Chemical and Catalytic Reactor Modeling - American Chemical Society

20 Poisoning Effects in Temperature-Increased Fixed-Bed Reactor Operation A Comparison of Experimental Results and Model Simulation GEORGE J. FRYCEK and J. B. BUTT Downloaded by FUDAN UNIV on November 19, 2016 | http://pubs.acs.org Publication Date: December 9, 1984 | doi: 10.1021/bk-1984-0237.ch020

Department of Chemical Engineering, Northwestern University, Evanston,IL60201

A variation of the constant conversion policy has been used to investigate the poisoning of a Nikieselguhr catalyst by thiophene. Hydrogenation of benzene was used as the model exothermic reaction. This alternative policy required that the conversion be allowed to decline under deactivating conditions in an isothermal fixed bed reactor. Upon reaching a lower conversion l i m i t , the feed impurity was removed from the feed and the bed temperature was increased to bring the average bed activity to a higher level and thus also the conversion of benzene. Comparison of experimental conversion vs. time and average isothermal bed temperature were made against a one-dimensional pseudohomogeneous plug flow reactor model u t i l i z i n g separable kinetics. Generally, excellent agreement between experiment and simulation were obtained for the f i r s t deactivation cycle. However, beyond the f i r s t cycle, the match between model and experiments was less satisfactory. In particular, the experimental results declined in a convex manner while the simulation decayed exponentially; temperature increase requirement (TIR) and decay times were reasonably well predicted for most cycles. I n d u s t r i a l l y , c a t a l y s t a c t i v i t y maintenance i s o f t e n screened v i a "temperature increase requirement" (TIR) experiments. In these experiments, constant conversion i s e s t a b l i s h e d and the r a t e o f temperature increase required to do so i s used as a measure o f the r e s i s t a n c e o f the c a t a l y s t t o d e a c t i v a t i o n . However, t h i s type o f o p e r a t i o n may mask the e f f e c t o f p a r t i c l e s i z e , temperature, temperature p r o f i l e , and heat o f r e a c t i o n on poison coverage, poison p r o f i l e , and the main r e a c t i o n r a t e . This masking may be p a r t i c u l a r l y important i n complicated r e a c t i o n s and r e a c t o r systems where the TIR experiment may produce p o s i t i v e feedback. The a l t e r n a t i v e approach presented here has been used i n an attempt to determine the importance o f some o f the above parameters. Another primary o b j e c t i v e o f t h i s research has been to determine how w e l l one might model the dynamic behavior o f the system i n an a p r i o r i fashion where the parameters used i n the 0097-6156/84/0237-0375$06.00/0 © 1984 American Chemical Society Dudukovi and Mills; Chemical and Catalytic Reactor Modeling ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

CHEMICAL AND CATALYTIC REACTOR MODELING

376

modeling were determined v i a independent experimentation, as reported i n p r i o r work (2-4).

Downloaded by FUDAN UNIV on November 19, 2016 | http://pubs.acs.org Publication Date: December 9, 1984 | doi: 10.1021/bk-1984-0237.ch020

Experimental A s t a i n l e s s s t e e l tubular r e a c t o r , which was approximately 16mm I.D., was used t o maximize the r a t e o f heat t r a n s f e r to and from a g l a s s bead d i l u t e d bed. The d i l u t e d bed was used to minimize l o c a l heat generation and was approximately 50cm i n length. Two temperature c o n t r o l l e r s and f i v e auto trans for mer s were used to c o n t r o l the r e a c t o r temperature p r o f i l e along the a x i a l length o f the r e a c t o r as shown i n Figure 1. The c o n t r o l l e r thermocouples were placed a t the top and bottom o f the r e a c t o r . The top c o n t r o l l e r was r e s p o n s i b l e f o r m a i n t a i n i n g a constant i n l e t feed temperature. The bottom c o n t r o l l e r and the f i v e v a r i a c s were used to maintain the desired temperature p r o f i l e . A c e n t e r l i n e thermocouple was used t o measure t h i s a x i a l temperature p r o f i l e as w e l l as t o serve as a guide f o r changes i n v a r i a c s e t t i n g s . These changes were a r e s u l t o f a moving a c t i v e zone from poisoning. Thermocouples used were o f type J, 0.040"O.D., and were grounded to minimize response time. An outer thermowell was o c c a s i o n a l l y used to give an i n d i c a t i o n o f r a d i a l gradients. S o l u t i o n s o f benzene and benzene-thiophene were fed v i a a p a r a l l e l - r e c i p r o c a l syringe pump. These s o l u t i o n s were then vaporized and mixed with hydrogen. E i t h e r the poisoned or pure feed streams could be passed through the r e a c t o r , as shown i n Figure 2, through the use o f a four-way valve and a by-pass r e a c t o r . Gas compositions were measured by a thermal c o n d u c t i v i t y gas chromatograph. Experimental c o n d i t i o n s are given i n Table I.

Table I. Run

Experimental Operating Conditions 6

7

8

9

10

υ

Average pressure (10- )[Pa]

1.1961

1.1961

1.2198

1.1961

1.1851

1.2063

Bed length (10 )[m]

4.667

4.659

4.667

4.699

4.699

4.667

Benzene molarflow benze (10')[Kg-mole/min]

8.795

8.795

22.069

8.720

8.720

22.147

Mole f r a cCtt liQo!n benzene (10^)

2.151

2.151

5.397

2.132

2.132

5.416

Mole f r a c t i o n hydrogen ( 1 0 )

9.784

9.784

9.457

9.783

9.783

9.457

134

134

336

341

341

122

5

1

1

PPM thiophene

A l l experiments: Cross s e c t i o n a l area f o r flow = 2.026 2

x10""^[m ]; c a t a l y s t weight = 2.00 g; glass bead = 150.0g; e x i t v o l u m e t r i c flow = 1000

weight

[ml/min]+55S (STP)

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

FRYCEK AND BUTT

Poisoning Effects: Experiment vs Model


20.

Figure 1.

Temperature Maintenance Schematic.


377


rt

Traps

m

GC Gas Chromatograph TC Thermocouple ΔΡ Pressure Gauge FV Four-way Valve BP: Bypass

F i g u r e 2.

Experimental

Apparatus.


Benztnt/ Thiophene

20.

379


FRYCEK AND BUTT

The hydrogénation c a t a l y s t used f o r t h i s study was Harshaw NÎ-0104T. The c a t a l y s t i s 58% by weight Ni on k i e s e l g u h r and was crushed to 60-70 mesh (250-210 microns) f o r use in these experiments. The granules were then mixed with s i m i l a r sized g l a s s beads and reduced f o r 4 hours i n hydrogen at 375°C.

Reactor Model A one-dimensional pseudohomogeneous plug flow r e a c t o r model assuming i s o t h e r m a l i t y was used to simulate experimental r e s u l t s . The c o n t i n u i t y and k i n e t i c expressions used were as f o l l o w s :


Benzene C o n t i n u i t y Equation

V

3Γ"

s

'

3z

V

z

(1)

Thiophene C o n t i n u i t y Equation

-P

(2) 3 x C

J£ ε·ρ

g.

x

In equations (1)&(2) "s" i s the a c t i v i t y v a r i a b l e and i s assumed to be separable from the i n t r i n s i c k i n e t i c s [1]. The a c t i v e s i t e balance i s as f o l l o w s , where the d e a c t i v a t i o n k i n e t i c s were determined by [ 2 ] and may be found i n Table I I :

f

=

r

=

D

=

-1^° exp

[-E /RT] . s D

P

^

V

The hydrogénation r a t e equation f o r benzene, as i n v e s t i g a t e d by [33 i s : k°K° exp[(Q-E)/RT] _

=

r

Β

M 1 + K°exp[Q/RT] Ρ Χ Table I I .

(4)

β

K i n e t i c Parameters

k L Kg-mole/Kg-s-Pa]

7.350 6E-3

K[1/Pa]

2.2652E-9

E[J/Kg-mole]

5.3398E+7

Q[J/Kg-mole]

4.65 85E+7

Ed[J/Kg-mole] kd[1/(Pa-s)] *M [Kg-mole/Kg] T

4.53 4E +6 1.802E-4 8.1E-4

M was determined e m p i r i c a l l y and found to be w e l l w i t h i n the bounds found by previous r e s e a r c h ers [2,5,6]. T


380


The benzene k i n e t i c parameters used for the above expression were those reported by Onal [4] and are given i n Table I I . The boundary c o n d i t i o n s used in the s i m u l a t i o n were: z=0 t=0

X

B

X

" B°


s(z) =

;

X

T

X

" T°

(5)

1.0

A major assumption i n the above modeling was that one could assume that the poisoning process was slow enough that the assumption o f quasi steady-state could be made. This e l i m i n a t e d the accumulation terms i n equations (1)&(2) and allowed them to be solved by a 4th order Runge-Kutta method. Further, by assuming that the thiophene term i n equation (3) was a constant, one could preserve i t s i n t e g r a t e d exponential c h a r a c t e r i s t i c s . The i n t e g r a t e d a c t i v i t y balance was then solved for each i n t e r i o r point w i t h i n the r e a c t o r . The time step s i z e was governed by the r a t e at which the c a t a l y s t d e a c t i v a t e d . Ihe c r i t e r i o n used to c o n t r o l the time step allowed a maximum drop i n conversion o f no more than 7%. V i o l a t i o n o f t h i s requirement r e s u l t e d in a h a l v i n g o f the time step. Further assumptions were that the poisoning process was i r r e v e r s i b l e , the benzene c o n c e n t r a t i o n was low enough t h a t volume changes were n e g l i g i b l e and that heat and mass t r a n s f e r l i m a t i o n s were absent. These r e s t r i c t i o n s were a l l obeyed i n the experimentation reported here. Reactor Dynamics and

Simulation

S t a r t Up Dynamics. Introduction o f benzene i n t o a f r e s h bed resulted" in an i n i t i a l non-isothermal temperature p r o f i l e . Through the use o f the au to trans formers, one was able to o v e r r i d e the system c o n t r o l and thus l i n e out the temperature p r o f i l e . Figure 3 d e p i c t s a t y p i c a l r e a c t o r s t a r t up for a f r e s h bed. F i r s t Cycle Poisoning Dynamics. Upon a t t a i n i n g an isothermal temperature p r o f i l e for an i m p u r i t y free feed, the next phase o f the experiment was begun. Ihiophene was introduced into the benzene feed as the feed impurity. The thiophene a l s o caused another problem which manifested i t s e l f as a m i g r a t i n g temperature depression, a r e s u l t o f the a c t i v e zone moving towards the end o f the r e a c t o r . This movement o f the r e a c t i o n zone decreased the heat generation at the upper s e c t i o n o f the r e a c t o r and t h e r e f o r e a temperature depression r e s u l t e d . A comparsion between the simulated a c t i v i t y p r o f i l e and the temperature migration i n Fig.4 i l l u s t r a t e s the above phenomena. Temperature Increase Dynamics a f t e r the F i r s t Cycle. As with the s t a r t up o f the bed, subsequent temperature c y c l e s r e s u l t e d i n the formation o f a m i l d hot spot. Ihe occurrence o f t h i s temperature f l u c t u a t i o n i s undesirable s i n c e the past h i s t o r y o f the c a t a l y s t may be a l t e r e d . The adsorption o f thiophene upon the a c t i v e hydrogénation s i t e s was assumed to be i r r e v e r s i b l e and t h e r e f o r e unaffected by temperature. However, as w i l l become apparent l a t e r , the e f f e c t o f temperature may have a l t e r e d the poison coverage/or p r o f i l e . Lyubarski, et. a l [71 determined t h a t , as a r e s u l t o f the hydrogénation o f thiophene and subsequent hydrogenolysis to butane, the adsorption c a p a c i t y o f a suported Ni


FRYCEK AND BUTT



20.

Figure 3. Representative A x i a l Temperature P r o f i l e s : S t a r t Up Run 6: Low Benzene.


381



382

Figure 4. A x i a l P r o f i l e s During F i r s t Cycle D e a c t i v a t i o n Run 6: Low Thiophene.


20.


FRYCEK AND BUTT

383


was a l t e r e d for temperatures o f 100-150° C. This temperature s i m u l a t i o n and experimental TIR and conversion decay p r o f i l e s . Decay Behavior and S i m u l a t i o n . Comparison o f experimental versus simulated r e s u l t s y i e l d s some very i n t e r e s t i n g i n s i g h t s i n t o the behavior o f both the model used to simulate the d e a c t i v a t i o n and the experimental system i t s e l f . Parametric values used i n S i m u l a t i o n of the dynamics of the system may be found i n Tables 1 and I I . Each experiment may be d i v i d e d into d i s t i n c t sections which w i l l be termed c y c l e s . The end o f a c y c l e was f o l l o w e d by a temperature increase and a concomitant increase i n conversion. Good agreement was found to e x i s t between s i m u l a t i o n and experiment during the f i r s t c y c l e as shown i n Figure 5 through Figure 10. This agreement supports the k i n e t i c model which was used to simulate the system. Temperature and decay times were reasonably w e l l p r e d i c t e d f o r both regimes o f slower and f a s t e r d e a c t i v a t i o n . Figure 8, which d e s c r i b e s Run 9· shows that the c a t a l y s t prematurely d e a c t i v a t e d with r e s p e c t to the model. Examination o f Run 9 Figure 8, and Run 10, Figure 9 shows that f o r the f i r s t c y c l e the experimental data are below the s i m u l a t i o n curve. This may i n d i c a t e that the assumption o f quasi steadys t a t e may have been questionable at t h i s point. However, r e p r o d u c i b i l i t y f o r a l l experiments was very good. This r e p r o d u c i b i l i t y i s important when experiment and model begin to deviate. Such i s the case f o r the c y c l e s a f t e r the f i r s t . Looking at Figure 5 through Figure 10 one observes that the s i m u l a t i o n temperature p r e d i c t i o n f o r a d e s i r e d conversion i n the 2nd and some 3rd c y c l e s was u s u a l l y good. However, the conversion decay p r o f i l e s deviate s u b s t a n t i a l l y from each other. The experimental r e s u l t s decay i n a convex manner while the simulated r e s u l t s decay e x p o n e n t i a l l y . This d e v i a t i o n may have been a r e s u l t o f the temperature excursions which r e s u l t e d on each TIR. Thus, as mentioned e a r l i e r , the past h i s t o r y o f the c a t a l y s t may have been a l t e r e d . Despite t h i s , the decay times are f a i r l y w e l l p r e d i c t e d . Some 3rd c y c l e p r e d i c t i o n s completely f a i l e d as shown i n Figures 8 and 10. These appear to be a r e s u l t o f the temperature a f f e c t i n g the chemisorption o f the thiophene on the active catalyst. In separable k i n e t i c terms, the r e s i s t a n c e to d e a c t i v a t i o n i n the above outwardly manifests i t s e l f as an increase i n the a c t i v a t i o n energy f o r d e a c t i v a t i o n . As the bed temperature i s increased, the c a t a l y s t appears to have a higher r e s i s t a n c e to poisoning. t

t

Conclusions The s i n g l e c y c l e , f a l l i n g conversion TIR experiment seems to be a r e l i a b l e measure o f decay r a t e . However, f o r m u l t i p l e c y c l e s , t h i s system appears to be v u l n e r a b l e to problems s i m i l a r to those o f the constant conversion TIR p o l i c y . E l i m i n a t i o n o f temperature upsets and regulated temperature increases need to be addressed i n the f u t u r e . The k i n e t i c model used to simulate the r e a c t i o n and poisoning process appears to be adequate f o r the experiments performed. However, the i n f l u e n c e exerted by s m a l l temperature upsets a f t e r each c y c l e may n e c e s s i t a t e a more d e t a i l e d s i m u l a t i o n o f heat t r a n f e r e f f e c t s i f more q u a n t i t a t i v e agreement with experiment i s required.



I.Oi

0.9 0.8-1 \


0.7 0.6 0.5 κ

0.4

\

0.3 0.2 0.1 0.0 380 370 360

•

350

Expérimental Data Simulation Results

•· · 340

0

30

60

-L

_L

90

120

150

180 210

± 240 270

Time (minutes) Figure 5. Simulation of Run 6: Low Benzene and Low Thiophene.


\ \

H

V \


•

385


FRYCEK AND BUTT

l \ . I\ I \ I \ I \ I \ I \ I \

\

•\

\

\ \

•

Experimental Simulation

30

60

90

120 Time

150

180

210

Data

Results

-L 240

(minutes)

Figure 6. Simulation of Run 7: Low Benzene and Low Thiophene.


270

386


0.8-1

ΟΤ

Ι

0.6


i S

0.5-

I

0.4-

o o \

0.3-

\

\

0.2

0.1 0.0

440-1

B S

4 0 0 J

•

380H

—


Data

Results

J

360

15

30

45

60 Time

75

90

-L

J-

105

120

(minutes)

Figure 7. Simulation of Run 8: High Benzene and High Thiophene.


135

FRYCEK AND BUTT

387

\


\


•


10

20

30

40 Time

50

60

70

Data

Results

80

90

(minutes)

Figure 8. Simulation of Run 9: Low Benzene and High Thiophene ( s u f f i c i e n t conversion to conduct a 3rd Cycle was not obtained f o r Τ up to 490°K).



'•°T 0.9-

0.8-|

0.7-


g

0.6-

m

.S

Ο Ο 0.3-

0 2^

0.1 0.0385-1

375-1

3 365-1

•

355-1

— —

345+

Experimental D a t a Simulation

Results

-e-J ΙΟ

20

30

40

L

•Χ

50

60

70

80

90

Time (minutes)

Figure 9. Simulation of Run 10: Low Benzene and High Thiophene.



FRYCEK AND BUTT

\

389

\

•

\

\


\

\

\

•

\

\ \

•\

\ \

V

• ·· ·

Γ7"

-L 30

45

60

75 90 120 Time (minutes)

150

180

210

Figure 10. Simulation of Run 11: High Benzene and Low Thiophene ( s u f f i c i e n t conversion to conduct a 3rd Cycle was not obtained f o r Τ up to 490°K).


390


Acknowle dgment s T h i s research was supported by the National Science under Grant CPE-791523 * and the Mobil Foundation.

Foundation

2

NOMENCLATURE E,E

A c t i v a t i o n energies f o r benzene hydrogénation and thiophene poisoning, r e s p e c t i v e l y (J/Kg-mole)

D

k°

Pre-exponential f a c t o r f o r benzene hydrogénation r a t e constant (Kg-mole/Kg(catalyst)-sec-Pa)

kS

Pre-exponential constant f o r thiophene (Pa-sec)-

poisoning


T

K°

Pre-exponential f a c t o r f o r adsorption r a t e constant (Pa)" 1

L M

C a t a l y s t bed length (m) Adsorption c a p a c i t y o f c a t a l y s t f o r thiophene (thiophene)/Kg(cat))

T

MWg

Molecular weight o f the gas phase

Ρ

Average pressure (Pa)

Q

Benzene heat o f adsorption (J/Kg-mole)

-r

B

-r

D

-r

T

(Kg/Kg-mole)

Rate o f benzene hydrogénation (Kg-mole/Kg(catalyst)sec) and (Kg-mole/m^ ( p a r t i c l e ) - s e c ) Rate o f d e a c t i v a t i o n (sec~^) Rate o f thiophene chemisorption (Kg-mole(thiophene) /Kg( c a t a l yst ) -sec )

R

U n i v e r s a l gas constant

s

Catalytic

t

Time (sec)

Τ

Temperature (°K)

V

(Kg-mole

(J/Kg-mole-°K)

activity

I n t e r s t i t i a l gas v e l o c i t y

z

(m/sec)

Χβ,Χ^,Χ-ρ

Mole f r a c t i o n o f benzene, hydrogen, and thiophene

Xg,X^

I n i t i a l mole f r a c t i o n o f benzene and thiophene

ζ

r e a c t o r a x i a l p o s i t i o n (m)

Greek Symbols ε

Bed void f r a c t i o n (m^ void/m^ t o t a l )


20.

Pg Peat

F R Y C E K AND B U T T

391


Density of gas phase (Kg/nr) Bulk density of catalyst (Kg/m^)


Literature Cited 1.

Szepe, S.; Levenspiel, O.; Proc. 45h Euro. Symp. Chem. Reac. Eng., 1968: 265- Pergamon Press, Oxford.

2.

Weng, H.S.; Eigenberger, G.; Butt, J.B.; Chem. Eng. S c i . 1975 30, 1281.

3.

Kehoe, J.P.G.; Butt, J.B. J . Appl. Chem. Biotechnol. 1972, 23, 22.

4.

Onal, I., Ph.D. Dissertation, Northwestern University, Evanston, IL, 1981. Available from University Microfilms.

5.

P r i c e , T.H.; Butt, J.B. Chem. Eng. S c i . 1977,

6.

B i l l i m o r i a , R.M.; Butt, J.B. Chem. Eng. J . 1981, 22,

7.

Lyubarski, G.D.; Ardeeva, L.B.; Kul'kova, N.V. Kinet. and Catal. 1962, 3, 102.

32,

393.

R E C E I V E D August 22, 1983


71.

Chemical and Catalytic Reactor Modeling - American Chemical Society

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