Basic Concepts in Reactor DesignLecture # 01

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Introduction Objectives

Learning Objectives

1 Different types of reactors

2 Fundamental concepts used in reactor design

3 Design equations of different types of reactors

4 Design of network of reactors

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Introduction Reactor types

Types of Reactors - Tank Reactors

Tank reactorsbatchsemibatchcontinuous

Tubular reactorsplug-flowpacked-bed

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Introduction Reactor types

Batch reactors

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Introduction Reactor types

Continuous stirred tank reactor (CSTR)

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Introduction Reactor types

Cascade of CSTR

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Introduction Reactor types

Tubular reactors

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Introduction Reactor types

Semibatch reactors

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Introduction Fundamental concepts used in reactor design

A quote from the book

. . . The bread-and-butter tools of the practicing chemical engineer are thematerial balance and the energy balance. In many respects, chemicalreactor design can be regarded as a straightforward application of thesefundamental principles. . .

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Introduction Fundamental concepts used in reactor design

Material Balance

A material balance on a reactant species of interest for an element ofvolume ∆V can be written as:

A shorter form:

input = output + disappearance by reaction + accumulation

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Introduction Fundamental concepts used in reactor design

Special forms of the equation

Batch reactor: flow terms are omitted

Continuous reactor -steady state operation: accumulation is omitted

Continuous reactor -unsteady state operation and semibatch reactor:all four terms are retained

tubular flow reactor: the equation takes a differential form (Why?)

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Introduction Fundamental concepts used in reactor design

Energy balance

The rate of reaction is temperature dependent. If the temperature is notconstant energy balance is necessary.

Energy balance for an element of volume ∆V over a time increment ∆t is:

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Introduction Some terms associated with reactor design

Space time

τ = VRV

VR : reactor volume; V: volumetric flow

A reference condition, usually the inlet condition, is selected to measurethe volumetric flow rate. Reference condition is emphasized by the use ofthe subscript zero:

τ = VRV0

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Introduction Some terms associated with reactor design

Space time vs average residence time

The two quantities are equal only if all of the following conditions are met:1 Pressure and temperature are constant throughout the reactor2 The density of the reaction mixture is independent of the extent of

reaction3 The reference volumetric flow rate is evaluated at reactor inlet

conditions

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Introduction Some terms associated with reactor design

Space Velocity

Space time:S = 1

τ= V0

VR

When heterogeneous catalyst is involved WHSV or VHSV is used:

WHSV = ρV0W

VHSV = V0W

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Reactor design Batch reactor

Mole balance

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Reactor design Tubular reactor

Assumptions- PFR

1 no longitudinal mixing of fluid elements as they move through thereactor

2 all fluid elements take the same length of time to move from thereactor inlet to the outlet

3 plugs of material move as units through the reactor, and thisassumption is conveniently expressed in terms of a requirement thatthe velocity profile be flat as one traverses the tube diameter

4 Each plug of fluid is assumed to be uniform in temperature,composition, and pressure - radial mixing is infinitely rapid

5 there may well be variations in composition, temperature, pressure,and fluid velocity as one moves in the longitudinal direction

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Reactor design Tubular reactor

Mole balance

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Reactor design Tubular reactor

Algebraic form and graphical determination

VRFA0

=∫ fAout

fAin

dfA(−rA)

This is known as a Levelspiel plotKBK (ChE) Ch. 8 19 / 32

Reactor design Tubular reactor

Residence time in plug flow reactor

t̄ =∫ VR

0

dVRV

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Reactor design Tubular reactor

Combinations of tubular reactors

Series of PFRs in a Levelspiel plot - How would they look??

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Reactor design Tubular reactor

DIY

Equation for a packed bed reactor??

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Reactor design CSTR

Basic assumptions

. . . the reactor contents are perfectly mixed so that the properties of thereacting fluid are uniform throughout. The composition and temperatureof the effluent are thus identical with those of the reactor contents. . .

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Reactor design CSTR

Scheme

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Reactor design CSTR

Algebraic form and graphical determination

VRFA0

= fA,out − fA,in(−rAF)

Levelspiel plot

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Reactor design CSTR

Mean residence time in a CSTR

τ̄ = VRVF

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Reactor design Relative Size

relative size requirements

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Reactor design Cascades of Stirred-Tank Reactors

Cascades of Stirred-Tank Reactors

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Reactor design Cascades of Stirred-Tank Reactors

Graphical solution for intermediate concentrations

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Reactor design Cascades of Stirred-Tank Reactors

Graphical solution for best combination

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Reactor design Cascades of Stirred-Tank Reactors

Graphical solution for best combination

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Reactor design Combination of Reactors

Series Combination

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