Advanced Chemical Reaction Engineering

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

2. 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 Syllabus

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 Separation Raw Chemical Separation By products Process material process Process 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 : • 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. BatchReactor • 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. Sodium hydroxide + ethyl acetate = sodium acetate + ethanol CH3COOC2H5 NaOH C2H5OH CH3COONa and Unreacted NaOH CH3COOC2H5 Example of a liquid phase batch reaction.

9. Typical Laboratory Glass Batch Reactor

10. Typical Laboratory High Pressure Batch Reactor (Autoclave)

11. Typical Commercial Batch Reactor

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

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

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

25. 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. Rate of disappearance of A, [moles/time] 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. 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. dVr CA0 FA0 XA0= 0 CAf FAf XAf FA XA FA+dFA XA+dXA 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 . 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. 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. 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 Factors Involved in Reactor Design

31. 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 Example Reactor Types

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