intro notes
TRANSCRIPT
Chemical Reactor Design An engineer seeks to design reaction
vessels, to carry out a particular chemical reaction or set of reactions This requires a knowledge of the reaction and
hence reaction kinetics Questions relate to operating conditions, heat
release/absorption/ reaction rate/ reaction order/phase change
A knowledge of the appropriate type of reactor to achieve this goal Scale, duration of reaction, conditions
CHG 3127: Reaction Engineering © E.J. Anthony 2012 1
Contact Details I have two e-mail accounts you can use
I can be contacted by telephone, but this is not reliable Tel: 613 996 2868
Course Marking Structure Tutorials and Assignments will be worth
15% of the mark (5% Tutorials and 10% Assignments) 6 Assignments remit in groups of 2
Mid-term 35% of the Mark Final 50% of the Mark
CHG 3127: Reaction Engineering E.J. Anthony 2012 3
TA for Class Xue Wang Email address: [email protected] Office: CBY D218 Office hours to be announced.
CHG 3127: Reaction Engineering © E.J. Anthony, 2012 4
I. Reviewing the Basics Introduction
basic definitions, industrial reactors
Mole Balances and Stoichiometry fractional conversion & extent of reaction mole balance equation stoichiometric tables systems with phase changes
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Basic Definitions
Chemical Species any chemical compound or element with a given identity (determined by kind, number, and configuration of atoms)
Chemical Reaction said to occur when detectable number of molecules of one or more species have lost their identity and assumed a new one
There may be intermediates, which maintain a steady concentration during the process of conversion
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Basic Definitions Decomposition Reaction
Combination Reaction
Isomerization Reaction
C H C H H2 6 2 4 2
C H H C H2 4 2 2 6
cis trans 2 butene 2 butene
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Basic Definitions
Homogeneous Reaction Reaction involving only one phase
Heterogeneous Reaction Reaction involving more than one phase, with the reaction usually occurring at or near the interface between phases
CHG 3127: Reaction Engineering © E.J. Anthony 2012 9
Basic Definitions
Series Reactions
Parallel Competitive Reactions
Independent Reactions
A B C
A BA C
A BC D
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Basic Definitions Chemical Reaction Rate, rj
rate of formation of species j, by chemical reaction, per unit volume (mass of catalyst, surface area of catalyst)
N.B. the reaction rate is not necessarily equal to dCj /dt This is true only for a constant volume batch reactor
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Basic Definitions
Elementary Reaction a reaction in which the reaction order of each
species is identical with the stoichiometric coefficient of that species within the reaction equation as written Note here that -rA is the rate of consumption of species A
A B Cr k C CA A B
22
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Basic Definitions
Non-elementary Reaction reaction rate is not discernible directly from
overall stoichiometry of reaction
22
21221
22 2
BrHBrkBrHkr
HBrBrH
HBr
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Basic Definitions
Molecularity the number of atoms, ions, or molecules involved
(colliding) in the rate-limiting step of the reaction
unimolecular: reaction involving one atom or molecule (e.g., radioactive decay)
bimolecular: reaction involving two atoms or molecules
termolecular: reaction involving three atoms or molecules
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Basic Definitions
Reaction Order relates to the exponents of the concentrations in the reaction rate expression of the general form reaction is order with respect to A, order with respect to B, order overall N.B. some complex reaction rate expressions yield meaningful orders under limiting conditions only
r kC CA A B ...
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Basic Definitions The Arrhenius law
assume the rate law given by
specific reaction rate constant, k, accounts for temperature dependence of reaction rate (Arrhenius expression):
N.B. in some complex cases k and F functions not separable:
k T Ae E RT /
r k T F C CA A B, ,...
2
2
2 *1 O
ONON Ck
kCr
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Classifying Industrial Reactors Classification by Reactor Operation
Batch Reactor all reactants enter at start high conversions, high labour costs limited scale-up potential
Continuous Reactor reactants introduced while products removed
continuously used extensively in large scale operations reactor may be tank, tube or vertical tower
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Classifying Industrial Reactors Classification by Reactor Operation
Semi-Continuous Reactor
either reactants or products removed continuously
remaining components entered batch-wise limitations of batch, except better control of
unwanted side reactions through control of specific component concentrations
used in two-phase reactions where gas is bubbled through liquid
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Classifying Industrial Reactors
Homogeneous batch reactor
hand holes for recharging reactor
connection for heating or cooling jacket
agitator
From Fogler, Elements of Chemical Reaction Engineering, 3rd Edition
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Classifying Industrial Reactors
Semi-batch reactor
reactant B
reactant A and product
heater or cooler
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Classifying Industrial Reactors Classification by Reactor Design
Features
Tank Reactor most common reactor type; ideal: CSTR operated in batch, continuous, or semi modes frequently includes mixing devices and heat transfer
components
Tubular Reactor common in gas phase reactions can contain packing, catalysts heat transfer jackets or shell and tube systems ideal form: plug flow reactor
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Classifying Industrial Reactors Classification by Reactor Design Features
Slurry Reactor vertical tower containing fine catalyst particles
suspended in liquid slurry gas phase reactant bubbled through high heat capacity of slurry maintains temperature control used in hydrocracking of fuel oils and in coal liquifaction
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Classifying Industrial Reactors CSTR
. Courtesy of Pfaudler, Inc
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Classifying Industrial Reactors
Longitudinal tubular reactor
From Fogler, Elements of Chemical Reaction Engineering, 3rd Edition
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Classifying Industrial Reactors Circulating fluidized bed reactor
feed
product
settling hopper
riser
standpipe
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Mole Balances and Stoichiometry Consider the reaction
generalized stoichiometric equation
extent of reaction fractional conversion
0 v B v C v S v Tb c s t... ...
v Ai ii
0
n nv
i i
i
0 X n nn
i i
i
0
0
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Mole Balances and Stoichiometry
relationship between and X
Xnv
lim,
lim
0
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Mole Balances and Stoichiometry Mole balance on species j in system of
volume V well-mixed system
not well-mixed
F F GdNdtj j j
j0
G r Vj j
G r V r dVj ji ii
m
jV
1
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Mole Balances and Stoichiometry General integral mole balance on
species j in system of volume V F F r dV
dNdtj j j
V
j0
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Stoichiometric Tables Objective: to find expressions for rj and
Nj
Consider the reaction
A as basis:
Rate of formation of C:
In general:
aA bB cC dD
A ba
B ca
C da
D
AAC racr
acr
ra
rb
rc
rd
A B C D
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Stoichiometric Tables
Consider the previous reaction with a batch system Moles A reacted at time t: Moles A remaining at time t:
A ba
B ca
C da
D
N XA0
N t N N X N XA A A A 0 0 0 1
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Stoichiometric Tables
Consider further the batch system with reaction Moles B reacted at time t: Moles B remaining at time t:
A ba
B ca
C da
D
moles reactedmoles reacted
moles reacted =BA
A ba
N XA 0
N t N ba
N XB B A 0 0
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Stoichiometric Tables Species Initial Change Remaining
A NA0 N XA0 N N XA A0 0
B NB0 ba
N XA0 N ba
N XB A0 0
C NC0 ca
N XA0 N ca
N XC A0 0
D ND0 da
N XA0 N da
N XD A0 0
I NI 0 — NI 0
Total NT0 N d c b a N XT A0 01
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Stoichiometric Tables
Total change in number of moles: Define parameter
represents change in total number of moles per mole of A converted
N N da
ca
ba
N XT T A
0 01
da
ca
ba
1
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Stoichiometric Tables
Concentration of species j given by For species B, for example,
CNVj
j
CN b
aN X
VB
B A
0 0
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Stoichiometric Tables
Define new variable to represent the inlet molar flow rate of species i relative to the inlet flow rate of the reference (limiting) reactant For species B, for example
ii
A
NN
0
0
CN b
aX
VB
A B
0
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Stoichiometric Tables For a constant volume process
Cases of constant volume process: dilute liquid phase reactions organic liquid phase reactions (except
polymerization reactions) gas phase reactions where number of reacting
molecules equals number of product ones, operating at constant T, P
CN
ba
X
V
Nba
X
VC
ba
XB
A B A B
A B
0 0
00
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Stoichiometric Tables
For flow problems, replace with
N Nj j and 0
F Fj j and 0
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Stoichiometric Tables Species Initial Change Remaining
A FA0 F XA0 F XA0 1
B FB0 ba
F XA0 F ba
XA B0
C FC0 ca
F XA0 F ca
XA C0
D FD0 da
F XA0 F da
XA D0
I FI 0 — FA I0
Total FT0 F XA0 F F XT A0 0
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Stoichiometric Tables
Local concentrations calculated from local molar flow rates and total volumetric flowrate C F
vii
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Stoichiometric Tables
In the case of negligible volume changes (most liquid phase reactions)
CFv
Fv
F Xv
C XAA A A
A
0
0
00
11
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Stoichiometric Tables
In the case of significant volume changes (most gas phase reactions) or Define a new symbol
N N N XT T A 0 0
NN
y XT
TA
001
y A0
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Stoichiometric Tables
Calculate the volume (volumetric flow rate) using the gas law: or, in terms of fractional conversion
V VPP
TT
zz
NN
T
T
0
0
0 0 0
V VPP
TT
zz
X
0
0
0 0
1
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Stoichiometric Tables
Concentration given by or, neglecting compressibility effects
0
00 1
/PP
TT
XXav
CC jjAj
XzzPPTTV
XavNV
XavNVN
C jjAjjAjj
1//
0000
00
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Stoichiometric Tables For a flow system
Concentration given by or, neglecting compressibility effects
v vPP
TT
zz
X
0
0
0 01
C C
v a XX
TT
PPj A
j j
0
0
01
CFv
F v a Xv
F v a X
v T T P P z z Xjj A j j A j j
0 0
0 0 0 0 1
Stoichiometric Tables In summary
For a batch system:
For a flow system:
For both:
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V VPP
TT
zz
X
0
0
0 0
1
v vPP
TT
zz
X
0
0
0 0
1
C Cv a X
XTT
zz
PPj A
j j
0
0 0
01
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Alternatives to X Fractional conversion not
appropriate for following situations: membrane reactions gas phase multiple reactions unsteady-state flow reactors
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Alternatives to X Express concentrations in terms of
molar flows in these cases
0
00 P
PTT
FF
CCT
jTj
00
0
0
00 RTZ
PvFC T
T
0
0
0
0
0
0
0
0
TT
PP
CF
zz
TT
PPF
Fvv
T
TT
T
(z=z0=1)
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Analysis of Reactions with Phase Change
Consider the reaction
Condensation occurs when
beyond this point each mole of D produced in the reaction condenses
A g B g C g D g l 2 ,
y PPD e
v
T,
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Analysis of Reactions with Phase Change
Create additional column in the stoichiometric table to account for balances after condensation begins
Let XC represent the conversion of A at which condensation begins
Use definition of mole fraction of D at condensation along with expressions for total molar flow rate to calculate XC
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Consider the example
Assume reactants only are introduced into a flow reactor according to their stoichiometric ratios:
A g B g C g D g l 2 ,
F FF F
B A
C D
0 0
0 0
20
Analysis of Reactions with Phase Change
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Analysis of Reactions with Phase Change
Species Remaining(Before Cond.)
Remaining(After Cond.)
A F XA0 1 F XA0 1
B F XA0 2 2 F XA0 2 2
C F XA0 F XA0
D F XA0 y FD e T,
Total F F XT A 0 3
F y F F F X
F F X yT D e T A A
T A D e
,
,. /
3 2
2 15 10 0
0
CHG 4127: Reaction Engineering © David G. Taylor, 2000 52
Calculate yD,e from operating conditions:
Equate total molar flow rates from two columns at point of condensation and solve for yD,e in terms of XC: or
y XXD eC
C,
3
y PPD e
vD
T,
F X F
XyA C A
C
D e0 03 2
151
.
,
Analysis of Reactions with Phase Change
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Analysis of Reactions with Phase Change to calculate the concentrations after
condensation we note that, at constant pressure and temperature
C CT T 0
CC
Fv
Fv
vFF
vT
T
T
T
T
T0 0
00
0
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Analysis of Reactions with Phase Change Knowing FT0 and FT, we can calculate
concentrations: where
T
TAAA F
FXvF
vFC 0
0
0 1
F FX
yF FT A
D eT A
2
151
30 0 0
.,
,
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Reversible Elementary Reactions Consider the reaction
assume rate constants written in terms of
benzene rate of disappearance of benzene by forward
reaction:
rate of appearance due to reverse reaction:
2 6 6 12 10 21 2C H C H Hk k,
r k CB forward B, 12
r k C CB reverse D H, 2
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Reversible Elementary Reactions
net rate of formation of benzene given by
net rate of consumption of benzene then equilibrium constant :
r r r r
r k C k C CB B net B forward B reverse
B B D H
, , ,
12
2
HDBHDBB CC
kkCkCCkCkr
1
2212
21
K kkc
1
2
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Summarizing So Far... Basic definitions:
chemical species, and reaction; types of reactions (e.g., decomposition, combination, etc.); reaction rate; elementary and non-elementary reactions; molecularity and reaction order; Arrhenius law
Characterizing industrial Reactors: by operation
batch, continuous, and semi-batch
by design tank , tubular, tower, fluidized bed, slurry
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Summarizing So Far...
generalized stoichiometric equation, extent of reaction, and fractional conversion
generalized mole balance equation well-mixed not well-mixed
CHG 3127: Reaction Engineering © E.J. Anthony 2012 59
Summarizing So Far...
relationships between reaction rates stoichiometric tables for batch systems tables for continuous (flow) systems definitions for and defining inlet moles (molar flow rates) in
terms of limiting reactant constant and variable volume processes
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Summarizing So Far... Use stoichiometric table to analyze phase
change Calculate critical mole fraction from vapour
pressure and operating pressure Add column for after condensation Use two expressions for total moles (or proper
ratio of molar flows) to determine critical X Expressions for reversible, elementary
reactions
Acknowledgement Slides taken and modified from David
Taylor
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