Basic Concepts in Reactor Design - Lecture #...
Transcript of Basic Concepts in Reactor Design - Lecture #...
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Basic Concepts in Reactor DesignLecture # 01
KBK (ChE) Ch. 8 1 / 32
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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
KBK (ChE) Ch. 8 2 / 32
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Introduction Reactor types
Types of Reactors - Tank Reactors
Tank reactorsbatchsemibatchcontinuous
Tubular reactorsplug-flowpacked-bed
KBK (ChE) Ch. 8 3 / 32
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Introduction Reactor types
Batch reactors
KBK (ChE) Ch. 8 4 / 32
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Introduction Reactor types
Continuous stirred tank reactor (CSTR)
KBK (ChE) Ch. 8 5 / 32
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Introduction Reactor types
Cascade of CSTR
KBK (ChE) Ch. 8 6 / 32
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Introduction Reactor types
Tubular reactors
KBK (ChE) Ch. 8 7 / 32
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Introduction Reactor types
Semibatch reactors
KBK (ChE) Ch. 8 8 / 32
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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. . .
KBK (ChE) Ch. 8 9 / 32
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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
KBK (ChE) Ch. 8 10 / 32
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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?)
KBK (ChE) Ch. 8 11 / 32
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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:
KBK (ChE) Ch. 8 12 / 32
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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
KBK (ChE) Ch. 8 13 / 32
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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
KBK (ChE) Ch. 8 14 / 32
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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
KBK (ChE) Ch. 8 15 / 32
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Reactor design Batch reactor
Mole balance
KBK (ChE) Ch. 8 16 / 32
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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
KBK (ChE) Ch. 8 17 / 32
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Reactor design Tubular reactor
Mole balance
KBK (ChE) Ch. 8 18 / 32
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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
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Reactor design Tubular reactor
Residence time in plug flow reactor
t̄ =∫ VR
0
dVRV
KBK (ChE) Ch. 8 20 / 32
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Reactor design Tubular reactor
Combinations of tubular reactors
Series of PFRs in a Levelspiel plot - How would they look??
KBK (ChE) Ch. 8 21 / 32
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Reactor design Tubular reactor
DIY
Equation for a packed bed reactor??
KBK (ChE) Ch. 8 22 / 32
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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. . .
KBK (ChE) Ch. 8 23 / 32
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Reactor design CSTR
Scheme
KBK (ChE) Ch. 8 24 / 32
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Reactor design CSTR
Algebraic form and graphical determination
VRFA0
= fA,out − fA,in(−rAF)
Levelspiel plot
KBK (ChE) Ch. 8 25 / 32
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Reactor design CSTR
Mean residence time in a CSTR
τ̄ = VRVF
KBK (ChE) Ch. 8 26 / 32
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Reactor design Relative Size
relative size requirements
KBK (ChE) Ch. 8 27 / 32
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Reactor design Cascades of Stirred-Tank Reactors
Cascades of Stirred-Tank Reactors
KBK (ChE) Ch. 8 28 / 32
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Reactor design Cascades of Stirred-Tank Reactors
Graphical solution for intermediate concentrations
KBK (ChE) Ch. 8 29 / 32
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Reactor design Cascades of Stirred-Tank Reactors
Graphical solution for best combination
KBK (ChE) Ch. 8 30 / 32
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Reactor design Cascades of Stirred-Tank Reactors
Graphical solution for best combination
KBK (ChE) Ch. 8 31 / 32
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Reactor design Combination of Reactors
Series Combination
KBK (ChE) Ch. 8 32 / 32