Dynamic Behavior of an Industrial Scale Fixed-Bed Catalytic Reactor

10 Dynamic Behavior of an Industrial Scale FixedBed Catalytic Reactor L . S. KERSHENBAUM and F. LOPEZ-ISUNZA

1

Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch010

Imperial College of Science and Technology, Department of Chemical Engineering and Chemical Technology, London SW7, England

T r a n s i e n t and steady s t a t e a x i a l and r a d i a l temperature measurements were made during the c a t a l y t i c air o x i d a t i o n o f o-xylene t o p h t h a l i c anhydride over a V 2 O 5 / T i O 2 c a t a l y s t in an i n d u s t r i a l s c a l e fixed-bed r e a c t o r , t o determine the e f f e c t s o f v a r i a t i o n s o f j a c k e t temperature and feed composit i o n and temperature on the dynamic behaviour o f the r e a c t o r . F o r small p e r t u r b a t i o n s , the e x p e r i mental r e s u l t s are c o n s i s t e n t with the p r e d i c t i o n s from a heterogeneous two-dimensional model o f the r e a c t o r and give i n s i g h t i n t o the behaviour o f r e a c t o r s with small t u b e - t o - p a r t i c l e diameter r a t i o s . However, somewhat l a r g e r p e r t u r b a t i o n s lead t o a s l i g h t , partially reversible deactivat i o n o f the c a t a l y s t which makes a comparison with model p r e d i c t i o n s difficult. A dynamic model f o r o n - l i n e e s t i m a t i o n and c o n t r o l o f a f i x e d bed c a t a l y t i c r e a c t o r must be based on a thorough experimental program. I t must be able t o p r e d i c t the measured experimental e f f e c t s o f the v a r i a t i o n of key v a r i a b l e s such as j a c k e t temperat u r e , feed flow r a t e , composition and temperature on the dynamic behaviour o f the r e a c t o r ; t h i s , i n t u r n , r e q u i r e s the knowledge of the k i n e t i c and " e f f e c t i v e " t r a n s p o r t parameters i n v o l v e d i n the model. Due t o the strong i n t e r a c t i o n between the p h y s i c a l and chemical mechanisms, p a r t i c u l a r l y when c a t a l y s t d e a c t i v a t i o n i s present, the parameter e s t i m a t i o n becomes very d i f f i c u l t . The k i n e t i c parameters are normally obtained from l a b o r a t o r y s c a l e r e a c t o r s and when used i n p i l o t p l a n t s t u d i e s , have t o be tuned (1, 2) o r even re-evaluated (3, 4) to o b t a i n reasonable p r e d i c t i o n s . The t r a n s p o r t parameters are estimated 1

Current address: Universidad Autonoma Metropolitana-Iztapalapa, Depto. de Ingenieria, Apdo. Postal 55-534, Mexico 09340. 0097-6156/82/0196-0109$06.00/0 © 1982 American Chemical Society In Chemical Reaction Engineering—Boston; Wei, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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CHEMICAL REACTION

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e i t h e r from steady s t a t e or dynamic experiments without r e a c t i o n , to o b t a i n approximate v a l u e s under r e a c t i o n c o n d i t i o n s . To i n v e s t i g a t e the behaviour of the present r e a c t o r , a s e r i e s of steady s t a t e and dynamic experiments were performed, c o n s i s t i n g of r e a c t o r s t a r t - u p , step changes i n feed composition and ramp changes i n feed and j a c k e t temperature. Heat t r a n s f e r experiments without r e a c t i o n were a l s o performed. Some of the r e s u l t s are compared w i t h model s i m u l a t i o n s , u s i n g , whenever p o s s i b l e , £ p r i o r i v a l u e s of model parameters.


Experimental System The schematic flow diagram of the p i l o t p l a n t i s shown i n F i g u r e 1. The r e a c t o r i s a s i n g l e tube of 25 mm i n t e r n a l diameter, 2.5 mm w a l l thickness and 3 m length, packed with a 2.6 m column o f V2O5/T1O2 c a t a l y s t p e l l e t s , and i s immersed i n an a g i t a t e d bath of molten s a l t . The c a t a l y s t used was developed by Chemische F a b r i k von Heyden, and c o n s i s t e d of an i n e r t s p h e r i c a l c a r r i e r of 8.2 mm diameter, covered w i t h a t h i n a c t i v e c o a t i n g which contained V2O5 and T1O2 ( 5 ) . There are 26 a x i a l sampling p o i n t s of which 5 were used to measure composition by o n - l i n e chromatography, and the r e s t to measure temperature u s i n g 3 mm OD p l a t inum r e s i s t a n c e thermometers. For measurement of r a d i a l temperat u r e - p r o f i l e s around the hot spot, the platinum r e s i s t a n c e thermometers could be r e p l a c e d by 1.5 mm OD Chrome 1-Alumel sheathed thermocouples which were f r e e to move r a d i a l l y w i t h i n the r e a c t o r . The r e a c t a n t mixture, c o n s i s t i n g of 1 mole % o-xylene i n a i r was r a i s e d to a temperature of 105-110°C by a v a p o r i z e r l o c a t e d up-stream of the r e a c t o r , before e n t e r i n g the top of the bed. The gas stream l e a v i n g the r e a c t o r passed to a condenser where the p h t h a l i c anhydride sublimated. The r e s i d u a l gas was conveyed to a s t r i p p e r where the organic m a t e r i a l was washed out b e f o r e being vented to atmosphere. Theoretical

Developments

K i n e t i c Scheme The k i n e t i c s used i n t h i s study are based on the work of Calderbank and co-workers CI) s i n c e t h e i r r e s u l t s have been found to apply to a v a r i e t y of commercial c a t a l y s t s . At low r e a c t a n t c o n c e n t r a t i o n , the proposed r e a c t i o n scheme can be summarised as comprising s i x major r e a c t i o n s : 2

OX OT PT PA

O-xylene O-tolualdehyde Phthalide P h t h a l i c Anhydride

In Chemical Reaction Engineering—Boston; Wei, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


Figure I.

Schematic diagram of o-xylene oxidation pilot plant


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112

CHEMICAL REACTION

ENGINEERING

A 'redox' type o f k i n e t i c model was developed i n which the r a t e of r e a c t i o n f o r any o f the Ν s p e c i e s , i s expressed i n the form

6

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~N Σ k.p. . , i*is 1=1 with u common f o r a l l r e a c t i o n s and species and Rj f i r s t order i n reactant p a r t i a l pressure. F o r t h i s work, i t was assumed that CO2 and CO were formed i n the r a t i o o f 3:1. Dynamic Model A two-dimensional heterogeneous dynamic model was developed, which d e s c r i b e s the mass and energy balances i n both phases. In dimensionless form, f o r the vfi component and the temperature i n the gas phase, 1

3x

. +

η

3x η

1 3^x 1 , η

-

+

1 3x 1 η.

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/1 \ (1)

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(4)

These can be solved n u m e r i c a l l y given the usual i n i t i a l and boundary c o n d i t i o n s , i n c l u d i n g the thermal boundary c o n d i t i o n at the r e a c t o r w a l l , r = l : - 3y/3r

=

B i (y - y ) w

w

(5)

E a r l i e r s i m u l a t i o n s t u d i e s (6, 7) assessed the importance o f a r a d i a l v e l o c i t y p r o f i l e f o r t h i s system and showed that the increased v e l o c i t y near the w a l l d i d not have a s i g n i f i c a n t e f f e c t on the p r e d i c t i o n of the r e a c t o r ' s behaviour. Subsequent work has assumed a uniform r a d i a l v e l o c i t y p r o f i l e .


10.

K E R S H E N B A U M A N D LOPEZ-isuNZA

Industrial Fixed-Bed Reactor

113


In order to reduce the complexity o f the model two a d d i t i o n a l s i m p l i f y i n g assumptions were made, (a) With t y p i c a l r e s i d e n c e times o f 1 second, p a r t i c l e Reynolds numbers o f 800 and tube-top a r t i c l e diameter r a t i o s o f 3, one would expect small v a l u e s o f the w a l l B i o t number; thus, a small number o f r a d i a l f i n i t e d i f f e r e n c e (or c o l l o c a t i o n ) p o i n t s should be adequate f o r the numerical s o l u t i o n o f the equations ( 8 ) . (b) I t was assumed that the dynamic term f o r the accumulation o f mass a t the c a t a l y s t p e l l e t s (eqn. 3) could be n e g l e c t e d (9^, 10). Numerical S o l u t i o n Numerical s o l u t i o n s of eqns. (1) - (5) based on the above assumptions a r e r e p o r t e d elsewhere ( 7 ) . Using orthogonal c o l l o c a t i o n i n the r a d i a l d i r e c t i o n (one i n t e r i o r c o l l o c a t i o n p o i n t ) equations (1) and (2) were reduced from p a r a b o l i c t o h y p e r b o l i c form. The method o f c h a r a c t e r i s t i c s enabled f u r t h e r r e d u c t i o n t o a coupled s e t o f o r d i n a r y d i f f e r e n t i a l equa t i o n s and n o n - l i n e a r a l g e b r a i c equations. T h i s system of equations was solved u s i n g orthogonal c o l l o c a t i o n on f i n i t e elements (11) ( a l s o c a l l e d g l o b a l s p l i n e c o l l o c a t i o n (12)) i n the a x i a l d i r e c tion. The e n t i r e domain 0 < ζ 4 1 i s d i v i d e d i n t o s e v e r a l subi n t e r v a l s and orthogonal c o l l o c a t i o n i s a p p l i e d a t i n t e r i o r p o i n t s w i t h i n these s u b - i n t e r v a l s . The s i z e and number o f s u b - i n t e r v a l s , and the number o f i n t e r i o r c o l l o c a t i o n p o i n t s a t each s u b - i n t e r v a l , were chosen to s u i t the steepness of the temperature p r o f i l e obtained. G e n e r a l l y , four o r f i v e s u b - i n t e r v a l s , with 4 i n t e r i o r p o i n t s f o r each one, were used f o r the whole r e a c t o r l e n g t h . F i n a l l y , the r e s u l t i n g equations - coupled o r d i n a r y d i f f e r e n t i a l equations (one f o r each c o l l o c a t i o n p o i n t ) p l u s s e t s o f coupled l i n e a r and n o n - l i n e a r a l g e b r a i c equations - were solved by a f o u r t h - o r d e r Runge-Kutta method together with Gaussian e l i m i n a t i o n techniques and an implementation o f Broyden's method (13).

R e s u l t s and D i s c u s s i o n D e t a i l s o f the r e a c t o r o p e r a t i n g c o n d i t i o n s which correspond to t y p i c a l i n d u s t r i a l o p e r a t i o n are given i n Table I . A l l the experiments reported here were performed a f t e r f o u r weeks o f con tinuous running o f the p l a n t under steady c o n d i t i o n s i n order t o allow the c a t a l y s t a c t i v i t y t o s t a b i l i z e . Steady-State Behaviour The dashed l i n e i n F i g u r e 2 shows a t y p i c a l experimental a x i a l temperature p r o f i l e f o r c o n d i t i o n s l i s t e d i n Table I . The banded r e g i o n i n the v i c i n i t y o f the hot spot i n c l u d e s those p o i n t s ( l a b e l l e d a, b and c) i n which r a d i a l temperature p r o f i l e s were a l s o measured u s i n g moving thermocouples. There, the upper and lower l i n e s represent the h i g h e s t measured temperature and the w a l l temperature, r e s p e c t i v e l y , a t those a x i a l points. The measured r a d i a l p r o f i l e s a r e i l l u s t r a t e d i n F i g u r e 3 and show a remarkable r e p r o d u c i b i l i t y d e s p i t e the low t u b e / p a r t i c l e diameter r a t i o . The magnitude of the r a d i a l gradient (up t o 40 C o


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CHEMICAL REACTION ENGINEERING

Table Reactor Operating 3


A i r flow r a t e : 4 m /hr (STP) O-xylene flow r a t e : 176 g/hr Feed temperature: 370°C I n l e t Pressure: 1.4 bar

I Conditions Bath Temperature: 380°C Bed Voidage: 0.5 C a t a l y s t Bulk D e n s i t y : 1300 kg/m 3

i n a J " r a d i a l d i s t a n c e ) g i v e s v a l u a b l e i n f o r m a t i o n about the heat t r a n s f e r p r o p e r t i e s of beds i n t h i s important, but p o o r l y charact e r i z e d regime. The r e s u l t s w i l l be presented i n more d e t a i l at a l a t e r date (7, 14). The asymmetry i n the r a d i a l p r o f i l e i s caused by the conductive heat l o s s e s along the sampling tube which i s welded to the r e a c t o r tube and through which the thermocouple enters the r e a c t o r . Only the l e f t - h a n d s i d e s of the r a d i a l p r o f i l e s i n F i g u r e 3 are s i g n i f i c a n t . The r e s u l t s of the steady-state model f o r the r e a c t o r under the same o p e r a t i n g c o n d i t i o n s are d i s p l a y e d as the s o l i d l i n e s i n F i g u r e 2. The p r e d i c t e d c a t a l y s t and gas temperatures are shown at each of the a x i a l c o l l o c a t i o n p o i n t s . As d i s c u s s e d e a r l i e r , a p r i o r i values of k i n e t i c parameters were used (1, 2); s i m i l a r l y , heat and mass t r a n s f e r parameters (which are l i s t e d i n Table I I ) were taken from standard c o r r e l a t i o n s (15, 16, 17) or from e x p e r i mental temperature measurements i n the r e a c t o r under non-reactive c o n d i t i o n s . The agreement with experimental data i s encouraging, c o n s i d e r i n g the u n c e r t a i n t y which e x i s t s i n the c a t a l y s t a c t i v i t y and i n the heat t r a n s f e r parameters f o r beds w i t h such l a r g e particles. Dynamic Behaviour Reactor behaviour d u r i n g s t a r t - u p i s i l l u s t r a t e d i n F i g u r e 4. The r e a c t o r was o p e r a t i n g i n i t i a l l y at normal c o n d i t i o n s but without o-xylene i n the f e e d , when the o-xylene flow r a t e was r a i s e d to 152 g/hr. The hot spot developed q u i c k l y ( w i t h i n 3 minutes) at the r e a c t o r e x i t and propagated upstream as heat t r a n s f e r and chemical r e a c t i o n e f f e c t s l e d to the h e a t i n g of the c a t a l y s t p e l l e t s to t h e i r steady-state temperatures . F i g u r e 5 shows the l e s s d r a s t i c response to a step i n c r e a s e i n feed composition, and subsequently, a step decrease of the same magnitude. P e r t u r b a t i o n s i n the form of step changes up to 10%, caused r e v e r s i b l e i n c r e a s e s or decreases i n the magnitude of the hot spot but no change i n i t s p o s i t i o n . F i g u r e 5 a l s o shows the t r a n s i e n t response p r e d i c t e d by the s i m u l a t i o n . Larger i n c r e a s e s i n the feed c o n c e n t r a t i o n , however, l e d to a p a r t i a l d e a c t i v a t i o n of the c a t a l y s t near the r e a c t o r i n l e t . T h i s was r e f l e c t e d by the movement of the hot spot down towards the middle of the r e a c t o r ; i t was not p o s s i b l e to p r e d i c t t h i s behaviour without the a r b i t r a r y i n c o r p o r a t i o n of c a t a l y s t a c t i v i t y p r o f i l e s i n the bed.


10.




560

AXIAL POSITION

(CM)

Figure 2. Typical steady-state axial temperature profiles. Key: - Λ - Δ - , simu lated catalyst surface temperature; - V - V - , simulated gas temperature; and - - 0 - - 0 - - , experimental results.

Figure 3.

Steady-state radial temperature profiles corresponding to Figure 2.


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CHEMICAL REACTION

ENGINEERING

Table I I Transport Parameters f o r Reactor

Simulation

E f f e c t i v e Wall B i o t Number, B i =0.84 E f f e c t i v e R a d i a l P e c l e t Numbers: P^ = 0.083 ; P = 0.075 E f f e c t i v e G a s / S o l i d Heat and Mass T r a n s f e r c o e f f i c i e n t s : h = 264. W/m °C ; k = 0.161 m/s Dimensionless G a s / S o l i d Transport Parameters: a^ = 34.6 ; w

m

2

g


am = 17.1 ; a

g

s

= 0.00945

The s e n s i t i v i t y o f the a x i a l temperature p r o f i l e t o the feed and s a l t bath temperatures i s shown i n F i g u r e s 6 and 7, r e s p e c t i v e l y . F i g u r e 6 shows the response to a ramp decrease i n the feed temperature by 2 C over a p e r i o d of 10 minutes. F o r small p e r t u r b a t i o n s (up to 5°C), the hot spot t r a v e l s downstream and passes through a maximum value before reaching i t s new steadys t a t e . When the disturbance i s r e v e r s e d , the hot spot moves upstream i n a s i m i l a r manner and r e t u r n s t o the former steadystate. F i g u r e 7 shows the e f f e c t of a 1°C increase i n the s a l t bath temperature. As b e f o r e , the hot spot t r a v e l s upstream; however, i n t h i s case", i t passes through a maximum temperature which could be h i g h enough t o d e a c t i v a t e that r e g i o n of the bed, e s p e c i a l l y when a newly charged, h i g h l y a c t i v e c a t a l y s t i s being used. Conclusions Dynamic experiments have shown that f o r t h i s r e a c t o r system, feed composition and flow r a t e can be used to a l t e r the p o s i t i o n and magnitude of the hot spot w i t h i n f a i r l y t i g h t l i m i t s . When feed and s a l t bath temperature disturbances are of a somewhat l a r g e r s c a l e , s i g n i f i c a n t departures from the o r i g i n a l steady-state are observed, some o f which can lead to c a t a l y s t d e a c t i v a t i o n . Model c a l c u l a t i o n s based upon some experimentally determined heat t r a n s f e r parameters p l u s k i n e t i c schemes and other parameters taken from the l i t e r a t u r e give reasonably good p r e d i c t i o n s o f the steady-state and dynamic behaviour o f the r e a c t o r when p e r t u r b a t i o n s are s m a l l . Serious l i m i t a t i o n s e x i s t f o r the p r e d i c t i o n of the response t o l a r g e p e r t u r b a t i o n s s i n c e the observed v a r i a t i o n s i n c a t a l y s t a c t i v i t y are not contained i n the k i n e t i c scheme and parameter e s t i m a t i o n becomes very u n c e r t a i n .


10.



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Dynamic Behavior of an Industrial Scale Fixed-Bed Catalytic Reactor

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