lecture notes chap 5 chem4005 2014 2015

8/18/2019 Lecture Notes Chap 5 CHEM4005 2014 2015

http://slidepdf.com/reader/full/lecture-notes-chap-5-chem4005-2014-2015 1/35

CHAPTER FIVE

MULTIPLE REACTIONS

1



Multiple reactions

Usually, there are more than one reaction occur in

a chemical reactor.

One of the key factors in the economic success of

a chemical plant is the minimization of undesired

side reactions that occur along with the desired

reaction.

2



Tpes o! "ultiple reactions

Parallel reaction (competing reactions)

Series reactions (consecutive reactions)

ndependent reactions

E#a"ple

!! CH CH −

O

NCH HOCH NH CH HOCH NH HOCH NH O E O E

"!!

..

!!!

..

!!" )()( → → →+

#esired product A $ C

k 1

k 2

A

C

$

k 2

k

1

!!! OCH CH +=

O H CO !! !! +

!! CH CH −

O

E#a"ple

A $k 1 C %

k 2

E#a"ple

T&e crac'in( o! cru)e oil to !or" (asoline

*Fee) stoc's containin( "an reactants+

,



Contactin( Patterns - $atc&

.



Contactin( Patterns

/



Parallel reactions

Purpose$ ma%imizing the desired product in

parallel reactions

&he rate of disappearance o! A0

A %

k D

A Uk U

*)esire)+

*un)esire)+

rate la'α

A D D C k r =!α

AU U C k r =

!' α α AU A DU D A C k C k r r r +=+=−

#efine a rate selectivity parameter, S

!' α α −== A

U

D

U

D DU C

k

k

r

r S

e ant t&is 3alue as lar(e as possi4le6



α1 7 α2

keep the concentration of as high as possile

in the gas phase$ without inerts and at high pressure

in the li*uid phase$ minimize the use of diluents

a atch or a plug+flow reactor is preferred

ecause - starts at a high value and drops progressively

cf$ - in a perfectly mi%ed -S& is always at its lowest value.

(i.e. the outlet concentration). /O& recommended.

Ma#i"i8in( t&e rate selecti3it para"eter 192

!' α α −= A

U

D DU C

k

k S

0!' >−α α

AC

:



∀α1 ; α2

1 keep the concentration of as low as possile diluting the feed with inerts

a -S& reactor is preferred

a recycle reactor in which the product stream acts as a

dilutent could e used to maintain the enteringconcentrations of at a low value.

!' α α −

= A

U

D

DU C k

k S

!' α α −

= A

U

D

DU C k

k S

2hat aout this it3

&his is actually affected y the reaction temperature.<

Ma#i"i8in( t&e rate selecti3it para"eter 292



Te"perature e!!ects 192

!' α α −

= A

U

D

DU C k

k S

Arr&enius E=uationRecall

−−= RT

E E

U

D

U

D

U D

e A

A

k

k )(

ratio o! !e=uenc !actor

)i!!erences 4eteen t&e acti3ation ener(

>



−

−= RT

E E

U

D

U

D

U D

e

A

A

k

k )(

4# 5 4U

1 &he reaction should e operated at the highest

possile temperature to ma%imize S#U.

4# 6 4U

1 &he reaction should e operated at a low

temperature to ma%imize S#U.

1 /ot so low that the desired reaction does not

proceed to any significant e%tent.1?

Te"perature e!!ects 292



Parallel reactions e#a"ple 192

eactant decomposes y three simultaneously reactions to form three

products$ one that is desired, #, and two that are undesired, 7 and U.

&hese gas+phase reactions, together with their corresponding rate laws, are

A D C T

r

D A

−=

→

'

"00

'!8000e%p00'!.0

9.''

"00

'!9000e%p00':.0 AU C

T r

U A

−=

→

9.0'"00'9000e%p00;9!.0 AQ C

T r

Q A

−=

→

<ow and under what conditions should the reactions aove e carried out

to minimize the concentrations of the unwanted products U and 7311

P ll l ti l 292



A D C T

r

D A

−=

→

'

"00

'!8000e%p00'!.0

9.''

"00

'!9000e%p00':.0 AU C

T r

U A

−=

→

9.0'

"00

'9000e%p00;9!.0 AQ C

T r

Q A

−=

→

%ue to its lo acti3ation ener(@ t&e rate

o! !or"ation o! ill 4e ne(li(i4le it&

repect to t&e rates o! !or"ation o! % an)

U at &i(& te"peratureB

QU

DUQ D

r r

r S

+==

&i(& te"perature

9.0

'

"00

''000

=

88.0

A

T

U

DUQ D

C

e

r

r S

−

=≈

lo concentration o! A

1B <igh temperatures.

!. >ow concentration of , which may e accomplished y

a. adding inerts

. using low pressures (if gas phase)

c. using a -S& or a recycle reactor

12

Parallel reactions e#a"ple 292



&at a4out to reactants 19,

A D $ %k D

A D $ Uk U

*)esire)+

*un)esire)+

rate la

'' β α

B A D D C C k r =!! β α

B AU U C C k r =

!'!' β β α α −−== B A

U

D

U

D DU C C

k

k

r

r S

1,



1.

Fi(ure 6B, %i!!erent

reactors an) sc&e"es!or "ini"i8in( unante)

pro)uct

& t 4 t t t t 29,



2hen α' 5 α!, β' 5 β!?aintain oth - and -@ as high as possile

tuular reactor or a atch reactor

<igh pressures (if gas phase)

2hen α' 5 α!, β' 6 β!?ake - high and -@ low

semiatch reactor in which @ is fed slowly into a large amount of

tuular reactor with side streams of @ continuously fed to the reactor

series of small -S&s with fed only to the first reactor and small

amounts of @ fed to each reactor.n this way, @ is mostly consumed efore the -S& e%it stream flows into the

ne%t reactor.

!'!' β β α α −−== B A

U

D

U

D DU C C

k

k

r

r S

1/

&at a4out to reactants 29,

&at a4out to reactants ,9,



2hen α' 6 α!, β' 6 β!?aintain oth - and -@ as low as possile

-S&, or a tuular reactor in which there is a large recycle ratio

feed dilute with inerts

>ow pressures (if gas phase)

2hen α' 6 α!, β' 5 β!

?ake - low and -@ high

semiatch reactor in which is fed slowly into a large amount of @

tuular reactor with side streams of continuously fed to the reactor

series of small -S&s with @ fed only to the first reactor and smallamounts of fed to each reactor.

!'!' β β α α −−== B A

U

D

U

D DU C C

k

k

r

r S

16

&at a4out to reactants ,9,



Series reactions

Purpose$ ma%imizing the desired product in seriesreactions

*)esire)+ *un)esire)+

&en to stop A % U

k 1

k 2

Ti"e is t&e 'e !actor &ere555

f the first reaction is slow and the second reaction is fast, it will e e%tremely difficult to

produce #.

f the first reaction is fast and the second reaction is slow, a large yield of # can e otain.

f the reaction is allowed to proceed for a long time in a atch reactor, or if the tuular flow

reactor is too long, the desired product # will e converted to U.1:



E#a"ple 6-. 19.

Mu#i"i8in( t&e iel) o! t&e Inter"e)iate

Pro)uct&he o%idation of ethanol to form acetaldehyde is carried out on a catalyst of ; wtA -u +

! wtA -r on l!O". Unfortunately, acetaldehyde is also o%idized on this catalyst to form

caron dio%ide. &he reaction is carried out in a threefold e%cess of o%ygen and in dilute

concentrations (ca. 0.'A ethanol, 'A O!, and B:.BA /!). -onse*uently, the volume

change with the reaction can e neglected.

#etermine the concentration of acetaldehyde as a function of space+time,

&he reactions are irreversile and first+order in ethanol and acetaldehyde, respectively.

!!

9

"!

'

)(!" !!!

COCHOCH OH CH CH OO

g → → ++

-H 2 O -2H

2 O

1<



A $ Ck 1

k 2

Mole 4alance on A0 A

A r dW

d =

Rate la !or A0 A A C k r '−=

Stoichiometry ( change of volume is neglected)$ 0vC A A =

A A C k

dW

dC v '0 −=

0'

0

v

W k

A A eC C

−

=

?@ C A C A?

τ ′−= '

0

k

A A eC C 0v

W =′τ 1>

E#a"ple 6-. 29.

E#a"ple 6 . ,9.



A $ Ck 1

k 2

Mole 4alance on $0 B

B

r dW

d

=Rate la !or $0

B A B C k C k r !' −=

Stoichiometry ( change of volume is neglected)$ 0vC B B

=

B A B C k C k

dW

dC v !'0 −−=

τ ′−

= '

0

k

A A eC C 0v

W =′τ

τ

τ

′−=+′

'

0'!

k

A B B eC k C k

d

dC

Inte(ral !actor@ e'2τG

τ τ

τ

′−′

=′

)(

0''!

! )( k k

A

k

B eC k d

eC d τG ?@ C$ ?

−−

=′−′−

'!

0'

!'

k k

eeC k C

k k

A B

τ τ

2?

E#a"ple 6-. ,9.



−

−

=

′−′−

'!0'

!'

k k

eeC k C

k k

A B

τ τ

Opti"u" iel)&he concentration of @ goes through a ma%imum at a point along the reactor. &o find the optimum

reactor length, we differentiate the aove e*uation$

( ) 0!'

!''!

0'

=+−

−=′

′−′− τ τ

τ

k k A B ek ek k k

C k

d

dC

!

'

!'

ln'

k

k

k k !ptim"m −

=′τ !

'

!'

0 lnk

k

k k

vW !ptim"m −

= 21

E#a"ple 6-. .9.



Co"ple# reactions

-omple% reactions consist of cominations of parallel and series reactions. t is usually easier to solve the prolems using moles N # or mole flow rate

# rather than conversion.

Procedure$

numer each reaction

mole alances for each species

determine the rate laws for each species in each reaction.

relate the rate of reaction of each species to the species with known rate laws (ased

on stoichiometry)

determine the net rate of formation of each species

comine all of the aove and solve the resulting set of coupled differential oralgeraic e*uaitons

22



Net rates o! reaction

Sum up the rates of formation for each reaction inorder to otain the net rate of formation.

f $ reactions are taking place

% B A

E B

E C A

DC B A

$A

B

A

A

k

k

k

k

→ +

→ +

→ +

+ → +

!

'

...

;"!

"!

"

"

!

'

∑

∑

=

=

=+++=

=+++=

$

i

iB$B B B B B

$

i

iA$A A A A A

r r r r r r

r r r r r r

'

"!'

'

"!'

...

...

∑=

=$

i

i# # r r '

reaction nu"4er

species2,



&he rate laws for each of the individual reactions

are e%pressed in terms of concentrations, - C, of

the reacting species.

4%ample $

"!

#k i#i# C C k r −=

Rate las

2.



elate the rate law for a particular reaction and species toother species participating in that reactionB

Stoic&io"etr0 relati3e rates o! reaction

E#a"ple0

dDcC &BaA +→+d r

cr

&r

ar DC B A ==−=−

i

iD

i

iC

i

iB

i

iA

d

r

c

r

&

r

a

r

==−=− !or t&e it& reaction

2/



E#a"ple 6B6 19.

Co"ple# reactions in PFR

Consi)er t&e !olloin( set o! reactions0

O H N NO NH k

!!" 898; ' + → +

!!!! O N NOk + →

!!! !! " NOO N k → +

9.'

'' " NO NH NO NO C C k r =−

!

!! !! NO N N C k r =

!

"" !!!! O N OO C C k r =−

2rite the rate law for each species in each reaction and then write the netrates of formation of /O, O!, and /!.

2rite the mole alances on a PD in terms of molar flow rates for each specie26

E#a"ple 6B6 29.



O H N NH NO k

!!"8

9

"

!' + → +

!!

!! O N NOk

+ → !!!

"

!

' NO N O

k → +

9.'


!

!! !! NO N N

C k r =

!

"" !!!! O N OO C C k r =−

Reaction 109.'


'8

9"

!'

!!" '''' O H N NH NOr r r r

==−

=−

9.'

''' ""

"

!)(

"

! NO NH NO NO NH C C k r r =−=−

9.'

''' "! 8

9)(

8

9 NO NH NO NO N C C k r r =−=

9.'

''' "! NO NH NO NOO H C C k r r =−= 2:

p



E#a"ple 6B6 .9.



Dor gas+phase reactions, the concentration of species # is $T

T

'

'

C C

T

#

T #0

0

0=

Dor no pressure drop and isothermal operation,

T

#

T #

C C

0=

!

!

0!

9.'

9.!

0'

!

!

9.'

'

!

"

!"

!

!

−

−=

−−==

T

NOT N

T

NO

T

NH

T NO

NO N NO NH NO NO NO

C k

C k

C k C C k r d(

d

Mole 4alances on all t&e species

Mole 4alance on NO

!

"

0"

!

!

0!

9.'

9.!

0'

!

"

!

!

9.'

'

!!

!!

"

!!!!"!

!

!

'

8

9

!

'

8

9

−

+

=

−+==

T

O

T

N

T O

T

NO

T N

T

NO

T

NH

T NO

O N O NO N NO NH NO N

N

C k

C k

C k

C C k C k C C k r d(

d Mole 4alance on N2

f all the mole alance e*uations are otained, the using the fact , the system can e

determined.

BBB

∑=

=)

#

#T '

2>

p

E l 6 : 19.



E#a"ple 6-: 19.

H)ro)eaI'lation o! Mesitlene in a PFR

&he production of m+%ylene y the hydrodealkylation of mesitylene over a <oudry #etrol

catalyst involves the following reactions$

m+Eylene can also undergo hydrodealkylation to form toluene$

&he second reaction in undesirale, ecause m+%ylene sells for a higher price than toluene.

2e therefore want to ma%imize the production of m+%ylene.

%esire) pro)uct

Un)esire) pro)uct

,?

E#a"ple 6-: 29.



&he hydrodealkylation of mesitylene is to e carried out isothermally at '900 F and "9 atm

in a packed+ed reactor in which the feed is 88.G molA hydrogen and ""." molA mesitylene.

&he volumetric feed rate is ;G8 ft"=h and the reactor volume is !": ft".

&he rate laws for reactions ' and ! are, respectively,

where the suscripts are$ ? H mesitylene, E H m+%ylene, & H toluene, ?e H methane,

and < H hydrogen.

t '900 F, the specific reaction rates are$

&he ulk density of the catalyst has een included in the specific reaction rate. #etermine

the concentrations of hydrogen, mesitylene, and %ylene as a function of space time.

9.0

!! H * T C C k r =

9.0

'' H + + C C k r =−

,m!l l& -t k

,m!l l& -t k

=)=(!."0

=)=(!.99

9.0"

!

9.0"

'

=

=

,1

p

E#a"ple 6-: ,9.



Reaction 10 +e * H + +→+

Reaction 20 +eT H * +→+

1B Mole 4alances0

H H H r

d(

d =

M

+e +e r

d(

d =

T T r

d(

d =T

* * r

d(

d =

Me

+ + r

d(

d =

''''

'''' +e * H + r r r r ==

−=

−

''''

!!!! +eT H * r r r r ==

−=

−

2B Rate las !or eac& species0

* H + H H H H C C k C C k r r r 9.0

!

9.0

'!' −−=+=

+ H + + C C k r r 9.0

'' −==

* H + H * * * C C k C C k r r r 9.0

!

9.0

'!' −=+=

* H T T C C k r r 9.0

!! ==

* H + H +e +e +e C C k C C k r r r 9.0

!

9.0

'!' +=+=

,B Con3ert F to C 0

H H C v 0=

*PFR+ *PFR+

+ + C v 0=

* * C v 0=

)()( 000 H H H H +eC C v −=−=

)( 000 * + + * + + T C C C v −−=−−=

reactant

reactant

inter"e)iate pro)uc

Ori(inall !ro" M Le!t as inter"e)iate pro)uct

,2

p

E#a"ple 6-: .9.



* H + H H C C k C C k

d(

C vd 9.0

!

9.0

'0 )(

−−=

+ H

+ C C k d(

C vd 9.0

'

0 )(−=

* H + H * C C k C C k

d(

C vd 9.0

!

9.0

'0 )(

−=

* H * + + C C k

d(

C vC vC vd 9.0

!0000 )(

=−−

* H + H

H H C C k C C k

d(

C vC vd 9.0

!

9.0

'

00 )(0 +=−

0v

(

=τ

space ti"e

* H + H H C C k C C k

d

dC 9.0

!

9.0

' −−=τ

+ H

+ C C k d

dC 9.0

'−=τ

* H + H * C C k C C k

d

dC 9.0

!

9.0

' −=τ

OB%BEB sol3er CH

C

CM

Opti"u"

,,

E#a"ple 6 < 192



E#a"ple 6-< 192

Co"ple# reactions in CSTR

&he same reactions as the previous e%ample, ut now in -S& 1B Mole 4alances0

H H H H r

(

−=

−0

M

+e +e r (

=

T T r

(

=T

* * r

(

=

Me

+ + + r

(

−=

−0

2B Rate las !or eac& species0

* H + H H H H C C k C C k r r r 9.0

!

9.0

'!' −−=+=

+ H + + C C k r r 9.0

'' −==

* H + H * * * C C k C C k r r r 9.0

!

9.0

'!' −=+=

* H T T C C k r r 9.0

!! ==

* H + H +e +e +e C C k C C k r r r 9.0

!

9.0

'!' +=+=

,B Con3ert F to C 0

H H C v 0=

*CSTR+ *CSTR+

+ + C v 0=

* * C v 0=

)( 000 * + + * + + T C C C v −−=−−=

reactant

reactant

inter"e)iate pro)uc

Ori(inall !ro" M Le!t as inter"e)iate pro)uct

,)()( 000 H H H H +e

C C v −=−=

E#a"ple 6-< 292



0v

( =τ

space ti"e

* H + H H H C C k C C k

C C 9.0

!

9.0

'0 +=

−τ

+ H

+ +

C C k

C C 9.0

'

0

=

−τ

* H + H * C C k C C k

C 9.0

!

9.0

' −=τ

CH

C

CM

Opti"u"

2e have dealt with prolems 2&<OU& volume change and

we used IconcentrationJ as the dependent variale.

2e are going to solve the prolems 2&< volume change and we should e using Imolar flow rateJ as the dependent variale.

,/

p

lecture notes chap 5 chem4005 2014 2015

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