Heat Transfer in Packed Reactor Tubes Suitable for Selective Oxidation

41 Heat Transfer in Packed Reactor Tubes Suitable for Selective Oxidation T. W E L L A U E R and D. L . CRESSWELL Technisch-Chemisches Labor, E.T.H. Zentrum, CH-8092 Zurich, Switzerland

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch041

E . J. NEWSON Schweiz. Aluminium A G , CH-8212 Neuhausen am Rheinfall, Switzerland

Extensive experimental determinations of overall heat transfer coefficients over packed reactor tubes suit able for selective oxidation are presented. The scope of the experiments covers the effects of tube dia meter, coolant temperature, air mass velocity, packing size, shape and thermal conductivity. Various predictive models of heat transfer in packed beds are tested with the data. The best results (to within ±10%) are obtained from a recently developed two-phase con tinuum model, incorporating combined conduction, con vection and radiation, the latter being found to be significant under commercial operating conditions. S e l e c t i v e hydrocarbon o x i d a t i o n r e a c t i o n s are c h a r a c t e r i s e d by both high a c t i v a t i o n energies and heats o f r e a c t i o n . I f the d e s i r e d p a r t i a l o x i d a t i o n products a r e to be safeguarded and the c a t a l y s t i n t e g r i t y ensured i t i s e s s e n t i a l t h a t c l o s e temperature c o n t r o l be maintained. In s p i t e o f the obvious a t t r a c t i o n s o f the f l u i d bed f o r t h i s purpose, mechanical c o n s i d e r a t i o n s normally d i c t a t e that a m u l t i - t u b u l a r fixed-bed r e a c t o r , comprising small diameter tubes between 2-4 cms. d i a m e t e r be used. Heat t r a n s f e r s t u d i e s on f i x e d beds have almost i n v a r i a b l y been made on tubes o f l a r g e diameter by measuring r a d i a l temperature p r o f i l e s ( 1 ) . The c o r r e l a t i o n s so obtained i n v o l v e l a r g e e x t r a p o l a t i o n s o f tube diameter and are o f questionable v a l i d i t y i n the design o f many i n d u s t r i a l r e a c t o r s , i n v o l v i n g the use o f narrow tubes. In such beds i t i s only p o s s i b l e to measure an a x i a l temperature p r o f i l e , u s u a l l y that along the c e n t r a l a x i s ( 2 ) , from which an o v e r a l l heat t r a n s f e r c o e f f i c i e n t (U) can be determined. The o v e r a l l heat t r a n s f e r c o e f f i c i e n t (U) can be then used i n one-dimensional r e a c t o r models to o b t a i n a p r e l i m i n a r y impression o f l o n g i t u d i n a l product and temperature d i s t r i b u t i o n s . y

0097-6156/82/0196-0527$06.00/0 © 1982 American Chemical Society Wei and Georgakis; Chemical Reaction Engineering—Boston ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

528

CHEMICAL REACTION ENGINEERING

The ranges o f experimental v a r i a b l e s covered (Table I) span those of s e v e r a l hydrocarbon o x i d a t i o n r e a c t i o n s , among which are o-xylene, benzene and η-butane. Only minor e x t r a p o l a t i o n s are r e q u i r e d i n ethylene o x i d a t i o n , and these are s a f e l y r e a l i s e d by the model developed i n the second p a r t of t h i s paper.


Experimental A schematic diagram o f the experimental equipment i s shown i n F i g . 1. Heat t r a n s f e r measurements were made i n v e r t i c a l l y - m o u n t e d s t e e l tubes of 21 mm and 28 mm I.D. and 4 metres l e n g t h , which had been constructed as p a r t of an experimental r e a c t o r p i l o t p l a n t . The tubes were contained w i t h i n a molten s a l t bath, equipped w i t h s t i r r e r and i n t e r n a l heat exchanger. B o l t e d onto each tube, but thermally i n s u l a t e d from i t , was a "water-cooled" calming s e c t i o n of the same I.D. as the r e a c t o r tube. A 2 mm s t e e l hypodermic tube was i n s e r t e d along the c e n t r a l a x i s of the r e a c t o r tube and calm ing s e c t i o n , p r i o r to packing, and l o c a t e d c e n t r a l l y by a number of spacers, which were removed before the experiments were s t a r t e d . The tube contained a s l i d i n g thermocouple, the p o s i t i o n of which could be a c c u r a t e l y measured. The r e a c t o r tube was f i r s t f i l l e d with about 2.5 metres of i n e r t packing followed by about 1.5 m. o f the packing under t e s t , to provide a continuous length of packed bed extending to the top of the calming s e c t i o n . A small s e c t i o n of the r e a c t o r tube, contained between the calming section and the top cover of the s a l t bath, was wrapped w i t h e l e c t r i c a l h e a t i n g tape and maintained near to s a l t - b a t h temperature. " S t i l l - a i r " experiments were conducted to examine the maximum e r r o r s i n temperature readings due to a x i a l conduction along the thermo couple guide tube. These were estimated t o be about 2°C. A i r was passed downwards through the bed at a known r a t e and, when steadys t a t e c o n d i t i o n s were reached, bed and s a l t - b a t h l o n g i t u d i n a l temperatures were recorded. The l a t t e r were always found to be uniform. F u r t h e r sets of readings were taken a t a number of d i f f e r e n t flow r a t e s . Some measured a x i a l temperature p r o f i l e s are d i s p l a y e d i n F i g . 2 a t v a r i o u s a i r flow r a t e s . The data are p l o t t e d as In θ v s . bed depth z, where 0=Tg-T(z), and ζ i s the a x i a l d i s t a n c e measured from the top of the r e a c t o r tube ( F i g . 1). At bed depths between 20 and 30 cms. r a d i a l temperature and v e l o c i t y p r o f i l e s become f u l l y developed and a l l the p l o t s become l i n e a r . The o v e r a l l heat t r a n s f e r c o e f f i c i e n t (U) can then be obtained simply from the slope of the l i n e s , s i n c e S

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Previous one-phase continuum heat t r a n s f e r models ( 1 ) , ( 5 ) , (10), (11), which are a l l based upon " l a r g e diameter tube" heat t r a n s f e r data, f a i l to e x t r a p o l a t e to narrow diameter tubes. These equations s y s t e m a t i c a l l y underpredict the o v e r a l l heat t r a n s f e r c o e f f i c i e n t by 40 - 50%, on average. When allowance i s made i n the one-phase model f o r the e f f e c t o f tube diameter on the apparent s o l i d con d u c t i v i t y ( k ^ ) , Eqn. ( 7 ) , the mean e r r o r i s reduced t o 18%. However, the best p r e d i c t i o n s by f a r (to w i t h i n 6.8% mean e r r o r ) are obtained from the heterogeneous model equations. r

s


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diameter o f sphere o f equal s o l i d volume/surface area tube i n s i d e diameter p a r t i c l e emissivity s u p e r f i c i a l mass v e l o c i t y f l u i d t o packing heat t r a n s f e r c o e f f i c i e n t apparent r a d i a t i o n heat t r a n s f e r c o e f f i c i e n t w a l l t o f l u i d heat t r a n s f e r c o e f f i c i e n t apparent w a l l heat t r a n s f e r c o e f f i c i e n t zero-order B e s s e l f u n c t i o n o f the r e a l k i n d molecular thermal c o n d u c t i v i t y of a i r thermal c o n d u c t i v i t y o f c a t a l y s t support e f f e c t i v e r a d i a l c o n d u c t i v i t y o f packed bed effective radial conductivity of f l u i d e f f e c t i v e radial conductivity of s o l i d within "central core"

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CHEMICAL REACTION ENGINEERING

Note: Packed bed heat t r a n s f e r parameters based upon u n i t t o t a l c r o s s - s e c t i o n a l area normal t o d i r e c t i o n o f heat t r a n s f e r (solid + void). Acknowle dgement s


Our thanks are due t o Schweizerische Aluminium AG ( A l u s u i s s e ) f o r p r o v i d i n g experimental f a c i l i t i e s and p a r t i a l f i n a n c i a l support d u r i n g t h i s p r o j e c t . We are a l s o g r a t e f u l to I.C.I. Petrochemicals and P l a s t i c s D i v i s i o n f o r p r o v i d i n g some o f the c a t a l y s t support packings. We much a p p r e c i a t e the advice and c o n t r i b u t i o n made by D. T r o j a n o v i c h d u r i n g the experimental phase o f the p r o j e c t .

Literature Cited 1. 2. 3. 4.

Specchia, V.; Baldi; Sicardi. Chem. Eng. Commun. 1980, 4, 361. Agnew, J . B . ; Potter. Trans. Inst. Chem. Eng. 1970, 48, T15. Dixon, A.G.; Cresswell. A.I.Ch.E.J. 1979, 25, 663. Dixon, A.G.; Cresswell; Paterson. A.C.S. Symposium Series 1978, No. 65, 238. 5. Kulkarni, B.D.; Doraiswamy. Cat.Rev.Sci.Eng. 1980, 22, 431. 6. Kunii, D.; Smith. A.I.Ch.E.J. 1960, 6, 71. 7. Dwivedi, P.N.; Upadhyay. I&EC Proc. Des. Dev. 1977, 16, 157. 8. Yagi, S.; Kunii. A.I.Ch.E.J. 1960, 6, 97. 9. Paterson, W.R. Ph.D. Thesis, 1975, University of Edinburgh, Scotland. 10. Bauer, R.; Schlünder. Int. Chem. Eng. 1978, 18, 181. 11. Schlünder, E.U.; Hennecke. C.I.T. 1973, 45, 277. Received April 27, 1982.


Heat Transfer in Packed Reactor Tubes Suitable for Selective Oxidation

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