A Model for Rate of Nitration of Toluene Under Heterogeneous

12 A Model for Rate of Nitration of Toluene Under Heterogeneous Conditions J.

GILES

ICI Limited, Organics Division, Huddersfield, England C. H A N S O N and Η. A. M. I S M A I L

Downloaded by CORNELL UNIV on June 4, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0022.ch012

Schools of Chemical Engineering, University of Bradford, England

Liquid phase aromatic mononitration under normal industrial conditions is an example of mass transfer with simultaneous chemical reaction. The problem of determining the magnitude and nature of the resistance to interphase transfer has been avoided in much research on nitration kinetics by the simple expedient of working in a solvent with which a l l reactants are miscible. Early work on the rate of the two liquid phase heterogeneous process has been reviewed by Albright and Hanson(1-4). They pointed out that, contrary to the assumption commonly made previously, all mass transfer resistance has not necessarily been eliminated i f the reaction rate becomes constant at high levels of agitation. Subsequently, Hanson and co-workers(5,6) studied the heterogeneous mononitration of both toluene and chlorobenzene in a miniature CFSTR (45ml capacity) using nitrating acid typical of that employed in industry (15 mole %HNO ,30 mole % H SO , 55 mole % H O). The results were shown to be inconsistent with a simple kinetic model and i t was concluded that diffusional resistances play a more important part in determining nitration 3

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r a t e s i n the two l i q u i d phase r e g i o n than had h i t h e r t o been supposed. T h i s i s i n agreement with the extensive r e s u l t s , a l s o f o r s m a l l s c a l e r e a c t o r s , p u b l i s h e d d u r i n g the l a s t few years by Strachan and co-workers(7-11), who have demonstrated t h a t the o v e r a l l r a t e can be c o r r e l a t e d by a simple k i n e t i c model a t low s u l p h u r i c a c i d concentrations but t h a t d i f f u s i o n a l r e s i s t a n c e s become important a t higher s u l p h u r i c a c i d c o n c e n t r a t i o n s . T h i s i s c o n s i s t e n t with the e f f e c t o f s u l p h u r i c a c i d on the k i n e t i c s of homogeneous n i t r a t i o n r e a c t i o n s . The s u l p h u r i c a c i d i s seen as an i o n i z i n g medium f o r n i t r i c a c i d to nitronium ions (assumed the t r u e n i t r a t i n g agent) and so the a v a i l a b i l i t y o f the l a t t e r increases with s u l p h u r i c a c i d c o n c e n t r a t i o n . Any attempt to i n t e r p r e t observed heterogeneous n i t r a t i o n r a t e s i n terms of mass t r a n s f e r with simultaneous chemical r e a c t i o n demands a knowledge of the i n t e r f a c i a l area a v a i l a b l e . The CFSTR i n which the o r i g i n a l work was performed was too small

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Albright and Hanson; Industrial and Laboratory Nitrations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


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to allow f o r i n s e r t i o n of photographic probes and so i t was decided t o b u i l d a l a r g e r r e a c t o r which would a l s o allow the e f f e c t of s c a l e to be i n v e s t i g a t e d . A one stage CFSTR was chosen i n preference to a l t e r n a t i v e forms of r e a c t o r i n the hope of g a i n i n g r e s u l t s which would be comparable with those from t y p i c a l i n d u s t r i a l n i t r a t o r s . A photographic method was adopted f o r measurement o f drop s i z e and hence i n t e r f a c i a l area. I t was chosen i n preference to a l t e r n a t i v e s such as l i g h t transmittance because o f the l a r g e hold-up o f d i s p e r s e d organic phase n e c e s s i t a t e d by the stoichiometry o f the r e a c t i o n . I t had the a d d i t i o n a l advantages o f being independent of the p h y s i c a l p r o p e r t i e s o f the system (which change with degree o f conversion) not i n f l u e n c e d by c o l o r (colored by-products are formed), and o f g i v i n g the drop s i z e d i s t r i b u t i o n r a t h e r than j u s t a mean v a l u e . The r e s u l t s o f the drop s i z e s t u d i e s have been reported elsewhere (12). P r o v i s i o n was made f o r measurement of power i n p u t i n a d d i t i o n to a g i t a t i o n speed to give more i n f o r m a t i o n a p p l i c a b l e to scale-up. Experimental Reactor. The r e a c t o r , which was 2.5 l i t r e i n c a p a c i t y , was designed as c l o s e l y as p o s s i b l e to the Standard Tank Configuration(13) and was constructed i n EN58J s t a i n l e s s s t e e l . I t i s shown i n F i g u r e 1. C o o l i n g was provided by means of a c o i l supported i n the tank b a f f l e s . Two s e t s o f probes were f i t t e d near the tank w a l l : one s e t opposite the i m p e l l e r and the other midway between the i m p e l l e r and the top of the r e a c t o r . A microscope camera was f i t t e d onto one probe of each p a i r , while a high speed f l a s h was i n s e r t e d i n the o t h e r . The camera probe was b u i l t from a microscope and i n c o r p o r a t e d a g r a t i c u l e . After c a l i b r a t i o n , the l a t t e r was used to measure drop s i z e s . By measuring a t l e a s t 300 drops, a maximum e r r o r of 3% was i n v o l v e d i n determination o f the Sauter mean drop diameter. The a g i t a t o r (a s i x blade f l a t - b l a d e turbine) was d r i v e n by a 1 HP, 800rpm d i r e c t c u r r e n t e l e c t r i c motor, the speed of which was c o n t r o l l e d by a t h y r i s t o r d r i v e u n i t . The power i n p u t was measured mechanically. The motor was supported v e r t i c a l l y by two bearings attached to the d r i v e s h a f t i n such a way t h a t the motor c a s i n g was f r e e to r o t a t e . To prevent r o t a t i o n of the c a s i n g , an arm was attached which acted on a load c e l l at a known d i s t a n c e from the s h a f t . The torque was r e g i s t e r e d d i r e c t l y on a meter. A b e a r i n g was provided i n the base of the r e a c t o r . There was a l i q u i d s e a l i n the l i d through which the e f f l u e n t was discharged, so a v o i d i n g any p o s s i b l e v o r t e x . The r e a c t o r was f i t t e d with an iron-constantan thermocouple housed i n a s t a i n l e s s s t e e l sheath. A s m a l l amount o f o i l was maintained i n the l a t t e r to ensure good thermal c o n t a c t . The thermocouple was wired t o a temperature compensated recorder and


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provided measurement t o * 0.25°C. The temperature o f the r e a c t o r contents was c o n t r o l l e d by v a r y i n g the flow of water to the c o o l i n g c o i l . The r e a c t a n t s were s t o r e d i n g l a s s a s p i r a t o r s and were f e d to the r e a c t o r by means o f metering pumps. The c a p a c i t i e s o f the l a t t e r gave a range o f r e s i d e n c e times i n the r e a c t o r from 6 to 60 minutes. The flow r a t e s were checked before and a f t e r each run. F u r t h e r d e t a i l s of the equipment are a v a i l a b l e elsewhere(14). M a t e r i a l s . Because o f the l a r g e q u a n t i t i e s r e q u i r e d , the reagents had to be purchased i n commercial q u a n t i t i e s . The b e s t commercially a v a i l a b l e grades were employed. The toluene was i n d u s t r i a l n i t r a t i o n grade. GLC a n a l y s i s showed i t to c o n t a i n 0.21 wt.% p a r a f f i n s but no d e t e c t a b l e benzene or x y l e n e s . The acids analysed as f o l l o w s : Density Sulphuric A c i d N i t r i c Acid

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(gm/cc a t 30 C) 1.827 1.362

Normality 35.10 12.88

N i t r a t i n g a c i d was prepared by mixing the c o n s t i t u e n t s i n a s t a i n l e s s s t e e l bucket submerged i n a water bath, the temperature being maintained below 20°C to reduce the p o s s i b i l i t y of forming nitrosylsulphuric acid. Procedure. The r e a c t o r was i n i t i a l l y f i l l e d with water. The a g i t a t o r was switched on a t the r e q u i r e d speed. The reactants were then introduced a t the a p p r o p r i a t e r a t e u s i n g the metering pumps. The c o o l i n g water flow was adjusted to keep the temperature constant. When steady s t a t e was reached (usually a f t e r about f i v e r e s i d e n c e t i m e s ) , the power input was noted, the d i s p e r s i o n photographed and samples o f the product taken. The l a t t e r were drawn through a sampling tube connected t o the main e f f l u e n t pipe near the e x i t from the r e a c t o r . The tube was purged t o remove any m a t e r i a l from the previous run. Three separate samples were taken d u r i n g each run: (i) the organic phase washed with water, ( i i ) the a c i d i c organic phase, and ( i i i ) the aqueous phase. The former was obtained by running the sample i n t o water and then s e p a r a t i n g the two phases. The organic phase was given a second water wash. Samples o f the unwashed phases were obtained by running some of the r e a c t i o n products i n t o a s e p a r a t i n g f u n n e l and a l l o w i n g the phases t o separate. T h i s took p l a c e q u i t e r a p i d l y so the extent o f r e a c t i o n d u r i n g the sampling would be s m a l l . Samples of the separated phases were removed q u i c k l y from areas w e l l away from the i n t e r f a c e . Depending on the degree of conversion, e i t h e r o r both the product phases were found t o be c o l o r e d , i n d i c a t i n g the presence of by-products. The c o l o r disappears i f the product i s


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c o l l e c t e d i n water and so i t has not been n o t i c e d by many previous workers s i n c e i t i s normal t o quench the r e a c t i o n by running the sample i n t o an excess o f water. However, the c o l o r a t i o n i s commonly observed i n i n d u s t r y and causes an e f f l u e n t problem when the organic product i s washed to e f f e c t i t s removal. Because of the obvious complexity o f the process and the i n d u s t r i a l importance o f the by-product formation, a f a i r l y comprehensive a n a l y s i s of the products was undertaken i n the hope o f throwing f u r t h e r l i g h t on the problem.


Analysis The organic product phase was analysed f o r toluene, the various mononitrotoluene isomers, n i t r i c , n i t r o u s and s u l p h u r i c a c i d s . The aqueous phase was anlysed f o r n i t r i c and n i t r o u s a c i d s , t o t a l a c i d i t y and mononitrotoluene. The presence o f s u l p h u r i c a c i d i n the organic phase was checked by the standard barium c h l o r i d e method but none could be detected. Since the t o t a l a c i d i t y o f the organic phase could be accounted f o r by the n i t r i c and n i t r o u s acids found, the s u l p h u r i c content was assumed n e g l i g i b l e . The organic phase was a l s o analysed f o r water by the C a r l F i s c h e r technique but none was found. This was confirmed by i n f r a - r e d a n a l y s i s o f the organic phase, where no water absorption peaks were found. The toluene and mononitrotoluene isomer concentrations i n the organic phase were obtained by GLC a n a l y s i s u s i n g a Pye 104 Gas-Liquid Chromatograph f i t t e d with a Kent Chromalog i n t e g r a t o r . A 1 2 f t column packed with Apiezon L on Chromasorb Ρ o f 60-80 mesh was employed and gave good s e p a r a t i o n working at a temperature o f 163°C. N-tetradecane was used as i n t e r n a l standard. Regular checks on performance were made u s i n g a sample of known composition. A n a l y t i c a l accuracy was w i t h i n + 1%. N i t r i c a c i d i n the aqueous phase was determined by the method used by Barduhn and Kobe(15), i . e . t i t r a t i o n a g a i n s t standard ferrous s u l p h a t e . N i t r o u s a c i d was analysed u s i n g formation o f a diazonium s a l t between m - n i t r o a n i l i n e and n i t r o u s a c i d . Standard m - n i t r o a n i l i n e s o l u t i o n was t i t r a t e d against a known volume of the sample a c i d i f i e d with d i l u t e h y d r o c h l o r i c a c i d u n t i l t e s t i n g with s t a r c h i o d i d e paper demonstrated the absence of f r e e n i t r o u s a c i d . The t o t a l a c i d i t y of the aqueous phase was obtained by t i t r a t i o n a g a i n s t standard sodium hydroxide. In the case o f the organic phase, the n i t r o u s a c i d content was found by f i r s t e x t r a c t i n g i n t o an excess of water and then u s i n g the above method on the aqueous e x t r a c t . To o b t a i n the t o t a l a c i d i t y of the organic phase, a known volume was washed with water and the mixture made a l k a l i n e . A f t e r shaking f o r f i v e minutes to ensure e x t r a c t i o n o f a l l the a c i d from the



12.

GILES E T A L .

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Figure 2. Distribution of nitric acid between aqueous and organic phases at 30°C

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9 0 0 Figure 3.

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Effect of agitator speed on fractional conversion


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organic phase, the excess a l k a l i i n the aqueous l a y e r was determined by t i t r a t i o n with standard h y d r o c h l o r i c a c i d . The t o t a l a c i d i t y was assumed due to n i t r i c and n i t r o u s a c i d s o n l y . Reproducible r e s u l t s ( i . e . + 2%) c o u l d only be obtained when the o r g a n i c phase samples were s t o r e d a t low temperature. At ambient temperature the a c i d i t y was observed to decrease s l o w l y . The mononitrotoluene c o n c e n t r a t i o n i n the a c i d phase was determined by u l t r a - v i o l e t spectroscopy. With a l l the above analyses, i t was found p o s s i b l e to o b t a i n good o v e r a l l mass balances (within +_ 4%) .


Results and

Discussion

Phase Compositions. I t was found t h a t c o n s i d e r a b l e q u a n t i t i e s o f n i t r o u s a c i d are formed d u r i n g the n i t r a t i o n r e a c t i o n and i t was p o s s i b l e to c o r r e l a t e t h i s with p r o d u c t i o n of by-products. T h i s q u e s t i o n i s d i s c u s s e d i n d e t a i l elsewhere(14). The comprehensive a n a l y s i s undertaken showed t h a t c o n s i d e r a b l e q u a n t i t i e s o f n i t r i c a c i d are e x t r a c t e d i n t o the organic phase and are thus t e m p o r a r i l y " l o s t " to the r e a c t i o n . At h i g h conversions, the c o n c e n t r a t i o n i n the organic phase was comparable with t h a t i n the aqueous. As might be expected, the amount e x t r a c t e d was found t o i n c r e a s e with aqueous phase acidity. The r e s u l t s are i l l u s t r a t e d i n F i g u r e 2. E f f e c t o f A g i t a t i o n . The e f f e c t o f a g i t a t i o n was s t u d i e d f o r an a c i d mixture with a composition t y p i c a l o f t h a t used f o r i n d u s t r i a l mononitrations (15 mole % HNO3, 30 mole % H2SO4) and an a c i d to toluene volumetric feed r a t i o of 1.5:1. The maximum p o s s i b l e f r a c t i o n a l conversion under these c o n d i t i o n s i s 0.775. With a residence time o f 10 minutes and a temperature of 30°C, the f r a c t i o n a l conversion v a r i e d with a g i t a t i o n , as shown i n F i g u r e 3. The p l o t a g a i n s t power consumption shown i n F i g u r e 4 demonstrates a sharp d i v i s i o n i n t o two r e g i o n s . That i n which conversion r i s e s r a p i d l y with a g i t a t i o n has o f t e n been assumed t o i n v o l v e s u b s t a n t i a l mass t r a n s f e r r e s i s t a n c e s , i n c r e a s e i n a g i t a t i o n reducing these by i n c r e a s i n g the i n t e r f a c i a l area. However, i t i s a l s o p o s s i b l e t h a t the hold-up o f d i s p e r s e d phase changes with a g i t a t i o n i n t h i s r e g i o n . Since r e a c t i o n takes p l a c e predominantly i n the aqueous phase, any change i n the phase r a t i o w i t h i n the r e a c t o r f o r a given o v e r a l l flow r a t e w i l l cause a change i n the space time o f the aqueous phase and hence i n the c o n v e r s i o n . This was found, as i l l u s t r a t e d i n F i g u r e 5. I t w i l l be seen from t h i s t h a t the j u n c t i o n between the two regions i n F i g u r e s 3 and 4 c o i n c i d e s with attainment o f a steady hold-up i n F i g u r e 5. In the second r e g i o n , conversion i s r e l a t i v e l y i n s e n s i t i v e t o a g i t a t i o n and i t has o f t e n been claimed t h a t t h i s i n d i c a t e s the e l i m i n a t i o n of mass t r a n s f e r r e s i s t a n c e s . However, i t w i l l be seen t h a t conversion does continue t o i n c r e a s e , even though


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A Model for Rate of Nitration of Toluene Under Heterogeneous

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