CRE Unit 1 Comp

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UNIT 1

C H E 3 0 0 6

S AH LU B AK ER

CHEMICAL REACTION

ENGINEERING


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Introduction

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Chemical reaction engineering is at the heart of virtuallyevery chemical process. It separates the chemical engineerfrom other engineers (Fogler)

Initiated and evolved primarily to accomplish the task ofdescribing how to choose, size, and determine the optimaloperating conditions for a reactor

Knowledge and experience in thermodynamics, chemicalkinetics, fluid mechanics, heat and mass transfer, and

economics

CRE is the synthesis of all these factors with the aim ofproperly designing and understanding the chemical reactor(J. Wood at Bham Univ).


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Chemical Thermodynamics /Chemical Kinetics

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y The two questions that must be answered for achemically reacting system are:

(1) what changes are expected to occur and

(2) how fast will they occur

y the constraints placed on a reacting system bythermodynamics should always be identified first

Enthalpy

Fugacity

Activity


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Chemical Identity

The identity of a chemical species is determined bythe kind, number, and configuration of thatspecies atoms

A chemical species is said to have reacted when ithas lost its chemical identity. Three basic ways are:

1. Decomposition

2. Combination

3. Isomerization


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Classification of Reaction type

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y Two types: Homogeneous and Heterogeneous

A reaction is homogeneous if it takes place in one phase alone

A reaction is heterogeneous if it requires the presence of at

least two phases to proceed


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y The reaction rate is the rate at which a specieslooses its chemical identity per unit volume.

y

The rate of a reaction (mol/dm3

/s) can beexpressed as either

the rate of Disappearance: -rA

or asthe rate of Formation (Generation): rA

Reaction Rate


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Reaction Rate

Consider the isomerization AB

rA= the rate of formation of species A perunit volume

-rA= the rate of a disappearance of species A perunit volume

rB = the rate of formation of species B perunit volume


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Reaction Rate

y EXAMPLE: ABIf Species B is being formed at a rate of

0.2 moles per decimeter cubed per second

rB = 0.2 mole/dm3/s

Then A is disappearing at the same rate:

-rA= 0.2 mole/dm3/s

The rate of formation (generation of A) is

rA= -0.2 mole/dm3/s


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Reaction Rate

y For a catalytic reaction, we refer to -rA',

which is the rate of disappearance of

species A on a per mass of catalyst basis.

(mol/gcat/s)

NOTE: dCA/dt is not the rate of reaction


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Reaction Rate

y Consider species j:

y rj is the rate of formation of species j per unit

volume [e.g. mol/dm3

/s]y rj is a function of concentration, temperature,

pressure, and the type of catalyst (if any)

y rj is independent of the type of reaction system

(batch, plug flow, etc.)

y rj is an algebraic equation, not adifferential equation -rA= k(T)CA


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General Mole Balance

FA0= Entering molar flow rate of A (mol/time)FA= Exiting molar flow rate of A (mol/time)GA= Rate of generation(formation) of A (mol/time)V = Volume (vol e.g. m3)rA= rate of generation(formation) of A (mole/timevol)NA= number of moles of A inside the system Volume V(mols)


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Ideal Reactor Types-Batch

y It has neither inflow nor outflow of reactants orproducts which the reaction is being carried out.

y Perfectly mixed

y No variation in the rate of reaction throughout thereactor volume

BATCH


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Batch Reactor

y All reactants are supplied to the reactor at the outset. Thereactor is sealed and the reaction is performed. No additionof reactants or removal of products during the reaction.

y Vessel is kept perfectly mixed. This means that there will be

uniform concentrations. Composition changes with time.y The temperature will also be uniform throughout the

reactor - however, it may change with time.

y Generally used for small scale processes, e.g. Fine chemicaland pharmaceutical manufacturing.

y Low capital cost. But high labour costs.

y Multipurpose, therefore allowing variable productspecification.


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Batch Reactor Mole Balance


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Typical Laboratory Glass

Batch Reactor


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Typical Laboratory High

Pressure Batch Reactor(Autoclave)


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Typical Commercial Batch Reactor


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Ideal Reactor Types-CSTR

y Normally run at steady state.

y Quite well mixed

y Generally modelled as having no spatial variations

in cencentration, temperature, or reaction ratethroughout the vessel

CONTINUOUS STIRRED TANK REACTOR (CSTR)

BACKMIX REACTOR


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Usually employed for

liquid phase reactions.

Use for gas phaseusually in laboratory

for kinetic studies.

Schematic representation of a CSTR

Assumption: Perfect mixingoccurs.

FA0(CA0)

FA(CA)

CACA

CA

Vr, lVr, g

Backmixed, Well mixed or CSTR


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Characteristics

y Perfect mixing: the properties of the reaction mixture areuniform in all parts of the vessel and identical to theproperties of the reaction mixture in the exit stream (i.e. CA,outlet = CA, tank)

y The inlet stream instantaneously mixes with the bulk of thereactor volume.

y A CSTR reactor is assumed to reach steady state. Thereforereaction rate is the same at every point, and timeindependent.

y 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


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CSTRMole Balance


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Cutaway view of a PfaudlerCSTR/ Batch Reactor


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PLUG F

LOW REACTOR (

PFR), TUBU

LAR REACTO

Ideal Reactor Types-PFR

y Normally operated at steady state

y No radial variation in concentration

y Referred to as a plug-flow reactor

y The reactants are continuously consumed as theyflow down the length of the reactor.


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PFR, Tubular reactor

y There is a steady movement of materials alongthe 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 theradial 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.


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y The plug flow assumptions tend to hold when there is good radialmixing (achieved at high flow rates Re >104) and when axial mixingmay be neglected (when the length divided by the diameter of thereactor > 50 (approx.))

y In the case of a gas phase reaction, the pressure history of thereaction must be noted in case the number of moles change duringthe reaction. e.g. Ap B + C

y As the reaction progresses the number of moles increases. Thereforeat constant pressure, fluid velocity must increase as conversion

increases.


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Plug Flow Reactor MoleBalance

PFR:

The integral form is: V!dF

A

rA

FA 0

FA

This is the volume necessary to reduce the entering molarflow rate (mol/s) from FA0 to the

exit molarflow rate ofFA.


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PBR, Tubular reactor

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y Tubular reactor packedwith solids usuallycatalyst

y Heterogeneous system

y Similar to PFR no radialgradients in conc, tempor reaction time

y Mass of solid determinerate of product formation


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Packed Bed ReactorMole Balance

PBR

The integral form to find the catalyst weight is: WdF

A

drAFA0

FA

FA0F

A dr

AdW

dNA

dt


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Selection of Reactors

y Batch small scale production of expensive products (e.g. pharmacy) high labor costs per batch difficult for large-scale production

y 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 conversionsy 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 flowreactors difficult to control temperature within the reactor hot spots can occur

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