Ch 3 - Chain Reaction
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Transcript of Ch 3 - Chain Reaction
8/13/2019 Ch 3 - Chain Reaction
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A D V A N C E D
C H E M I C A L R E A
C T I O N E N G I N E E R I N G
Chain Reactions
Example
Initiation step
Propagation
Termination
Propagation steps occur faster than
initiation and termination steps
FACULTY OF CHEMICAL ENGINEERING CPE624 2
CH
COCH CHOCH 43
23623
32233
311343
3
][ 2
][
]][[
][ ,
CH k r H C CH
COCH k r CH COCOCH
CH Ak r COCH CH CH A
Ak r CHOCH A
t t
p p
p p
i i
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A D V A N C E D
C H E M I C A L R E A
C T I O N E N G I N E E R I N G
Pseudo – steady – state approximation
PSS – setting time derivative equal to zero
Mass balance equations are developed by
assuming steady state, so that PSS of intermediate
species is in steady state and concentration is small.
CPE624FACULTY OF CHEMICAL ENGINEERING 3
Example: Explain the physical meaning of pseudo-steady-state approximation, when applied to a CSTR and a PFR.
Explanation:-PSS valid if the concentration of a species is small.- In PFR or batch system, the time derivative is set equal to zero,d[CH3CO.]/dt = 0, not the concentration equal to zero.Physical meaning: The concentration of free radical is constant.-For CSTR, PSS approximation is indeed that the concentration be small or[CH3CO.]/ = 0Physical meaning: The concentration of free radical is small.
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A D V A N C E D
C H E M I C A L R E A
C T I O N E N G I N E E R I N G
FACULTY OF CHEMICAL ENGINEERING CPE624 5
CH
{[CH3 CO• ]-[CH3 CO• ]o/ = kp1[CH3CHO][CH3 • ] - k p2[CH3CO • ]
{[CO]-[CO ]o/ = k p2[CH3CO • ]
{[CH4]-[CH4 ]o/
= kp1[CH3CHO][CH3 • ]
[CH3 •] and [CH3CO •] are very reactive, thus the concentrations are very low.
{[CH3 CO• ]-[CH3 CO• ]o/ = kp1[CH3CHO][CH3 • ] - k p2[CH3CO • ] = 0
kp1[CH3CHO][CH3 • ] = k p2[CH3CO • ]
[CH3CO • ] = kp1/k p2 [CH3CHO][CH3 • ] …………………………(1)
{[CH3 • ]-[CH3 • ]o/ = -k i[CH3CHO] – kp1[CH3CHO][CH3 • ] + k p2[CH3CO • ]- 2k t [CH3 • ]2= 0 …………………..(2)
Insert (1) into (2)Ki[CH3CHO] - kp1[CH3CHO][CH3 • ] + k P2 x kp1/k p2 [CH3CHO][CH3 • ] - 2k t [CH3 • ]2=0
Ki[CH3CHO] - 2k t [CH3 • ]2 = 0
[CH3 • ]2= k i/2k t [CH3CHO][CH3 • ] = (k i/2k t )
1/2[CH3CHO]1/2
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A D V A N C E D
C H E M I C A L R E A
C T I O N E N G I N E E R I N G Generic chain reaction
The reaction propagates by radical R·.
ni and nt are the number of molecules react in
initiation and termination steps respectively.
PSS approximation on CR yield the overall rateexpression
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C B A
X R n
R C BR A
R An
t
i
,
,
t
i
n
R t t
R A p p
n
Ai i
C k r
C C k r
C k r
t
i t
n
n
A p
n
t t
i C k k n
k r
1
1
eff n
Aeff C k r
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A D V A N C E D
C H E M I C A L R E A
C T I O N E N G I N E E R I N G
Characteristic of chain reactions Large temperature dependences.
Sensitive to trace impurities that can alter theinitiation and termination rates
Initiators and Scavengers (promoters and poisons)
have large influences.
Initiators
Initiated by adding species I that easily forms radical
Initiate the reaction faster than reactant.
Scavengers
Termination step - radical species decomposed /reactedwith other radical species to form an inactive species X
Adding scavengers S
X is important in determining overall reaction rate
CPE624FACULTY OF CHEMICAL ENGINEERING 8SR tsts C C k r X SR ,
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A D V A N C E D
C H E M I C A L R E A
C T I O N E N G I N E E R I N G
Wall termination reactions
Surface reaction steps important in controlling
chain reactions.
Wall termination reactions introduce a complexity
to all chain reactions – the overall reaction rate as
a function of the size of reactor.
In a small reactor, termination reactions on surface
keep the radical intermediate small and inhibit
chain reaction.
In large reactor, the termination rate is smaller.
CPE624FACULTY OF CHEMICAL ENGINEERING 9
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A D V A N C E D
C H E M I C A L R E A
C T I O N E N G I N E E R I N G Example: Briefly explain the terms initiator and
scavenger for a chain reaction
Initiator: Species (I) that can easily form radicals in a
chain reaction and can initiate the reaction faster
than the reactant.
Scavenger: Species (S) added into a chain reaction
that can readily scavenge the chain propagator to
terminate the reaction.
CPE624FACULTY OF CHEMICAL ENGINEERING 10
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A D V A N C E D
C H E M I C A L R E A
C T I O N E N G I N E E R I N G Autooxidation
Autooxidation – autocatalytic process andit is an oxidation that converts alkanes into
alkyl peroxides.
CPE624FACULTY OF CHEMICAL ENGINEERING 11
ROOH OH R 2
R ROOH H R O
ROOOR
H R H R
RO
,
2
• A research student forgot to close a bottle of an organic compound tightly and kept on a table. The student left for vacation and when he came bac
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A D V A N C E D
C H E M I C A L R E A C T I O N E N G I N E E R I N G Autooxidation
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A research student forgot to close a bottle of an organic
compound tightly and kept on a table. The student left for
vacation and when he came back after one month, he saw the
table and everything on the table destroyed. Illustrate the
reaction steps and mechanism that occurred in the bottle.
Example
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A D V A N C E D
C H E M I C A L R E A C T I O N E N G I N E E R I N G
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(a) Initiation: A → R• ; r i = k iCA
Propagation: A + R• → B + C + R•; r i = k pCA CR
Termination: R•→ X; r t = k tCR
(b) Overall reaction rate:CR/ = k iCA – k tCR = 0 CR = k i/k t x CA r = k pCACR
= k ik p/k t CA2
(c) Solving CSTR equation: r = CAo – CA = (CAo – CA)/r = (CAo – CA )/CA
2 x k t/k ik p
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A D V A N C E D
C H E M I C A L R E A C T I O N E N G I N E E R I N G
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= [2 moles/L - 2(1-0.98)]/(0.04)2 x 0.05/ s/0.002/s x 10 l/s
= 1225 x 2.5 = 3062.5 s
= 51 min
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A D V A N C E D
C H E M I C A L R E A C T I O N E N G I N E E R I N G Chain branching reaction
Produce more than one free radical species in
propagation step. Thus, the propagation steps
increase the concentration of radical species and
destabilize the kinetics.
Example:
Rapid rise in the concentration of radical species can
accelerate the reaction and possibly a chain-
branching explosion.
CPE624FACULTY OF CHEMICAL ENGINEERING 16
OH OR ROOH
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A D V A N C E D
C H E M I C A L R E A C T I O N E N G I N E E R I N G Model of chain branching reaction
Consider the reaction of :
The mechanisms are:
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C B A
X R
R C BR A
R A
,
,
R t t
R A p p
Ai i
C k r
C C k r
C k r
A pt
Ai R C k k
C k C )1(
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A D V A N C E D
C H E M I C A L R E A C T I O N E N G I N E E R I N G References
Schmidt, L.D. (2005). The Engineering of Chemical
Reactions, 2nd edition, New York: Oxford University
Press.
Fogler, H.S. (2006). Elements of Chemical ReactionEngineering , 4th Edition, New Jersey: Prentice Hall.
Levenspiel, O. (1999). Chemical Reaction
Engineering , 3rd Edition, New York: John Wiley.
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