Heat Transfer in Packed Reactor Tubes Suitable for Selective Oxidation

steel tubes of 21 mm and 28 mm I.D. and 4 metres length, which had been constructed as part of an ..... Salt circulation rates were such that the outs...
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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

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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.

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

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WELLAUER E T AL.

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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

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Legend o f Symbols Cp dp dp d

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

542

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

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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.

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