Chemical Reactors - American Chemical Society

4 Simulation of a Fluidized Bed Reactor for the Production of Maleic Anhydride

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 2, 2016 | http://pubs.acs.org Publication Date: September 21, 1981 | doi: 10.1021/bk-1981-0168.ch004

J. L. JAFFRÈS, W. IAN PATTERSON, C. CHAVARIE, and C.

1

LAGUÉRIE

Ecole Polytechnique, Montréal, Canada

The s i m u l a t i o n o f a fluidized bed p r e h e a t e r - f l u i d i zed bed r e a c t o r system f o r the c a t a l y t i c o x i d a t i o n of benzene to maleic anhydride was attempted. The experimental apparatus and r e s u l t s of K i z e r e t a l ( 7 ) together with the k i n e t i c s proposed by Quach e t al ( 8 ) formed the b a s i s f o r the s i m u l a t i o n . I t was determined that the r a t e constants and a c t i v a t i o n energies would not s u c c e s s f u l l y describe the exper i m e n t a l r e s u l t s , and these parameters were estimated using a p o r t i o n o f the r e s u l t s . The r a t e constants and a c t i v a t i o n energies found in this manner were c l o s e to those reported by other workers f o r s i m i l a r c a t a l y s t s . The s i m u l a t i o n using these estimated parameters gave reasonable agreement with the comp l e t e experimental r e s u l t s f o r conversion and selectivity as f u n c t i o n s o f temperature, a i r flow r a t e and bed h e i g h t , except f o r selectivity versus bed h e i g h t . An unsteady-state s i m u l a t i o n agreed q u a l i t a t i v e l y with the l i m i t e d data a v a i l a b l e . The production of maleic anhydride by the c a t a l y t i c o x i d a t i o n of benzene is an e s t a b l i s h e d i n d u s t r i a l process. While hydrocarbons are o f t e n suggested as a feedstock, it has been pointed out r e c e n t l y by De Maio ( 1 ) that they are an a l t e r n a t i v e but not n e c e s s a r i l y a s u b s t i t u t e . " The benzene o x i d a t i o n is done commerc i a l l y in f i x e d bed r e a c t o r s and, because of i t s e x o t h e r m i c i t y , is d i f f i c u l t t o c o n t r o l in any optimal sense. The process is thus a n a t u r a l candidate f o r a fluidized-bed r e a c t o r . The r e a c t i o n has been s t u d i e d in both f i x e d bed ( 2 , 3 ) and fluidized bed ( 4 - 7 ) r e a c t o r s . These s t u d i e s , with the exception o f that of K i z e r e t a l Ç7) do not give s u f f i c i e n t i n f o r m a t i o n f o r s i m u l a t i o n purposes. The a v a i l a b i l i t y of the r e a c t i o n data of K i z e r e t a l and the k i n e t i c s t u d i e s o f Quach e t a l ( 8 ) using a s i m i l a r c a t a l y s t suggested the p o s s i b i l i t y o f s i m u l a t i n g the process. -

1

I n s t i t u t du génie chimique, Toulouse,

France

0097-6156/81/0168-0055$05.00/0 © 1981 American Chemical Society Fogler; Chemical Reactors ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

CHEMICAL REACTORS

56


Reaction

Kinetics

The key to good r e a c t o r s i m u l a t i o n is undoubtedly a knowled ge of the r e a c t i o n k i n e t i c s . The k i n e t i c s of the c a t a l y t i c o x i dation of benzene to maleic anhydride has been studied f o r d i f f e rent c a t a l y s t s and conditions by many workers (8-13) however only Quach e t a l (8) examined a c a t a l y s t , FX203, of a type s i m i l i a r to that employed by K i z e r e t a l (FB203-S). Both c a t a l y s t s are f a b r i c a t e d by Halcon C a t a l y s t I n d u s t r i e s , but are of d i f f e r e n t formulation. Quach e t a l studied the c a t a l y s t ( i n the form of O.4 cm gra nules) in a Carberry-type r e a c t o r . Reaction conditions were: a temperature range of 280°C to 430°C and a benzene to a i r feed r a t i o v a r i a t i o n of O.45 to 8.23 mol percent. Their results dic tated a two-step o x i d a t i o n of the form: C H 6

+ 40

6

C H 0

2

4

2

+ CO + C 0

3

+ 2H 0

2

(1)

2

C.H 0_ + 20 -> 2C0 + 2C0 + H 0 4 2 3 2 2 2 o

o

o

o

Both r e a c t i o n s are exothermic and e s s e n t i a l l y i r r e v e r s i b l e . The maleic anhydride formation occurs only a t the c a t a l y s t surface while i t s degradation takes place in the gas phase ( 8 ) . I t is therefore expected that the s e l e c t i v i t y and the conversion w i l l be e q u a l l y important in the operation of fluidized bed r e a c t o r . Quach e t a l found that the benzene conversion rate was best des c r i b e d by the Langmuir-Hinshelwood r e l a t i o n : k

Γ β

=

k

P

1

/

k D 0 0

2

1 / 2

k

5

k

+ 4k_n O ^ B

P

where:

P

B 0 B 0

=

B

=

3

4

5

9

,

0

0

1 e

e

x

p

x

p

(-

2 4 6 0

R T

°/ )

6

(- *300/RT)

(

3

)

P

τ- = r e a c t i o n rate in gmol · g ^ · h D

The form of equation (3) i n d i c a t e s that oxygen d i s s o c i a t i o n occurs before i t s adsorption on the c a t a l y s t . When the r e a c t i o n has a be ne l a r g e excess of a i r ( *gg « χ i %) equation (3) can be r e w r i t t e n as: m

k

Γ

Β = -

o

p

B R =

V~

k

B

P

B

Vo and

f i r s t order k i n e t i c behaviour w i l l be observed. The gas phase degradation of the maleic anhydride is d e s c r i bed by: 1/2 Μ M M ; *Μ (-33400/RT) (5) —3 —1 where r = r e a c t i o n rate in gmol · m «h Γ

=

k

P

=

9

0

0

0

0

e

x

p

M

Fogler; Chemical Reactors ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

JAFFRES E T AL.

4.

Maleic Anhydride Production

57

P i l o t Reactor The r e a c t o r used by K i z e r and simulated in this workisi l l u s t r a t e d in Figure 1. I t c o n s i s t s of a fluidized bed preheater s e c t i o n feeding d i r e c t l y the fluidized bed r e a c t o r s e c t i o n . Each s e c t i o n was a O.4 m high c y l i n d e r of O.184 m diameter. The pre heater contained sand and was heated by an e x t e r n a l e l e c t r i c a l element. The FB203S c a t a l y s t is a powder o f O.173 mm diameter p a r t i c l e s (weight average) and has a minimum fluidization veloci t y , U f , of O.021 m · s ~ l a t normal temperature and pressure. The r e a c t o r was cooled by ambient a i r blown through a j a c k e t . The r e a c t o r d i s t r i b u t o r was made from a 50 mm t h i c k f i x e d bed of 5 mm diameter pebbles supported on a p e r f o r a t e d p l a t e with the benzene introduced a t i t s centre. N i c k e l p a r t i c l e s (O.53 mm diameter) to a depth o f 25 mm on top of a second p e r f o r a t e d p l a t e formed a second f i x e d bed and completed the d i s t r i b u t o r . The r e a c t o r was completely i n s u l a t e d with g l a s s wool.


m

Experimental

Results

The e f f e c t s of the r e a c t i o n temperature, T, the a i r flow rate F (reported a t 20°C and 1 atm), the depth of the c a t a l y s t bed, H ^ , and the molar concentration of benzene, c, on the conversion, s e l e c t i v i t y and production were reported by K i z e r e t a l (14). The experiments were performed according to a f a c t o r i a l p l a n o f 2^ ex periments w i t h i n the f o l l o w i n g l i m i t s : a

430°C £ Τ £ 490°C 4 £ F 3
=11900 Ε 3 =58100 f0 =2900

bo

B

f =10260 Eg =60000 D =O.420

bo

B

f =37500 E =66800 D =O.043

D

= 0

c

5

n

6 4 3

„ . ™ *°-J £° bo -

Optimized parameters

* These c o n d i t i o n s a r e the c e n t r a l p o i n t of the f a c t o r i a l p l a n and are counted four times f o r the purposes of o p t i m i z a t i o n .

Γ

λ

b

Eq. Y c 3 D (cm)

b

1.25

87

80

67

75

1.53

O.93

1.25

1.18

82

1 5 4 460

83

1.22

b

82

1 5 6 490

83

75

66

1 5 6 430

66

c D (cm)

1 5 6 460

*

75

Y

Eq. Y c 4 D (cm)

Eq. 4

a

mf ι F (m3h-1) T(°C)

H

c(mol %)

Y, % c

Eq. 3

η

OrcuttDavidson PF

OrcuttDavidson PM

Operating conditions

Kinetics:

_.

Case C

Case Β

Case A

1

Kizer s data

TABLE I I RESULTS OF OPTIMIZATION OF MODEL PARAMETERS



4.

JAFFRÈS E T AL.


!

^

67

Maleic anhydride degradation Iproducts

plus p iu

^

I

Emulsion

Bubble

Homogeneous oxidation of

phase

phase

maleic

anhydride

51 |* — g Emulsion

phase

[

g Maleic

Bubble phase

anhydride Catalytic oxidation of Benzene

Figure 5. Model of thefluidizedbed. Benzeneflowisshown by the heavy solid line and maleic anhydrideisrepresented by the heavy dashed line.


68

CHEMICAL REACTORS

1/2 k

k

P

where k_ = 11900

P

B O B 0

D

k k

P

0 0

1 / 2

+

e

4 k

-58100/RT (20)

65900/RT = 2900 e"

Q

P

B B

f o r the c a t a l y t i c o x i d a t i o n , and


r

™ M

=

k

ι 1/2 -30000/RT ™P™ > where k , = 237000 e M M M ,

0

0

7

Λ Λ Λ

(21)

x

I m p l i c i t in these equations are the s u c c e s s i v e o x i d a t i o n s of benzene and maleic anhydride. The d i r e c t o x i d a t i o n of benzene to water and carbon oxides is not permitted. The o p t i m i z a t i o n r e s u l t s reported in t a b l e I I i n d i c a t e that the a c t i v a t i o n energies are almost independent of the model chosen to represent the fluidized bed r e a c t o r . Furthermore, the a c t i v a t i o n energy obtained in this manner agree with those reported by Holsen, Steger and Germain et a l while those given by Quach are much s m a l l e r . The data are summarized in table I I I below. More over, it is known from the c a t a l y s t f a b r i c a t o r that f i x e d bed reactors having an i n l e t benzene c o n c e n t r a t i o n of 1.5% and a r e s i dence time of O.72 s. give conversions on the order of 93 to 95%. The k i n e t i c s r e q u i r e d f o r this r e s u l t c o i n c i d e with the k i n e t i c s obtained from the numerical experimentation. F i n a l l y , we note that the energy of a c t i v a t i o n f o r the homogeneous decomposition of maleic anhydride obtained from the o p t i m i z a t i o n is in good agree ment with the work of Quach et a l . TABLE I I I COMPARISON OF ACTIVATION ENERGIES Worker

Catalyst

Holsen

V 0 /A1 0

Steger

Ag 0, V 0 , Al 0 /SiC

2

5

2

2

2

2

325-450

3

5

Temperature range °C

MoO

Activation energy kJ/mole 81-82

450-530

63

380-500

92-42

3

Germain et a l

V 0 /Mo0

Quach et a l

V 0 /Si0

2

280-430

24

Our numerical optimization

V 0 /Si0

2

430-490

60-67

2

2

2

5

5

5

b i s c u s s i o n and Conclusion:

3

F l u i d i z e d Bed Model

The optimized values given in t a b l e I I i n c l u d e the values of the mean bubble diameter. These values are c o n s i s t e n t l y smaller than those c a l c u l a t e d from the Mori and Wen equation. For example, at the c e n t r a l p o i n t of the f a c t o r i a l p l a n , a value of = 2.1 cm is p r e d i c t e d by Mori and Wen's equation while the "optimized 11


4.

JAFFRÈS ET A L .

69


values f o r vary between 1 . 2 2 and 1 . 7 1 cm depending on the simul a t i o n case. This discrepancy is not e n t i r e l y unexpected s i n c e the bubble diameters i d e n t i f i e d from the fluidized bed models are apparent or e f f e c t i v e values i n t i m a t e l y l i n k e d to the mass t r a n s f e r mechanism of the model. The smaller bubble s i z e values obtained by our procedure may simply mean that the a c t u a l mass t r a n s f e r is l a r g e r than that suggested by the Orcutt-Davidson models. This is compat i b l e with the f a s t r e a c t i o n assumption that implies a disproport i o n a t e l y high conversion c l o s e to the d i s t r i b u t e r and a much higher mass t r a n s f e r rate in this zone. C a l c u l a t i o n s o f conversion and s e l e c t i v i t y have good general agreement with K i z e r s r e s u l t s as shown in F i g u r e 6. An exception is the s e l e c t i v i t y - b e d height r e l a t i o n s h i p . Our c a l c u l a t i o n s show s e l e c t i v i t y to be i n s e n s i t i v e to bed height, but K i z e r found a strong inverse r e l a t i o n between s e l e c t i v i t y and . K i z e r explains this by proposing a d i r e c t o x i d a t i o n of benzene to water and carbon oxides which is in compet i t i o n with the o x i d a t i o n to maleic anhydride. We note that the heterogeneous d e p l e t i o n of maleic anhydride may a l s o e x p l a i n the above behaviour. K i z e r e t a l ( 1 4 ) claimed that the combined e f f e c t s of bed height and flow r a t e could be replaced by the residence time. This implies that simple f i x e d bed models could be used to adequately describe this r e a c t o r . Table I I and Figure 4 shows that this could be the case f o r the Orcutt-Davidson PM model, however the model demands the u n r e a l i s t i c value of D^Q = O.043 cm (case A). The PF model requires operation away from the l i m i t i n g conv e r s i o n s and is thus in c o n f l i c t with K i z e r s claim, although more r e a l i s t i c values o f D are estimated. I t seems probable that the r e a c t o r operation is somewhere between that of a single-phase p e r f e c t l y mixed r e a c t o r and plug flow in the same r e a c t o r . I t is p r e c i s e l y in this region that both bed hydrodynamics and k i n e t i c s are important. Thus, it is not u s e f u l to f u r t h e r analyse our r e s u l t s without possessing indépendant knowledge o f the hydrodynamic or k i n e t i c parameters. A number of points have become apparent as a r e s u l t o f our e f f o r t s to simulate the fluidized-bed reactor-preheater system s t u d i e d by K i z e r . Two of the most important are: it is imperat i v e to have good k i n e t i c data f o r the r e a c t i o n ( s ) that occur. I t has been demonstrated that the i n t e r p r e t a t i o n o f the r e s u l t s is profoundly a f f e c t e d by r e l a t i v e l y small changes in the k i n e t i c s . The second important point is the r e c o g n i t i o n that there are r e gions of operation where both the r e a c t i o n k i n e t i c s and the bed hydrodynamics i n f l u e n c e the o v e r a l l performance o f the r e a c t o r . The coupling o f k i n e t i c and hydrodynamic e f f e c t s is strong such that both must be known to properly describe the r e a c t o r behaviour . We note that this model is not s u i t e d to process c o n t r o l purposes. The computational resources and time r e q u i r e d are simply too great to allow r e a l - t i m e c o n t r o l algorithms to use this


T

1

b o



65

4 0

4 5

460

460

490

490

T,°C

e

T, C

a

3

1

m /

mf,cm

mf,cm

Figure 6. Conversion and selectivity vs. catalyst temperature, airflowrate,and bed height. The results of Kizer et al. are the solid lines and our calculations are shown as the hatched area. Operating conditions are: F = 6m h' , H = 5 cm, Τ = 460°C, benzene concentration, c == 1 mol percent except when the variable appears on the abscissa.

430

55

- 5 0

ο

1

430

65 "

70

^60 > %

ο

1

~ 85^

c ο

c= 1%


8

H

M

ο

4. JAFFRÈS E T AL.


71

model in s p i t e o f the many s i m p l i f y i n g assumptions made to reduce the computer l o a d .


Legend of Symbols A a C c D Ε F, f G h H H ΔΗ K k, k Pr p Q Re R r Τ U V w X

= =

T

1

x

= -

-

a i r (at NTP; 20°C and 1 atm) benzene bubble, initial, mean convection fluid inlet

-

Y , Yp, Y 3 η λ y V ξ p c

s

z

c r o s s - s e c t i o n a l area o f r e a c t o r , m h e a t - t r a n s f e r area, m^ s p e c i f i c heat, c a l * g ~ l c o n c e n t r a t i o n , mol percent diameter, m a c t i v a t i o n energy, J · mol"*l volumetric flow r a t e , m^ · h~ mass flow r a t e , g · h " l heat t r a n s f e r c o e f f i c i e n t , c a l · m · s"" °C enthalpy, c a l · g " l height, m heat o f r e a c t i o n , c a l · mol~"l (kgRTW )/(AU), dimensionless r e a c t i o n r a t e r e a c t i o n r a t e constant, s e c ~ l P r a n d t l number, dimensionless ( p a r t i a l ) pressure, Ν · m~2 heat, c a l Reynolds number, dimensionless gas constant r e a c t i o n r a t e , mol · h " l temperature, °C v e l o c i t y , m · sec~"l volume, m^ mass, g (xH)/(U Vb)* number o f t r a n s f e r u n i t s , equations (10) and (11) o v e r a l l r a t e of exchange between bubble and dense phase r e a c t o r conversion, production and s e l e c t i v i t y 1 - ( U / U ) , equations (10) and (11), dimensionless parameter of equation (16) _^ thermal c o n d u c t i v i t y , c a l · sec · m~l · °C~1 v i s c o s i t y , Pa · sec""i s t o i c h i o m e t r i c c o e f f i c i e n t , dimensionless parameter of equation (16) d e n s i t y , g · cm"3 s

1

b

mf

Subscripts a Β b , bo, bm c f in


72

CHEMICAL REACTORS

summation i n d i c e s lost maleic anhydride mean minimal fluidization particle reactor, reaction separation solid

I M m mf


Ρ R S s

Acknowledgements The authors g r a t e f u l l y acknowledge the research grants pro vided by the Province of Quebec (FCAC), the N a t u r a l Sciences and Engineering Research C o u n c i l of Canada and the I n s t i t u t du génie chimique de Toulouse.

Literature Cited 1.

De Maio, D.A., " W i l l Butane Replace Benzene as a Feedstock for M a l e i c Anhydride", Chem. Eng., 1980, May 19, p. 104. 2. Germain, J.E., Graschka, F., Mayeux, Α., "Cinétique de l'oxyd a t i o n c a t a l y t i q u e du benzène sur oxydes de vanadium-molybdène", Bull. Soc. Chim. F r . , 1965, p. 1445. 3. Vaidyanathan, K., Doraiswamy, L.K., " C o n t r o l l i n g Mechanism in Benzene O x i d a t i o n " , Chem. Eng. Sci., 1968, 23, 537. 4. Badarinarayana, M.C., Ibrahim, S.M., Kuloor, N.R., " S i n g l e Step C a t a l y t i c Vapor Phase O x i d a t i o n of Benzene", Ind. J. Techn., 1967, 5, 314. 5. K u l l a v i n a j a y a , P., "Statistical Study of the Benzene O x i d a t i o n Process in a F l u i d i z e d Bed Reaction", Ph.D. t h e s i s , Ohio State U n i v e r s i t y , Columbus, 1966. 6. Ahmad, S.I., Ibrahim, S.M., Kuloor, N.R., " K i n e t i c Studies on the Oxidation of Maleic Anhydride", Ind. J. Techn., 1971, 9, 251. 7. K i z e r , O., Laguérie, C., Angelino, Η., "Experimental Study of the C a t a l y t i c Oxidation of Benzene to Maleic Anhydride in a F l u i d i z e d Bed", Chem. Eng. J o u r n a l , 1977, 14, 205. 8. Quach, T.Q.P., Rouleau, D., Chavarie, C., Laguérie, C., "Catalytic Oxidation of Benzene to M a l e i c Anhydride in a Continuous S t i r r e d Tank Reactor", Can. J. Chem. Eng., 1978, 56, 72. 9. Hammar, C.G.B., "Reaction K i n e t i c s of the C a t a l y t i c Vapor-Phase Oxidation of Benzene to Maleic Anhydride", Svensk. Kem. Tid., 1952, 64, 165. 10. Holsen, J.N., "An I n v e s t i g a t i o n o f the C a t a l y t i c Vapor Phase Oxidation of Benzene", Ph.D. t h e s i s , Washington U n i v e r s i t y , S t . L o u i s , M i s s o u r i , 1954.


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Chemical Reactors - American Chemical Society

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