Runaway in an Industrial Hydrogenation Reactor

12 Runaway in an Industrial Hydrogenation Reactor G E R H A R T EIGENBERGER and ULRIKE W E G E R L E

Downloaded by TUFTS UNIV on February 16, 2018 | https://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch012

BASF A G , D-6700 Ludwigshafen, Federal Republic of Germany

In a d i a b a t i c a l l y operated i n d u s t r i a l hydrogenation r e a c t o r s temperature hot spots have been observed under steady-state c o n d i t i o n s . They are a t t r i b u t e d to the formation of areas w i t h d i f f e r e n t fluid residence time due t o o b s t r u c t i o n s i n the packed bed. I t is shown that in a d d i t i o n t o these steady- s t a t e e f f e c t s dynamic instabilities may a r i s e which l e a d to the temporary formation o f excess temperatures w e l l above the steady-state l i m i t if a sudden l o c a l r e d u c t i o n of the flow r a t e occurs. An example of such a runaway in an i n d u s t r i a l hydrogenation r e a c t o r is presented together w i t h model c a l c u l a t i o n s which r e v e a l d e t a i l s of the onset and course of the r e a c t i o n runaway. The hydrogénation o f hydrocarbons i n v o l v e s h i g h l y exothermic r e a c t i o n s which are o f t e n c a r r i e d out a t elevated pressure (50-300 b a r ) . Because of the h i g h pressure not a multitube c o n s t r u c t i o n but a s i n g l e l a r g e diameter tube i s used as the r e a c t o r which means that the r e a c t i o n has to be run a d i a b a t i c a l l y . I f the hydrocarbon feed i s a l i q u i d , a three phase gasl i q u i d - s o l i d r e a c t i o n takes p l a c e i n the c a t a l y s t bed. Normally only p a r t i a l hydrogénation i s r e q u i r e d , b u t a t e l e v a t e d temperatures and w i t h a s u f f i c i e n t excess of hydrogen the r e a c t i o n may run away up to t o t a l methanation. The hydrogénation o f benzene to cyclohexane i s a simple example: C H 6

H

6

-t3 2*.C H 6

1 2

- *

H

2 - *

6CH

4

(1)

Because o f the c o n s i d e r a b l e heat generation and the p o t e n t i a l danger o f a r e a c t i o n runaway the temperature c o n t r o l of hydrogénation r e a c t o r s i s of prime importance. Since the r e a c t i o n i s c a r r i e d out a d i a b a t i c a l l y only two p o s s i b i l i t i e s e x i s t : e i t h e r the degree of hydrogénation can be c o n t r o l l e d by the residence time o r the a d i a b a t i c temperature r i s e can be l i m i t e d by d i l u t i n g the feed e.g. by c i r c u l a t i n g a l a r g e amount of hydrogen. 0097-6156/82/0196-0133$06.00/0 © 1982 American Chemical Society Wei and Georgakis; Chemical Reaction Engineering—Boston ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

134

CHEMICAL REACTION

ENGINEERING


The hydrogénation of a l i q u i d feed i n an a d i a b a t i c f i x e d bed hydrogen i n s u b s t a n t i a l excess w i l l now be considered.

with

Steady s t a t e e f f e c t s . In a simple a d i a b a t i c r e a c t i o n the existence of temperature hotspots seems to be impossible i n steady s t a t e . I t i s w e l l known however that strong temperature excursions have been observed i n i n d u s t r i a l hydrogénation r e a c t o r s (_^,2). S.B. J a f f e (2) has presented a reasonable e x p l a n a t i o n f o r these phenomena. In a l a r g e diameter packed bed, there i s l i k e l y to be flow m a l d i s t r i b u t i o n , the most s e r i o u s form of which can be caused by o b s t r u c t i o n s which b l o c k p a r t of the c r o s s s e c t i o n a l area f o r f l u i d flow. An example i s given i n Figure 1 where a c i r c u l a r d i s c w i t h a r a d i u s of 0.085 m i s thought to obs t r u c t the inner part of the r e a c t o r . Just behind the o b s t r u c t i o n the a x i a l flow r a t e vp i s reduced to 1/30 of i t s normal value but somewhat f u r t h e r downstream the flow r a t e r i s e s again due to the r a d i a l c r o s s flow. This means that there i s a very long residence time behind the o b s t r u c t i o n where the r e a c t i o n can proceed up to t o t a l conversion. I t i s shown i n F i g u r e 2 that i n t h i s r e g i o n the complete conversion of the l i q u i d feed ( c o n c e n t r a t i o n cp) r e s u l t s i n a pronounced temperature hot spot. Without the o b s t r u c t i o n only a small degree of conversion and a minor temperature i n c r e a s e would occur as i n the p e r i p h e r a l p a r t s (r = 0.15 m) of the r e a c tor. Dynamic I n s t a b i l i t i e s . We were reminded of the above r e s u l t s when we were confronted with s e v e r a l temperature runaways i n an i n d u s t r i a l hydrogénation r e a c t o r . A sketch of the r e a c t o r i s given i n F i g u r e 3. The l i q u i d feed and hydrogen i n h i g h excess (about 80 times more than the s t o i c h i o m e t r i c requirement) enter the r e a c t o r at the bottom, the gas bubbles throught the c a t a l y s t bed and both gas and l i q u i d move upwards at d i f f e r e n t v e l o c i t i e s . The r e a c t i o n i s p a r t i a l hydrogénation, b a s i c a l l y s i m i l a r to equ. (1), e s p e c i a l l y i n the f a c t that the r e a c t i o n can t u r n to t o t a l methan a t i o n i f the temperature runs out of c o n t r o l . However, due to the h i g h d i l u t i o n with hydrogen only a 30°C temperature r i s e has to be expected i n normal o p e r a t i o n . A number of temperature measurements are i n s t a l l e d along the a x i s of the r e a c t o r and F i g u r e 4 shows a t y p i c a l recorder s t r i p of these temperatures. I t can be seen that the r e a c t o r has been running very s t e a d i l y between 15:00 and 21:00. But at 21:15 a l l of a sudden one temperature takes o f f at more than 100°C/min and the two adjacent temperatures f o l l o w . For no obvious reason a sudden runaway of the r e a c t i o n occurs with excess temperatures of w e l l above 300°C. The automatic shut-down c o n t r o l stopped the r e a c t i o n by purging the r e a c t o r w i t h n i t r o g e n and the temperatures dropped again. The cause of t h i s behaviour was a mystery f o r some time, s i n c e the runaway occurred during completely steady operating c o n d i t i o n s . An obvious suggestion was to a t t r i b u t e the runaway to a switch i n the r e a c t i o n mechanism from p a r t i a l hydrogénation

Wei and Georgakis; Chemical Reaction Engineering—Boston ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Industrial Hydrogénation

Reactor

135


EIGENBERGER AND W E G E R L E

Figure 2.

Temperature (top) and liquid concentration c (bottom) behind obstruction. F


136

CHEMICAL

REACTION

ENGINEERING


0,6 m

(0,2m3/h) Figure 3.

Sketch of the hydrogénation reactor considered.

ρ r—

—

Ξι

(50m3/h)

ζζ*. ι:::::

ι—8

:~

1

"II

ιιι:

jfillZ

—

ΒΞΞ |Ξ

Ξζϊ

*""""T" Ξ

ΞΞΞ

1

: ΞΞ

ΞΞ

ΞΞ ΞΞ-

—

—

μΖ

ζζ£

—

Γ;::Ζ.

ΞΗΞΞΞ"

""ZI"

HZL

—rSjzzz: ζ:

ϋ

• — TCS\

Ξ ο

ς j

ΙΙΖ

"Ξ

ζ— cν-» — •ΞΞ D

—

ζ:

ζζ Ζ

...

L

τ (°c)

—

— " ΞΞΞ —

— •-s

ΖΓΖΖ

ΞΞ ΞΞζ :ζζ

ΖΖΓΓ

ΕΞ - Ζ Ζ Ζ Ζ Ζ — -• !%~ [— —

ζζζζ I

:~: τζζζ

'ΖΖΖ.,

— â ~ ι: _

Ξ

—

Ζ Ι Ζ ZI

—8

.ζζ

1

—J

I TP

—R

ι *

ί

L

— δ

•il8

J

g

W

1*4»

Time (h) Figure 4.

Recorder strip of reactor temperatures during runaway.



12.

EIGENBERGER A N D W E G E R L E


Reactor

137

to t o t a l methanation. However, i t could e a s i l y be shown that even with t o t a l methanation, the a d i a b a t i c temperature r i s e was below 80°C and not above 300°C as observed. Another suggestion was the i n f l u e n c e o f a flow r e d u c t i o n i n the r e a c t o r . T h i s would be d e t r i m e n t a l i n the case o f a reduced throughput o f hydrogen, s i n c e the l a r g e excess o f hydrogen i s used p r i m a r i l y t o remove the heat and keep the temperature i n crease s m a l l . Figure 5 shows some s i m u l a t i o n r e s u l t s based upon a simple one-dimensional two-phase model o f the r e a c t o r . Under normal o p e r a t i n g c o n d i t i o n s there i s a r a p i d consumption o f the l i q u i d feed component CF over the f i r s t meters o f the r e a c t o r . However, the temperature i n c r e a s e i s only a few degrees c e n t i grade because o f the great excess o f hydrogen. I f the hydrogen flow i s reduced t o 1/4, the heat removal i s only 1/4, and hence the temperature increase i s 4 times g r e a t e r . Therefore the l i q u i d c o n c e n t r a t i o n drops more r a p i d l y . These steady-state c a l c u l a t i o n s are o f course not s u f f i c i e n t to e x p l a i n a temperature excess o f more than 300°C. I t can be seen however, that during the t r a n s i e n t from the f i r s t steady s t a t e t o the s t a t e o f reduced throughput o f gas, the f l u i d c o n c e n t r a t i o n has t o drop c o n s i d e r a b l y , i . e . , a s u b s t a n t i a l amount o f the f l u i d holdup p r o p o r t i o n a l to the area between c ( t ^ 0 ) and c p C t - * * ) w i l l r e a c t during the t r a n s i e n t . T h i s i s a l a r g e q u a n t i t y compared with the amount o f f l u i d feed that enters the r e a c t o r during the same time. A dynamic c a l c u l a t i o n o f the t r a n s i e n t (Figure 6) r e v e a l s that i t i s so much that the hydrogen i s completely consumed temporarily and h i g h excess temperatures i n the range of 300°C occur. A sudden r e d u c t i o n of the hydrogen flow r a t e may thus be a reasonable e x p l a n a t i o n o f the phenomena observed. However, according t o the records the t o t a l gas flow r a t e was kept constant. The only e x p l a n a t i o n i s that a l o c a l and sudden flow m a l d i s t r i b u t i o n occurred w i t h i n the r e a c t o r , l i k e the one d i s c u s s e d i n the f i r s t section. Such a flow m a l d i s t r i b u t i o n can be caused by an o b s t r u c t i o n i n the packed bed due t o c a t a l y s t abrasions and the formation o f cracked products and i t was w e l l known that the c a t a l y s t used was s u s c e p t i b l e t o t h i s k i n d of problem. As a consequence o f t h i s e x p l a n a t i o n the r e a c t i o n runaway t o t o t a l methanation i s not a necessary c o n d i t i o n f o r the observed phenomenon. Any simple exothermic two phase r e a c t i o n i n an a d i a b a t i c r e a c t o r ought to show the same behaviour provided that one phase w i t h a h i g h throughput i s used t o c a r r y the heat out o f the r e a c t o r and the flow i s suddenly reduced. T h i s w i l l be shown i n the f o l l o w i n g s i m u l a t i o n r e s u l t s . Due t o problems with the numeric a l s t a b i l i t y o f the s o l u t i o n (see Apendix) only a moderate r e a c t i o n r a t e w i l l be considered. Reaction parameters a r e chosen i n such a way that i n steady s t a t e the l i q u i d c o n c e n t r a t i o n Cf drops from 4.42 to 3.11 kmol/m but the temperature r i s e i s only 3°C (hydrogen i n great excess). A t t = 0 the uniform flow p r o f i l e 0

F

3


ENGINEERING


CHEMICAL REACTION

Figure 6.

Transient behavior.



12.

EiGENBERGER A N D W E G E R L E


Reactor

139

changes t o one s i m i l a r t o F i g u r e 1 and a t r a n s i e n t develops that i s shown i n Figure 7. Due t o the increased residence time behind the o b s t r u c t i o n , the r e a c t i o n proceeds t o completion, causing the temporary formation of a pronounced hot spot. I n the f i n a l steady s t a t e the temperatures a r e f l a t again. Under more r e a l i s t i c r e a c t i o n parameters, only a s p a t i a l l y one-dimensional s o l u t i o n could be obtained. Here i t was assumed that the gas flow over the whole cross s e c t i o n a l area was reduced u n i f o r m l y t o 1/4 a t time zero. The r e s u l t i n g dynamics are given i n F i g u r e 8. I n t h i s case the t r a n s i e n t temperature i s so high that the f l u i d c o n c e n t r a t i o n i s completely consumed and the r e a c t i o n zone moves l i k e a f r o n t through the whole r e a c t o r . F i n a l l y a f l a t steady s t a t e temperature p r o f i l e i s e s t a b l i s h e d again. Obviously the a c t u a l runaway took p l a c e i n a s i m i l a r f a s h i o n . The gradual development o f the i n i t i a l hot spot must have taken p l a c e i n a p a r t o f the r e a c t o r where no thermocouple was l o c a t e d . Only the movement o f the f u l l y developed temperature f r o n t was recorded and caused the r a p i d i n c r e a s e o f the measured temperatures . Based upon t h i s p i c t u r e o f the causes o f the runaway, i t was p o s s i b l e t o take precautions that l e d t o the safe and steady o p e r a t i o n o f t h i s r e a c t o r without any f u r t h e r malfunction. Conclusions. In t u b u l a r multiphase r e a c t o r s w i t h an exothermic r e a c t i o n where one phase w i t h a h i g h throughput serves to c a r r y the heat o f r e a c t i o n out o f the r e a c t o r , a sudden flow r e d u c t i o n i n t h i s phase (whether accompanied by a s i m i l a r reduct i o n i n the other phases o r not) can l e a d t o a c o n s i d e r a b l e t r a n s ient temperature r i s e , w e l l above the new steady s t a t e temperat u r e . The maximum excess temperature depends i n a complex way upon the r a t e o f the flow r e d u c t i o n , the flow r a t e s i n the d i f f e rent phases, the heat c a p a c i t i e s and the r e a c t i o n r a t e s o f the system. P a r t i a l hydrogénation r e a c t i o n s o f a l i q u i d feed i n an a d i a b a t i c f i x e d bed w i t h hydrogen i n h i g h excess a r e e s p e c i a l l y s e n s i t i v e t o t h i s k i n d o f i n s t a b i l i t y . Even l o c a l flow reductions caused by a sudden o b s t r u c t i o n o f p a r t o f the packed bed can i n i t i ate these phenomena. Appendix; Mathematical models* The mathematical model f o r Figures 1,2 and 7 i s a modified Deans-Lapidus c e l l model ( 3 ) , s i m i l a r t o that used by J a f f e (2) except that a d i f f e r e n t flow scheme has been used. The f o l l o w i n g assumptions have been made throughout : 1) Volume f r a c t i o n s f o r gas ( € Q ) l i q u i d ( É ^ ) and c a t a l y s t (1 - €Q - BjJ a r e constant i n the bed. 2) No temperature d i f f e r e n c e between gas, l i q u i d and c a t a l y s t i s assumed. 3) The o v e r a l l r e a c t i o n r a t e depends upon the hydrocarbon c o n c e n t r a t i o n i n the l i q u i d c and the hydrogen concent r a t i o n i n the gas phase eg. 9

p


140

CHEMICAL REACTION ENGINEERING


300

Figure 7.

Transient behavior upon a sudden circular obstruction with radius 0.085 m.




Reactor

141


12.

Figure 8.

Transient behavior upon a sudden reduction of hydrogen flow rate to 1 /4. For model equations and parameters see Table I.


CHEMICAL

142

REACTION

Table I . Model Equations and Parameter

ENGINEERING

Values

f o r F i g u r e 8. Indices :

G - gas, F - l i q u i d , C - c a t a l y s t 9

c

C

F * Mass balance l i q u i d : g-j- - - v

F


f

C

V

" ~ T Tz

c

ν

τ

+ D

G

^ G Q

Energy balance; S ? Tt

JS

p

C

^ G ^ = - v

Mass balance gas:

2C

+ D

+

3 F g-^- - r 3

2C

6

y G— -* gF ^ . +

Tz*~

(

*

lAH

r

)

R '*F '

r

= C

+ C

?c Pc =

£

F F S F PF V

C

+

6

+

F *F '

V

P

£

F

?

G

C G

PG

C

G G ? G PG

Boundary c o n d i t i o n s : Danckwerts type

Reaction r a t e

fkmol 1 | [ m 3

F

J

r

=

3

9

>

1

5

.

1

6 0

F H ( )

/

5

ex ρ (-13000/T) ' G

C

C

Parameters : feed c o n d i t i o n s : enthalpy o f r e a c t i o n :

c° = 6.15 cf, = 4.42 C* m vj JP mr ("^^ 100 kcal/mol =

kcal

Î

C

?G PG C

fc P c void fractions: flow v e l o c i t i e s : eff. diffusivities:

T° = 220°C

£

=

4

2

-

3

5

?F

C

=

PF

4 3 0

-

5

= 400.

• 0.3; G v_ • 0.2

• 0.3

n

r 0.05 m/s; v_ • 0.0002 m/s 4

2

2

D_ - 0.112 · 10" m /s; D. - 0.0108 m /s r G e f f . heat c o n d u c t i v i t y : λ = 0.0048 kcal/m s Κ



12.



Reactor

143

T h i s means that the numerical r e s u l t s can give but a rough and q u a l i t a t i v e p i c t u r e o f some b a s i c e f f e c t s . I n a d d i t i o n , the c e l l model turned out t o possess s e v e r a l shortcomings, the most s e r i o u s o f which are the strong c o r r e l a t i o n between the number of c e l l s per u n i t volume and the a x i a l and r a d i a l flow c h a r a c t e r istics and the f a c t that an excessive number o f c e l l s i s needed to model a strong thermal runaway r e a c t i o n with h i g h temperature peaks. D i f f e r e n t mathematical formulations o f multidimensional f l u i d flow i n packed beds l i k e those developed by Jeschar (4) and Szekel e y (5) a r e l i k e l y t o overcome the f i r s t d i f f i c u l t y . Because o f the numerical problems mentioned above a s p a t i a l l y one-dimensional two-phase model has been used t o simulate the more r a p i d t r a n s i e n t s . The model equations f o r F i g u r e 8 and the p a r a meters used are given i n Table I . I n F i g u r e s 5,6 the decomposition r e a c t i o n t o methane has a l s o been considered.

Acknowledgments The runaway problem was brought t o our a t t e n t i o n by Dr. Toussaint, Dr. Wittwer and Dr. Wolff who provided us w i t h the necessary i n f o r m a t i o n and i n the d i s c u s s i o n s w i t h whom the b a s i c modelling assumptions emerged. T h e i r c o n t r i b u t i o n s are g r a t e f u l l y acknowledged.

Literature Cited 1. Weekmann, V.W.: Proc. 4th I n t e r n a t . Symp. Chemical React i o n Engineering, Heidelberg 1976, V o l . 2, ρ 615 - 646 2. J a f f e , S.B.: IEC Proc.Des.Dev. 1976, 15, 410 - 416 3. Deans, H.A. and Lapidus, L.: AIChE Jl. 1960, 6, 657 - 668 4. Radestock, J. and Jeschar, R: Chem.Ing.Techn. 1971, 43, 355 - 360 and 1304 - 1310 5. Szekeley, J . et.al.: AIChE Jl. 1974, 20, 974 - 980, 1975, 21, 769 - 775 and 1976, 22, 1021 - 1023 R E C E I V E D May

6,

1982.


Runaway in an Industrial Hydrogenation Reactor

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