CHEMICAL REACTION ENGINEERING -

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Lecture 3: General Mole Balance

1CRE-1 by Dr. S.M.Mhatre

Lecture 3: General Mole Balance Equation

By Dr. S.M.Mhatre

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General Mole Balance

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How to apply this to different reactors ?

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Reactors:3

� Equipments used to carry on chemical reactions

� Chemical Engineers design the reactor to optimized

the present values for a given reaction.

� Designer ensures: highest efficiency, maximum yield

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� Designer ensures: highest efficiency, maximum yield

of product and minimum cost to purchase

ClassificationOf reactors

Batch /Continuous

Isothermal / Non-isothermal

Ideal / Non-ideal

Homogenous/Heterogeneous

Multiple Reactors

Heterogeneous React

Non-isothermal

Non–Ideal

CRE-1 by Dr. S.M.MhatreIntroduction 4Ideal Reactors

Homogen. Reactors

Isothermal Reactors

Multiple Reactions

Multiple Reactors

Single Reactions

Up

hill Ta

sk

Constant density

Variable density

Ideal Reactor Types –Batch Reactor

� 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

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� 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.

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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.

Ideal Reactor Types – CONTINUOUS STIRRED TANK REACTOR /BACKMIX REACTOR

� Normally run at steady state.

� Quite well mixed

� Generally modelled as having no spatial

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� Generally modelled as having no spatial

variations in cencentration, temperature, or

reaction rate throughout the vessel

CONTINUOUS STIRRED TANK REACTOR (CSTR) /

BACKMIX REACTOR

� 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.

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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

Ideal Reactor Types –PLUG FLOW REACTOR (PFR), TUBULAR REACTOR

� Normally operated at steady state

� No radial variation in concentration

� Referred to as a plug-flow reactor

� The reactants are continuously consumed as they

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� The reactants are continuously consumed as they

flow down the length of the 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.

PFR, Tubular reactor

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temperature or composition change in the radial direction.

� No diffusion from one fluid element to another.

� All fluid element have same residence time.

Batch Reactor Mole Balance

If composition in uniform (i.e. perfect mixing) - material balance

No flow in or out of reactor. Terms (1) and (2) in

general mole balance = 0.

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If composition in uniform (i.e. perfect mixing) - material balance

can be written over whole reactor.

V = Reactor volume, but really refers to the volume

of fluid in reactor.

∫=1

01

A

A

N

NA

A

Vr

dNt

CSTR

Mole Balance

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Plug Flow Reactor

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Plug Flow Reactor Mole BalancePFR:

The integral form is:V =

dFA

rA

FA 0

FA

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

exit molar flow rate of FA.

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Packed Bed Reactor Mole

Balance

PBR

FA0 −FA + ′ r AdW =dNA

dt∫

The integral form to find the catalyst weight is: W =dF

A

′ r AFA 0F

A

∫

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Reactor Mole Balance Summary

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