(CA)
Vr, l CA CA
Vr, g CA
Assumption: Perfect mixing occurs. ?
Schematic representation of a CSTR

14. 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

15. Cutaway view of a Pfaudler CSTR/ Batch Reactor

16. PLUG FLOW REACTOR (PFR), TUBULAR 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

17. Used for either gas phase or liquid phase reactions.
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.

18. 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.

19. 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

20. Design equations for the ideal reactors: based on material balance

21. 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:

22. Material Balance for Any Simple Ideal Reactor - Isothermal
(1)
(2)
(3)
(4)
Element of reactor volume
Reactants leave
Reactants enter
Reactant accumulates
within the element
Reactant disappears due to reaction within the element
Reactants enter

23. 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
Rate of reactant A loss
by reaction in reactor = -
(1)
(4)

24. Rate of accumulation of A, [moles/time]
Where NA = moles of A in system
NA = NA0 (1-XA)
Where NA0 is the initial moles
XA = fractional conversion of A = (NA0-NA)/NA0
NA = moles of A at conversion XA
dNA = -NA0 dXA

25. Rate of disappearance of A, [moles/time]
Rate of disappearance = (-rA) Vr
where (-rA) = moles A reacting / (unit volume) (time)
Vr = Reactor volume, but really refers to the volume of fluid in reactor.
If system is constant volume, then
Where CA0 is the initial concentration of A (mol/m3)
Integrating the equation gives the design equation for the batch reactor

26. CSTR (at steady state) - NO ACCUMULATION.
Mole Balance - CSTR
CSTR (at steady state) - NO ACCUMULATION.
Accumulation = Input Output Disappearance by reaction 0 = FA in - FA out (-rA)Vr
FA = FA0 (1-XA)
FA0 = FA + (-rA)Vr
 FA0 XA = (-rA) Vr

27. Mole Balance - PFR dVr CA0 FA0 XA0= 0 FA XA FA+dFA XA+dXA CAf FAf XAf
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
FA+dFA
XA+dXA
CAf
FAf
XAf
INPUT
Input of A, moles/time = FA
Conversion of A = XA
OUTPUT
Output of A, moles/time = FA + dFA
Conversion of A = XA + dXA
Disappearance of A by reaction, moles/time = (-rA) dVr

28. PFR (at steady state) - NO ACCUMULATION.
Accumulation = Input Output Disappearance by reaction
0 = FA - (FA+dFA) (-rA)dVr
PFR (at steady state) - NO ACCUMULATION.
- dFA = (-rA)dVr
dFA = -FA0 (dXA)

29. 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.

30. 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

31. 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

32. 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)

33. Mole balances on 4 common reactors