Chemical and Catalytic Reactor Modeling - American Chemical Society

2 Influence of the Reactional System on the Irrigation Rate in Trickle-Bed Reactors Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 2, 2017 | http://pubs.acs.org Publication Date: December 9, 1984 | doi: 10.1021/bk-1984-0237.ch002

P. RUIZ, M. CRINE1, A. GERMAIN, and G. L'HOMME Laboratoire de génie chimique et de chimie industrielle, Groupe de Chimie Appliquée, Univer sité de Liège, Rue A. Stévart, 2, B-4000, Liège, Belgique When designing a trickle-bed reactor, i t is essential to know how the catalyst i r r i g a t i o n rate varies with the operating conditions. We previously developed a theoretical model based on percolation theory, relating i r r i g a t i o n rate to a hydrodynamic parameter characterizing the effective particle w e t t a b i l i t y , i.e., the one prevailing with the reactional system. In this paper, we present an experimental determination of the variations of this parameter with some characteristic variables of a specific reactional system: the hydrogenation of maleic acid into succinic acid on a Pd/Al2O3 supported catalyst. The data interpretation shows how temperature and concentrations may affect the catalyst i r r i g a t i o n rate, eviden cing the importance of carrying out hydrodynamic experiments under chemical operation. Catalytic fixed bed reactors with concurrent downflow of gas and l i q u i d reactants are widely spread i n the refining as well as i n the chemical industry (1). They are generally operated i n the t r i c k l i n g flow regime i.e. with the gas as the conti nuous phase and with the l i q u i d t r i c k l i n g on the fixed bed of catalyst. It i s well known that, i n such t r i c k l e bed reac tors, the performances strongly depend on : - transfers of reactants at the gas-liquid interface, - transfers of reactants at the l i q u i d - s o l i d interface, - gas-liquid i n t e r f a c i a l area, ~ l i q u i d - s o l i d i n t e r f a c i a l areas, - specific a c t i v i t y of the c a t a l y t i c system, including pore diffusion limitations. - hydrodynamics, (see for example r e f . 2 ) . In addition, strong interactions between these physical and chemical phenomena make the prediction of performances very difficult. This does not guarantee that the separate study Research associate, F.N.R.S.

1

0097-6156/84/0237-0015$06.25/0 ©1984 American Chemical Society Dudukovi and Mills; Chemical and Catalytic Reactor Modeling ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 2, 2017 | http://pubs.acs.org Publication Date: December 9, 1984 | doi: 10.1021/bk-1984-0237.ch002

16

CHEMICAL AND CATALYTIC REACTOR MODELING

of each step of the r e a c t i o n pathway i s s u f f i c i e n t to completely understand the behaviour of the r e a c t o r . A pure phenomenological model of such an i n t r i c a t e process, t a k i n g i n t o account a l l p o s s i b l e r e a c t i o n steps, i s t h e r e f o r e a powerful t o o l f o r the s c a l e up and the p r e d i c t i o n of p e r f o r mances of t r i c k l e - b e d r e a c t o r s . Such a model (3) has proved to be able t o c o r r e c t l y reproduce experimental data using only two a d j u s t a b l e parameters. I t has been checked i n several cases (hydrogénation of alphamethylstyrene {3}, hydrogénation of 2-butanone {A), h y d r o r e f i n i n g (J5) ) / with more or l e s s v o l a t i l e l i q u i d reactants and i t appeared to be a l s o u s e f u l to c a l c u l a t e a p o s t e r i o r i the extent of the d i f f e r e n t types of wetted c a t a l y s t area and t h e i r d i f f e r e n t e f f e c t i v e n e s s f a c t o r . Of course, the f i t t i n g of the data can give r i s e to some compensation e f f e c t s and the accuracy of the determination of the wetted areas ( r e l a t e d to the t o p o l o g i c a l d e s c r i p t i o n of the l i q u i d flow on the c a t a l y s t ) i s l i m i t e d by the knowledge of a l l the physico-chemical p r o p e r t i e s (vapour pressure, v i s c o s i t i e s etc....) and of the c o r r e l a t e d parameters (mass t r a n s f e r c o e f f i c i e n t e t c . . ) i n v o l v e d i n the model. Using a n o n - v o l a t i l e l i q u i d reactant would of course consider a b l y l i m i t the number of p o s s i b l e r e a c t i o n steps and, i n the f i t t i n g of the model, t h i s would e l i m i n a t e p o s s i b l e cross c o r r e l a t i o n s between the values which the model enable to c a l c u late. With t h i s i n mind, we t e s t a t r i c k l e bed r e a c t o r using the hydrogénation i n aqueous s o l u t i o n of maleic a c i d i n t o succ i n i c a c i d i n the presence of a supported palladium c a t a l y s t . °v yO 0 >0 T>J - HC = CH + Η — i - CH - CH - CH HO ^OH HO ^OH In the whole range of our experimental c o n d i t i o n s , maleic a c i d has no measurable v o l a t i l i t y and we hoped so to o b t a i n more accurate information on the wetting e f f i c i e n c y as w e l l as to check the model i n a much more constrained s i t u a t i o n . Moreover the d i r e c t comparison of t h i s r e a c t i o n with the very s i m i l a r hydrogénation of alphamethylstyrene (the v o l a t i l i t y of the reactant excepted) which we experienced p r e v i o u s l y , could d i r e c t l y put i n t o s i g h t the r o l e of dry or badly i r r i g a t e d c a t a l y s t particles. E X P E R I M E N T A L

S E C T I O N

PRELIMINARY KINETIC STUDY ?ËË £Ë

i

2£Ë

i _ fi Y Ë Ë

We f i r s t c o n d u c t e d a p r e l i m i n a r y k i n e t i c study i n order to determine the i n t r i n s i c and p a r t i c l e s c a l e apparent r e a c t i o n r a t e s . A powder c a t a l y s t was used to determine the i n t r i n s i c r e a c t i o n r a t e while c y l i n d r i c a l p a r t i c l e s were used both f o r the p a r t i -

Dudukovi and Mills; Chemical and Catalytic Reactor Modeling ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

2.

Reactional System Effect on Irrigation

RUIZ ET A L .

17

Rate

c l e s c a l e apparent r e a c t i o n r a t e determination and f o r the t r i c k l e - b e d experiments. T y p i c a l p r o p e r t i e s of these c a t a l y s t s are l i s t e d i n Table I.

TABLE I. P h y s i c a l p r o p e r t i e s of the c a t a l y s t


Bulk_form

Powder

Mean p a r t i c l e diameter, 10 m

_Cylinder_

2.5

Bulk dimensions, 10 m height diameter Carrier Palladium Palladium

3.52 3.24 Y -alumina

Y -alumina

0.5 20

0. 5

content, % dispersion, %

37

4

2

T o t a l surface, m /kg P a r t i c l e apparent d e n s i t y kg/m P a r t i c l e porosity, %

102 10

5.9 10

1.74 10

3

2.44 10

5

3

3

53.7

35.2

In a l l our experiments, the s e l e c t i v i t y of the r e a c t i o n was very good. The only by-product, fumaric a c i d , was produced i n very small amounts. High p u r i t y reactants were used : 99% maleic a c i d (Fluka AG), 99. 99% hydrogen and 99.8% nitrogen ( A i r Liquide). Water used as solvent was d i s t i l l e d i n our l a b o r a t o r i e s . The only m a t e r i a l s used i n the experimental apparatus were s t a i n l e s s s t e e l , glass and PTFE i n order t o avoid c o r r o s i o n e f f e c t s . Many m e t a l l i c s a l t s are indeed poisons f o r the c a t a l y s t . Further d e t a i l s on the experimental operating c o n d i t i o n s and procedures are given elsewhere ( 6 ) .

The i n t r i n s i c r e a c t i o n r a t e has been measured i n a d i s c o n tinuous s l u r r y s t i r r e d tank r e a c t o r and i n a continuous microt r i c k l e bed r e a c t o r (6). This l a s t one i s represented i n Figure 1 Both methods lead t o rather s i m i l a r expressions f o r the i n t r i n sic} r a t e equation r(mol/kg Pd. s) r

ο .ο , -1 = 2.58 χ 10 Λ

,39090 exp ( - g —

.

, 1 · λ

p

0.6

>*

E

( 1 )

The p a r t i c l e s c a l e apparent r e a c t i o n r a t e has been deter mined i n a discontinuous Carberry type r e a c t o r and i n a c o n t i nuous m i c r o - t r i c k l e bed r e a c t o r i n which c a t a l y s t p a r t i c l e s



18


F i g u r e 1. M i c r o t r i c k l e bed r e a c t o r . Key: a, gas i n l e t ; b, l i q u i d i n l e t ; c, t h e r m o c o u p l e ; d, j a c k e t ; e, g l a s s b e a d s ; f , c a t a l y s t , h, p o r o u s bed s u p p o r t ; i , gas o u t l e t ; j , l i q u i d o u t l e t .


2.

RUIZ E T A L .

Reactional System Effect on irrigation

19

Rate

-4 are d i l u t e d by i n e r t g l a s s beads (1.5x10 m diameter). Both techniques lead to s i m i l a r r e s u l t s represented by the f o l l o w i n g equation : ο λα m " ,24030 , 1 1. „0.06 „ 0.6 , r = 2.44x10 exp ( - J J — . -) ). . P (2)

L (7) m 1

1

< L > represents the bed s c a l e averaged value of the l i q u i d v e l o c i t y whereas L represents the minimum wetting v e l o c i t y i n an i s o l a t e d channel. T h i s v e l o c i t y corresponds t o the more s t a b l e flow c o n f i g u r a t i o n of t h i s channel. I t i s conse quently c l o s e l y r e l a t e d to the d i f f e r e n t sources of energe t i c a l d i s s i p a t i o n due to the c r e a t i o n of t h i s channel (surface energy, viscous drag,..). A c t u a l l y , t h i s parameter characte rizes the e f f e c t i v e w e t t a b i l i t y of the packing, i . e . , the w e t t a b i l i t y under the a c t u a l o p e r a t i n g c o n d i t i o n s . T h i s pro blem w i l l be analyzed l a t e r i n more d e t a i l . DETERMINATION OF THE BED

SCALE APPARENT REACTION RATE

In the preceeding s e c t i o n , we have shown how f l u i d flows through a packed bed may be observed at v a r i o u s l e v e l s , l e a d i n g to completely d i f f e r e n t i n t e r p r e t a t i o n s . A c t u a l l y , when model l i n g any k i n e t i c process, i t i s always necessary to consider at l e a s t two o b s e r v a t i o n l e v e l s i n a t r i c k l e bed r e a c t o r : the bed s c a l e and the p a r t i c l e s c a l e . The bed s c a l e c o r r e s ponds to the whole bed or to a volume c o n t a i n i n g a l a r g e number of p a r t i c l e s . That i s the l e v e l at which we want t o d e r i v e a model f o r the i n v e s t i g a t e d process. However, t h i s process i s g e n e r a l l y r u l e d by g a s - l i q u i d - s o l i d i n t e r a c t i o n s o c c u r r i n g at the p a r t i c l e s c a l e . The change of s c a l e or volume averaging between those two l e v e l s i s r u l e d by the p e r c o l a t i o n process, i . e . , by the v e l o c i t y d i s t r i b u t i o n d e f i n e d by Eq.3. I f φ i s an extension quan t i t y , i t s ^ bed scale averaged value i s given by : = τ Φα.)·α. (8) ι=ο To d e s c r i b e the i n v e s t i g a t e d process and to determine the r e l a t e d q u a n t i t y at the p a r t i c l e s c a l e we have t o adopt a r e p r e s e n t a t i o n of the t r a n s p o r t c e l l which i s a s s o c i a t e d t o


2.

RUIZ ET AL.

27

Reactional System Effect on Irrigation Rate

each bond i n the l a t t i c e d e f i n e d by the p e r c o l a t i o n process (see Figure 7), This c e l l i s assumed to be e x a c t l y the same a t any p o s i t i o n w i t h i n the bed. The randomnes of the process i s indeed accounted f o r by the p e r c o l a t i o n process. The f o l l o w i n g developments w i l l be r e s t r i c t e d t o laminar liquid flow with very low g a s - l i q u i d i n t e r a c t i o n s ; t h a t i s the flow regime p r e v a i l i n g with the operating c o n d i t i o n s adop ted i n the experimental work. A p p l i c a t i o n of Eq.8 with some s i m p l i f i c a t i o n s , w i l l be presented f o r the modelling of the different processes controlling the apparent reaction rate a t the bed s c a l e .


f

The l o c a l transport c e l l i s represented by a s t r a i g h t pore with an i n c l i n a t i o n to the v e r t i c a l c h a r a c t e r i z e d by angle θ (see Figure 8 ) . The curvatures of g a s - l i q u i d and l i q u i d - s o l i d i n t e r f a c e s are assumed t o be n e g l i g i b l e . The model a l s o assumes that the laminar l i q u i d flow i s motivated only by g r a v i t y . The apparent r e a c t i o n r a t e r a t the l e v e l of one pore - i . e . of one p a r t i c l e - r e s u l t s from mass exchange between the l i q u i d flow and the porous s t r u c t u r e of the c a t a l y s t p a r t i c l e as depicted i n the close-up of Figure 8. The complexity of the mass balance equations depends s t r o n g l y on the number of steps l i m i t i n g the mass t r a n s f e r processes as w e l l as on the complexity of the expression of r . With the i n v e s t i g a t e d experimental system, the limiting reactant i s hydrogen i n l i q u i d phase. Maleic a c i d i s indeed a non v o l a t i l e substance so that r e a c t i o n occurs only on the i r r i g a t e d zones of the c a t a l y s t . Furthermore, the apparent r e a c t i o n a l order f o r maleic a c i d i s very c l o s e to zero and the product « ~ where D represents the l i q u i d molecular d i f f u s i v i t y and the bulk liquid concentration - i s d e f i n i t e l y smaller f o r hydrogen than f o r maleic a c i d . In such a c o n d i t i o n , a s i n g l e mass balance equation has to be w r i t t e n f o r hydrogen as f o l l o w s : D

C

L

K

a

f

LS- - W

(

C

H,L

"

C

H,S

)

=

V

C

H , S '

™

1

"

ε

)

·

L

Ρ

may be determined as follows : =|^ (13) w The brackets mean that bed s c a l e averaged values are considered. is obtained by i n t r o d u c i n g Eqs. 10a and 10b i n t o Eq. 8. w oo a

>

Ό

< f

>

=

L,

a

i=

1

"

a

14

0

< >

The value of OL i s e x p l i c i t e d by Eqs. 3 and F i n a l l y , one obtains :
^ τ + L__

=

w

4.

( 1 5 )


30


i s an i n c r e a s i n g f u n c t i o n of , reaching a s y m p t o t i c a l l y u n i t y f o r very large values of < L> . C l e a r l y decreases when i n c r e a s i n g L . The apparent log-slope of < ï > versus < L> equals 1 That means t h i s slope ranges between 1 f o r a very small i r r i g a t i o n r a t e ( l i n e a r i t y ) and 0 f o r a n e a r l y complete i r r i g a t i o n (asymptotic v a l u e ) . The approximate averaged value < k > i s r e a d i l y obtained r e p l a c i n g Lj_ by < L> i n Eq.11. 7


L S

< V LS , * " V ti^

8

)

1

24

' -

a

1

/

3

_1/

- p^g £ > -

The mass balance equation f o r the i s then s i m i l a r to Eq.9. ^

Chemical and Catalytic Reactor Modeling - American Chemical Society

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