Chemical Reaction Engineering-Houston

2 Performances of Tubular and Loop Reactors in Kinetic Measurements G E R H A R D L U F T , R A I N E R R Ö M E R , and F R I T Z H Ä U S S E R

Downloaded by CORNELL UNIV on October 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch002

Institut für Chemische Technologie der Technischen Hochschule Darmstadt, 61 Darmstadt, Petersengstrasse 15, West Germany

I n d u s t r i a l reactors i n which highly exothermic,he terogenous c a t a l y t i c reactions are carried out are sen s i t i v e i f the reaction conditions or the cooling rates are suddenly changed. They can be operated only i n a small range i n order to avoid damage to the apparatus or to the catalyst by super heating, also to avoid loss i n y i e l d by side reactions, favoured at high tempera tures. In a d d i t i o n , poor accuracy in the rate data, as well as i n the mass and heat transfer parameters,do not allow to calculate the exact concentration and temperature p r o f i l e s inside the reactor. This leads to incorrect p r e d i c t i o n of the reactor's dynamic behaviour. There fore these data should be determined as accurately as possible. For the measurement of reaction rates, d i f f e r e n t i a l reactors having extremely short catalyst beds or i n t e g r a l reactors with r e l a t i v e long catalyst beds are often used. In the f i r s t type of experimental reactor, the concentration and temperature gradients within the catalyst beds are n e g l i g i b l y small. Due to t h i s fact, the reaction rate point data can be measured, provided the small concentration differences can be accurately analyzed. In the i n t e g r a l reactor, the change i n con centration i s much higher. There i s i n general no d i f f i c u l t y analyzing the concentrations of the reacting species but, the reaction rates have to be determined from the concentration curves by c a l c u l a t i o n and cannot often be related to the fast changing temperature. Be cause of these obvious disadvantages, the s o - c a l l e d loop reactors are being used more and more i n k i n e t i c studies. In loop reactors, the extremely small concen t r a t i o n and temperature gradients desired within the short catalyst bed, along with s u f f i c i e n t l y high con centration difference between the reactor i n l e t and the ©

0-8412-0401-2/78/47-065-015$05.00/0

Weekman and Luss; Chemical Reaction Engineering—Houston ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


CHEMICAL REACTION ENGINEERING—HOUSTO^

Integral Tubular Reactor

Loop Reactor Figure 1.

Differential Reactor

Stirred-Tank Reactor Types of laboratory reactors



2.

LUFT ET AL.

Tubular and Loop Reactors

17

o u t l e t , can be r e a l i z e d by r e c y c l i n g a p a r t o f t h e react i o n products. In o r d e r t o see how t h e s e advantages c o u l d be r e a l i zed i n p r a c t i c e , t h e performance o f a l o o p r e a c t o r was compared w i t h t h a t o f a c o n v e n t i o n a l l y - b u i l t i n t e g r a l r e a c t o r . I n t h i s comparison t h e c a p a b i l i t y t o handle a c t u a l i n d u s t r i a l c a t a l y s t s , t h e s e t t l i n g time o f changing experimental c o n d i t i o n s , the d i f f i c u l t y of the m a t h e m a t i c a l e v a l u a t i o n o f t h e measured d a t a were cons i d e r e d . The a c c u r a c y o f t h e d a t a s f o r s c a l e up p r o b lems was checked i n a p i l o t p l a n t . F o r t h e r e a c t i o n , the o x i d a t i o n o f o-xylene w i t h a vanadiumpentoxide c a t a l y s t , an i n d u s t r i a l l y important p r o c e s s , was chosen. Apparatus The d e s i g n o f t h e l o o p r e a c t o r ( F i g . 2) p e r m i t s changes i n t h e r e a c t i o n c o n d i t i o n s , such a s temperatur e , c o n c e n t r a t i o n and throughput i n a wide range. I t s core i s a d i f f e r e n t i a l r e a c t o r d i r e c t l y c o u p l e d t o t h e blower. I t sucks t h e r e a c t a n t s through t h e c a t a l y s t bed and r e c y c l e s p a r t o f i t . T h i s d e s i g n a l l o w s o n l y a s m a l l dead volume and a s m a l l p r e s s u r e drop a c r o s s t h e c a t a l y s t bed even a t h i g h f l o w r a t e s . F u r t h e r m o r e , t h e whole a p p a r a t u s i s compact and t h e r e f o r e i t can e a s i l y be m a i n t a i n e d a t c o n s t a n t temperature. The s m a l l temp e r a t u r e and c o n c e n t r a t i o n g r a d i e n t s w i t h i n t h e catalyst bed, n e c e s s a r y f o r t h e k i n e t i c measurements, can^be r e a l i z e d by r e c y c l i n g p a r t o f t h e gas about 12 m /h. I t i s v e r y l a r g e compared t o t h e feed and c o r r e s p o n d s t o r e c y c l e r a t i o s o f l o o t o 5oo, a l s o s u f f i c i e n t f o r t h e a p p r o p r i a t e study o f highly-exothermic reactions. The r e c y c l e r a t i o can be changed w i t h r e s p e c t t o t h e r e a c t i o n c o n d i t i o n s by changing t h e speed o f r o t a t i o n o f t h e blower. The blower i s d r i v e n by an asynchronousmotor whose r o t o r i s f i x e d t o t h e s h a f t o f t h e blower. I t i s separ a t e d from t h e s t a t o r by means o f a p r e s s u r e tube i n o r d e r t o e x c l u d e any l e a k a g e . The i n t e g r a l a p p a r a t u s ( F i g . 3) c o n s i s t s o f a tubul a r r e a c t o r o f 1 m-length. In o r d e r t o measure t h e conc e n t r a t i o n , about 2o sampling t a p s a r e i n s t a l l e d a l o n g the tube. Through each sampling t a p , a thermocouple i s passed t o determine t h e temperature p r o f i l e . The a i r i s f e d through d r i e r s , f l o w meters and h e a t e r s b e f o r e e n t e r i n g v a p o r i z e r , where x y l e n e i s e v a p o r a t e d . T h i s a i r - x y l e n e m i x t u r e , c o n t a i n i n g about o.9 i:ol% x y l e n e , i s f e d t o t h e t o p o f t h e t u b u l a r r e a c t o r . The p h t h a l i c a n h y d r i d e , l e a v i n g t h e r e a c t o r i s washed and condensed by water i n a s p r a y tower. The r e a c t i o n heat i s removed by an e f f i c i e n t c o o l i n g system i n which d i p h e n y l (Dow therm) i s v a p o r i z e d .



18

CHEMICAL REACTION ENGINEERING—HOUSTON

Figure 2.

Loop reactor



2.

LUFT ET AL.

Tubular and

19

Loop Reactors

j 1 2 3 4 5 6 7

Figure 3.

Xylene storage Filter Metering pump Reactor Spray absorber Vaporizer Air pre heater

8 9 10 11 12 13

Rotameter Adsorber - Dryer Air regulater Savety switch Filter

Tubular reactor for the oxidation of xylene



20


R e s u l t s and e v a l u a t i o n The measurements i n b o t h r e a c t o r s were c a r r i e d out at steady s t a t e . The c a t a l y s t a c t i v i t y was m a i n t a i n e d at a l l times, t e s t i n g a t r e g u l a r i n t e r v a l s f o r any l o s s in activity. The r e s u l t s o f a t y p i c a l experiment a r e shown i n F i g . 4 . The c o n c e n t r a t i o n o f t h e r e a c t a n t s i s p l o t t e d v e r s u s a m o d i f i e d r e s i d e n c e time. The r e s i d e n c e time c o u l d be v a r i e d a l o n g t h e range l - l o g.h/mole by changing t h e throughput and t h e q u a n t i t i y o f c a t a l y s t . The temperature was k e p t c o n s t a n t a t 41o°C. The x y l e n e c o n c e n t r a t i o n i n t h e feed c o u l d be i n c r e a s e d up t o 1 , 3 mol %, which i s h i g h e r than the lower e x p l o s i o n l i m i t . As i t can be seen from t h e c u r v e s , t h e concent r a t i o n of the xylene feed decreases s t e a d i l y with i n c r e a s i n g r e s i d e n c e time. The c o n c e n t r a t i o n o f t h e mai r e a c t i o n product p h t h a l i c a n h y d r i d e (PSA) i n c r e a s e s f i r s t , then d e c r e a s e s a t h i g h r e s i d e n c e t i m e s due t o i t * o x i d a t i o n forming CO and C G . A l s o t h e c o n c e n t r a t i o n ofth intermediate products tolualdehyde (TCL) and p h t h a l i d e (PI) which a r e c o n s i d e r e d t o g e t h e r f o r s i m p l i c i t y , pass through a maximum. In t h e i n t e g r a l r e a c t o r , t h e e x p e r i m e n t s c o u l d not be c a r r i e d out i s o t h e r m a l l y ( F i g . 5 ) . The temperature ( l e f t ordinate) r i s e s s t e e p l y i n the f i r s t part of the c a t a l y s t bed, p a s s e s through a d i s t i n c t maximum and dec r e a s e s a g a i n by the c o o l i n g . The x y l e n e i s almost c o m p l e t e l y c o n v e r t e d . The concent r a t i o n o f t h e PSA i n c r e a s e s a t f i r s t s t e e p l y , then t e n d s t o l e v e l o f f i n t h e lower p a r t o f t h e r e a c t o r . Carbonmonoxide (CO) and c a r b o n d i o x i d e as w e l l as malei c a n h y d r i d e which was d e t e c t e d a t a low c o n c e n t r a t i o n , i n c r e a s e s t e a d i l y a l o n g the c a t a l y s t bed whereas t h e curve o f t o l u a l d e h y d e and p h t h a l i d e show a maximum s i m i l a r t o t h e loop r e a c t o r e x p e r i m e n t s . The e v a l u a t i o n o f t h e e x p e r i m e n t s i n both r e a c t o r s was based on t h e mechanism o f t h e o x i d a t i o n . The conc e n t r a t i o n p r o f i l e s measured i n t h e i n t e g r a l r e a c t o r , as w e l l as t h e f i n i t e s l o p e i n t h e o r i g i n o f t h e conc e n t r a t i o n - r e s i d e n c e time c u r v e s from t h e loop r e a c t o r , r e v e a l t h a t , t h e x y l e n e i s c o n v e r t e d by s i m u l t a n e o u s r e a c t i o n s t o the products p h t h a l i c a n h y d r i d e , p h t h a l i d e , t o l u y l a l d e h y d e , CO and C G . The d i s t i n c t maximum o f t h e p h t h a l i d e - and t o l u a l d e h y d e - c o n c e n t r a t i o n curve i n d i c a t e s t h a t these a r e i n t e r m e d i a t e p r o d u c t s which a r e converted mainly t o phthalicanhydride i n a consecutive s t e p . From t h e d e c r e a s e o f t h e P S A - c o n c e n t r a t i o n a t h i g h r e s i d e n c e t i m e s i t may be concluded t h a t , t h i s s p e c i e s o x i d i z e t o CO, CG , as w e l l a s water and t o a lower extend, t o MSA. F o r t h e e v a l u a t i o n o f t h e ex2

2

2



2.

LUFT ET AL.

21


IrW/Ho Residence time Figure 4.

Results of loop-reactor experiments

450|

— —

Length

Figure 5.

Concentration and temperature distribution in integralreactor



22


p e r i d e n t a l r e s u l t s , i t i s considered u s e f u l to s i m p l i f y the mentioned r e a c t i o n scheme; t h u s , the concent r a t i o n s of t o l u a l d e h y d e and p h t h a l i d e were i n c l u d e d t o g e t h e r . The s m a l l q u a n t i t i e s of MSA were n e g l e c t e d . At the h i g h r e c y c l i n g r a t i o s the loop r e a c t o r o p e r a t e s as an i d e a l s t i r r e d - t a n k r e a c t o r . T h e r e f o r e , the r e a c t i o n r a t e can immediately be determined from the d i f f e r e n c e i n c o n c e n t r a t i o n between the feed and the o u t l e t , the throughput and the q u a n t i t y of c a t a l y s t . T h e r a t e e q u a t i o n , d e s c r i b i n g the consumption of x y l e n e and the f o r m a t i o n of the r e a c t i o n p r o d u c t s , are c o n s i d e r e d t o be pseudo f i r s t o r d e r . The parameter of the r a t e equations, vhich are the f r e q u e n c y f a c t o r s and the a c t i v a t i o n e n e r g i e s , are determined by l e a s t square methods. In the above f u n c t i o n ( F i g . 6b) r i s the measured r a t e , f i s c a l c u l a t e d w i t h e s t i m a t e d p a r a meters, w r e p r e s e n t a p p r o p r i a t e weight f a c t o r s and N i s the number of measured v a l u e s . Because the r a t e e q u a t i o n s c o u l d be d i f f e r e n t i a t e d w i t h r e s p e c t t o the unknown k i n e t i c parameters, the o b j e c t i v e f u n c t i o n was m i n i m i z e d by a s t e p w i s e r e g r e s s i o n . The s t e e p c o n c e n t r a t i o n and temperature p r o f i l e s i n the i n t e g r a l r e a c t o r d i d not a l l o w t o determine the r e a c t i o n r a t e s immediately. T h e r e f o r e , the o b j e c t i v e f u n c t i o n c o n t a i n s the measured and the c a l c u l a t e d conc e n t r a t i o n s i n s t e a d of the r e a c t i o n r a t e s , a l s o the t e m p e r a t u r e s because of the n o n i s o t h e r m a l r e a c t o r beh a v i o u r . The k i n e t i c parameters must be o b t a i n e d by d i r e c t s e a r c h t e c h n i q u e s l i k e the d e r i v a t i v e f r e e simp l e x method of N e l d e r and Mead. Comparison Comparing the two l a b o r a t o r y r e a c t o r s i t may be n o t i c e d t h a t the loop r e a c t o r i s more e x p e n s i v e . A l though the q u a n t i t y , o f c a t a l y s t and the volume of the l o o p r e a c t o r i s s m a l l , compared t o the i n t e g r a l r e a c t o r , the r e c y c l i n g of a l a r g e volume of gas r e q u i r e s a c o m p l i c a t e d blower. Hoivever, c e r t a i n advantages and d i s a d v a n t a g e s r e s u l t from the d i f f e r e n t c o n c e n t r a t i o n and temperature d i s t r i b u t i o n i n both r e a c t o r s . Because of the u n i f o r m c o n c e n t r a t i o n and temperature i n s i d e the loop r e a c t o r , the c o n c e n t r a t i o n of the r e a c t a n t s c o u l d be measured o n l y i n the r e a c t o r j n l e t and o u t l e t to determine the r e a c t i o n r a t e . The s t e e p c o n c e n t r a t i o n and temperat u r e g r a d i e n t s i n s i d e the i n t e g r a l r e a c t o r r e q u i r e measurements at many s p o t s a l o n g the tube. T h i s becomes r a t h e r e x p e n s i v e i n time i f s e v e r a l components are t o be a n a l y z e d as i n the o x i d a t i o n of x y l e n e . In the e v a l u a t i o n of the e x p e r i m e n t a l r e s u l t s the d i s t r i b u t i o n of c o n c e n t r a t i o n and temperature appear


2.

LUFT ET AL.

23


^L_ToUPI-^ Xylene \

§—••PSA 12 *C0*C02

3


Xylene

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-(k

x

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Phthali canhydride

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Tolu — aldehyde j r

Phthalide

CO, c o

TOL*PI

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= o3-*'

2

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3

Figure 6a.

R

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,

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k

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Rate equations

N #

=

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x

( r

x

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w

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Xylene PhthoJic anhydride Tolu—at deny de Phthalide

Figure 6b.

(r 2

C0,C0

N W ? r

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) 2 2

Number of runs Appropriate weight factor Reaction rate, calculated - * - , measured

Objective function


}


24


Length

[cm]

•

Figure 7. Comparison of the loop-reactor data with pilot plant experiments



2.

LUFT ET AL.

Tubular and

25

Loop Reactors

t o be the b a s i c f a c t o r s i n t r o d u c i n g d i f f i c u l t y . The l o o p r e a c t o r can be d e s c r i b e d by simple a l g e b r a i c e q u a t i o n s of which the c o e f f i c i e n t s , p e r t a i n i n g t o the unknown f r e q u e n c y f a c t o r s and a c t i v a t i o n e n e r g i e s , can be o b t a i n e d by s t e p w i s e r e g r e s s i o n . In the case of the i n t e g r a l r e a c t o r , the e s t i m a t i o n parameters are more c o m p l i c a t e d and r e q u i r e s more computation time because of the n e c e s s i t y f o r n u m e r i c a l i n t e g r a t i o n of a s e t of d i f f e r e n t i a l e q u a t i o n s . In o r d e r t o check the a c c u r a c y of the measured data and c o l l e c t i n f o r m a t i o n f o r s c a l e - u p , a d d i t i o n a l exp e r i m e n t s were c a r r i e d out i n a p i l o t p l a n t and the r e s u l t s were compared w i t h t h e s e p r e v i o u s l y o b t a i n e d i n the l a b o r a t o r y r e a c t o r s . The p i l o t p l a n t r e a c t o r c o n s i s t e d of a tube of 25 mm i n n e r d i a m e t e r and 4 m l e n g t h t a k e n from an i n d u s t r i a l m u l t i t u b e r e a c t o r . I t was f i l l e d w i t h about 1 kg c a t a l y s t p e l l e t s . The measured temperature- and c o n c e n t r a t i o n p r o f i l e s are p l o t t e d i n F i g . 7 v e r s u s the l e n g t h of the c a t a l y s t bed. The p o i n t s are e x p e r i m e n t a l l y determined whereas the t h i c k - l i n e c u r v e s have been c a l c u l a t e d u s i n g the k i n e t i c c o n s t a n t s o b t a i n e d i n the l o o p r e a c t o r exp e r i m e n t s . A c l o s e agreement between the e x p e r i m e n t a l r e s u l t s of the p i l o t r e a c t o r and the c a l c u l a t e d v a l u e s i s apparent. O n l y the c a l c u l a t e d CO and CG concentrat i o n s are a l i t t l e h i g h , c a u s i n g a l s o a h i g h e r temperature maximum. 2

Recommendations As the e x p e r i m e n t s i n the l o o p r e a c t o r can be carried out i s o t h e r m a l l y and at c o n s t a n t c o n c e n t r a t i o n s and the i n f l u e n c e of mass t r a n s f e r can be e x c l u d e d by a h i g h flow r a t e and s m a l l c a t a l y s t p e l l e t s , the l o o p r e a c t o r i s t o be recommendated f o r k i n e t i c s t u d i e s . F u r t h e r advantages are the f l e x i b i l i t y of the r e a c t o r w i t h r e s p e c t t o changes i n e x p e r i m e n t a l c o n d i t i o n s and l a s t but not l e a s t the u n c o m p l i c a t e d e v a l u a t i o n of the measured d a t a . The i n t e g r a l r e a c t o r shows some advantages i n the study of the p r o d u c t q u a l i t y and s e l e c t i v i t y because t e c h n i c a l c o n d i t i o n s can e a s i l y be i n c o r p o r a t e d . Furthermore, i t i s p o s s i b l e t o measure s i m u l t a n e o u s l y the heat cond u c t i v i t y i n the c a t a l y s t bed and the heat t r a n s f e r c o e f f i c i e n t through the r e a c t o r w a l l . The d i f f i c u l t i e s i n the e v a l u a t i o n of the e x p e r i m e n t s depend s t r o n g l y on the m a t h e m a t i c a l model which has t o be chosen. The e v a l u a t i o n i s c e r t a i n l y more comp l i c a t e d i f the r e a c t o r must be d e s c r i b e d by a twod i m e n s i o n a l model because o f s t e e p r a d i a l temperature g r a d i e n t s as we have observed i t i n the p h t h a l i c anhydrid reactor.


Chemical Reaction Engineering-Houston

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