Advanced Chemical Reaction Engineering Lecture 1 Lecturer : 郭修伯

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

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.

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 : 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

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. NaOH CH3COOC2H5 Sodium hydroxide + ethyl acetate = sodium acetate + ethanol C2H5OH CH3COONa and Unreacted NaOH

Typical Laboratory Glass Batch Reactor

Typical Laboratory High Pressure Batch Reactor (Autoclave)

Typical Commercial Batch Reactor

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

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

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

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

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.

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:

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

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)

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

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

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

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

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)

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