Chemical Reaction EngineeringâBoston - American Chemical Society

35 Exothermic Gas Absorption with Complex Reaction: Sulfonation and Discoloration in the Absorption of Sulfur Trioxide in Dodecylbenzene R. M A N N , P. KNYSH, and J. C . A L L A N

Downloaded by YORK UNIV on November 23, 2012 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch035

University of Manchester Institute of Science and Technology, Department of Chemical Engineering, Manchester, M60 1QD England Experimental measurements of absorption fluxes and colour development for the gas-liquid reaction between sulphur trioxide and dodecylbenzene have been carried out i n a s t i r r e d c e l l absorber. A model with two parallel reaction paths representing sulphonation and discolouration has been applied to analyse the exothermic absorption accompanying conversions up to 70%. The results show that the two reactions have similar activation energies and that temperature increases greater than 100°C occur at the interface during absorption. The absorption enhancement factor exhibits a maximum value as l i q u i d phase conversion proceeds. G a s - l i q u i d r e a c t o r s present a number o f i n t e r e s t i n g problems i n r e a c t o r a n a l y s i s and design which a r i s e from the c o u p l i n g o f mass t r a n s f e r and chemical r e a c t i o n processes. Thus, the d i f f i c u l t y o f r e s o l v i n g the r e l a t i v e c o n t r i b u t i o n s o f filmwise and bulkwise r e a c t i o n remains unsolved f o r a l l but the s i m p l e s t k i n e t i c s . Such d i f f i c u l t i e s are compounded when thermal e f f e c t s and s i g n i f i c a n t heat r e l e a s e accompany the a b s o r p t i o n and r e a c t i o n . W h i l s t e a r l y work i n d i c a t e d that f o r carbon d i o x i d e a b s o r p t i o n heat r e l e a s e c o u l d not be expected t o be s i g n i f i c a n t O ) , Van de Vusse had a l r e a d y remarked i n 1966 (2) that the a b s o r p t i o n o f c h l o r i n e i n t o a hydrocarbon c o u l d produce flames a t the i n t e r f a c e . Around that time the heat e f f e c t s accompanying ammonia a b s o r p t i o n were estimated t o g i v e i n c r e a s e s o f around 15°C across the mass t r a n s f e r f i l m (3)> and subsequently f u r t h e r treatments q u a n t i f i e d temperatures up t o 40°C (4,5,6). Systems i n v o l v i n g the a b s o r p t i o n o f S0~ i n t o organic l i q u i d s i n v o l v e s u r f a c e temperatures up t o 100°C higher than the bulk (7) and Beenackers has experimentally observed s u r f a c e b o i l i n g f o r a b s o r p t i o n o f SO^ i n benzene ( 8 ) . The i n d u s t r i a l sulphonation o f high b o i l i n g l i q u i d s l i k e l i n e a r a l k y l benzenes can i n theory give r i s e t o very h i g h temperatures s i n c e evaporative c o o l i n g does not occur before thermal degradation temperatures a r e reached. 0097-6156/82/0196-0441$06.00/0 © 1982 American Chemical Society In Chemical Reaction Engineering—Boston; Wei, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


442

CHEMICAL REACTION ENGINEERING

D i s c o l o u r a t i o n o f l i q u i d product i n the manufacture o f detergent sulphonates from liquids like dodecylbenzene is a problem a s s o c i a t e d w i t h the s e v e r i t y o f sulphonation c o n d i t i o n s . High p r o d u c t i v i t y o f r e a c t o r s tends to be l i m i t e d by the formation o f c o l o u r i n g agents which may moreover be malodourous. A lack o f r i g o u r i n i n t e r p r e t i n g mass t r a n s f e r effects i n gas-liquid sulphonation w i t h SO- has r e s u l t e d i n an ad-hoc v a r i e t y o f r e a c t o r types ranging from sparged r e a c t o r s (9)· through f a l l i n g film r e a c t o r s (1£) w i t h scraped s u r f a c e r e a c t o r s (JO and spray r e a c t o r s i n between. A l i q u i d - l i q u i d r e a c t o r u s i n g SO- d i s s o l v e d i n l i q u i d SOg has even been proposed (^2) t o by-pass problems o f g a s - l i q u i d systems. Precise quantitative design o f e f f i c i e n t direct sulphonation r e a c t o r s which minimise d i s c o l o u r a t i o n and permit high p r o d u c t i v i t y a t h i g h SO- c o n c e n t r a t i o n s , r e q u i r e s knowledge o f the b a s i c r e a c t i o n k i n e t i c s * and the heat and mass t r a n s f e r processes o c c u r r i n g a t the i n t e r f a c e . The i n t e r a c t i o n s o f heat and mass t r a n s f e r d i s p l a y c o m p l e x i t i e s analogous t o those observed i n nonisothermal c a t a l y s t p e l l e t s and m u l t i p l i c i t i e s a r e p r e d i c t e d t o occur a c r o s s the t r a n s f e r f i l m s (13)* Laminar J e t Experiments A f u l l s e t o f dodecylbenzene laminar j e t experiments a t c a r e f u l l y c o n t r o l l e d SO^ c o n c e n t r a t i o n s from 3$ t o 30% have been undertaken. The theory o f a b s o r p t i o n with pseudo f i r s t order r e a c t i o n , i n c o r p o r a t i n g the i n f l u e n c e o f i n t e r f a c e temperature i n c r e a s e on s o l u b i l i t y d r i v i n g f o r c e r e d u c t i o n , and u s i n g the s i m p l i f i c a t i o n t h a t the heat t r a n s f e r f i l m i s much t h i c k e r than the mass t r a n s f e r f i l m as shown i n F i g . 1, has been used t o produce the estimates o f i n t e r f a c i a l temperature shown i n F i g . 2. At the highest 30$ SO- composition, the i n t e r f a c e temperature a t the base o f the 14mm diameter j e t i s estimated t o be 114 C above the datum o f 25°C. The k i n e t i c parameters f or £he r a t e o f r e a c t i o n o f SO- have been estimated to be A = 1.24 X 10 and Ε = 24,700 c a l / m o l e " ( 14). For these k i n e t i c parameters the h a l f - l i f e o f SO- d u r i n g a b s o r p t i o n v a r i e s g r e a t l y with j e t l e n g t h as shown i n F i g . ? f o r 30$ SO-. The value o f the Hatta number a t the base o f the laminar j e t i n c r e a s e s from 1.78 f o r 3$ SO- t o 479 f o r 30$ SO-, p l a c i n g the r e a c t i o n i n the f a s t regime. ^ ^ 9

1:5

S t i r r e d C e l l Experiments A g a s - l i q u i d s t i r r e d c e l l r e a c t o r with a w e l l d e f i n e d plane i n t e r f a c e permits a study o f the heat and mass t r a n s f e r e f f e c t s throughout the e n t i r e range o f dodecylbenzene c o n v e r s i o n up t o 100$. Experiments have been c a r r i e d out u s i n g a 900ml charge o f l i q u i d dodecylbenzene (Dobane JN a l k y l a t e ) with continuous feed o f SO- d i l u t e d with n i t r o g e n . Samples o f the bulk l i q u i d phase were p e r i o d i c a l l y withdrawn and analysed f o r sulphonic a c i d and f o r degree o f d i s c o l o u r a t i o n by measuring the absorbance at a wavelength o f 420 nm. The conversion-time behaviour f o r v a r i a t i o n i n gas phase composition a t a f i x e d o v e r a l l gas flowrate and s t i r r e r speed i s

In Chemical Reaction Engineering—Boston; Wei, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

MANN ET AL.

Exothermic Gas Absorption


35.


443

444



0.1

0.2 gas phase mol

Figure 2.

0.3 f r a c t i o n o f SO

Interface temperature rise at the jet surface.


MANN ET A L .



35.

Figure 3. Reaction half-life of SO* at jet surface.


445


446


shown i n F i g . 4 ( a ) . I t i s c l e a r that gas phase composition a f f e c t s the absorption r a t e . F i g . 4(b) shows the e f f e c t o f v a r y i n g the s t i r r e r speed from 100 t o 500 rpm and i n t h i s case no s i g n i f i c a n t e f f e c t can be detected. On the other hand, F i g . 4(c) i n d i c a t e s t h a t the gas flowrate a t a f i x e d composition i n f l u e n c e s the f l u x and hence the conversion achieved i n a given time. Whilst t h i s might be taken t o i n d i c a t e gas-phase c o n t r o l l e d mass t r a n s f e r , care i s necessary i n drawing such a conclusion because heat release a f f e c t s s o l u b i l i t y (hence d r i v i n g f o r c e ) i n a complicated way and a l s o changes chemical enhancement f o r a f i n i t e a c t i v a t i o n energy. At any r a t e , i t i s necessary t o a c c u r a t e l y assess interfacial temperatures i n these experiments because o f the p o s s i b i l i t y that these could a f f e c t the d i s c o l o u r a t i o n r e a c t i o n s . F i g . 5 shows how the d i s c o l o u r a t i o n developed i n these semibatch experiments i s c o r r e l a t e d against conversion. At low conversion l e v e l s up to 50$, the r e a c t i o n and mass t r a n s f e r c o n d i t i o n s do not a f f e c t the extent o f d i s c o l o u r a t i o n achieved. Beyond 50$, there i s some evidence that under severe conditions ( i e . 30$ SO-) the degree o f d i s c o l o u r a t i o n i s a c c e l e r a t i n g . However f o r the purposes o f i n i t i a l assessment, the by-product colour can be represented by a p a r a l l e l r e a c t i o n where the sulphonation and discolouration reactions have similar activation energies. Brostrom's c o l o u r r e s u l t s a r e d i f f e r e n t , and shown i n F i g . 5 f o r comparison (15). Exothermic Absorption with Two P a r a l l e l Reactions The

r e a c t i o n scheme i s t h e r e f o r e product sulphonic

acid

d i s c o l o u r i n g by-product Within the mass transfer film dodecylbenzene i s described by 2 2 D

d

°A

dx

2

A

=

-D

d p

C

B

dx

=

the

reaction

(k^T) + k (T))C 2

A

of

C

SO-

B

with

(1)

2

2

06j"§

=

(ΔΗ

subject t o the boundary

c

A

= Cc .(T.)

Κ 1

^(Τ)

T»

. χ = 0

J

ÛH

conditions

A

Cg*

+

and

R 2

k (T))C 2

c

A

= c

C

B

=

Τ

C

A b

B b

A

C

B

(2)

l

X

J-^

=

(3)

Τ. χ = x„ 'b H The i n t e r f a c i a l and bulk boundary c o n d i t i o n s are assumed quasi-stationery with respect t o the timewise increase i n conversion o f Β (dodecylbenzene) and the v a r i a t i o n o f unreacted



Figure 4a. Influence of gas phase composition in a stirred cell reactor.


4^ ^4

1'

î

ξ

i

I

>

W H

Χ Χ

y*



ο


Figure 4c. Influence of gas massflowrate in a stirred cell reactor.


vo

î

3· /M

1

tanhjM

1

F i n a l l y , F i g . 8 i n d i c a t e s the s e n s i t i v i t y w i t h r e s p e c t t o v a r i a t i o n i n a c t i v a t i o n energy f o r the by-product r e a c t i o n . I f the two p a r a l l e l r e a c t i o n s d i d have d i f f e r e n c e s o f the order o f + 10 k c a l mol" , then v a r i a t i o n o f gas phase composition (at f i x e d Ν and G) as per F i g . 4 ( a ) , would give a wide spread o f absorbance behaviour, caused by d i f f e r e n c e s i n i n t e r f a c i a l temperature.


M A N N ET A L .

200 *·

Y

A b

453


- 0.098

170


140

110

À

80 20

Ô

40

time Figure 6a.

minutes

Interface temperature predictions as y b varies, when N = 400 rpm. A

100 rpm

120 time Figure 6b.

minutes

Interface temperature predictions as N varies, when yAI, = 0.049.


In Chemical Reaction Engineering—Boston; Wei, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982. Ah

F/gwre 7. Enhancement factor behavior through a semi-batch. Conditions: y , 0.049; N, 400 rpm; and G, 2.324 mol/s.


ta

I

ta

H

s

In Chemical Reaction Engineering—Boston; Wei, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982. 9

E = 35 kcal/mol; and —

, Et =

t

E.

Figure 8. Discoloration: sensitivity to activation energy. Key:

t

,E


= 15 kcal/mol; · · ·,

Ln

ϊ

I

?

Β*

s-

1

?

>

H

w

I

to

CHEMICAL REACTION

456

ENGINEERING

Legend o f Symbols A^

pre-exponential factor of i t h reaction pre-exponent


s t i r r e d c e l l s p e c i f i c surface

area

Cj

c o n c e n t r a t i o n o f component I

Dj

d i f f u s i v i t y o f component I

Ε

a b s o r p t i o n enhancement f a c t o r

E^^

a c t i v a t i o n energy o f i t h r e a c t i o n

G

gas molar flow rate

k

r e a c t i o n r a t e constant

k

l i q u i d phase mass t r a n s f e r c o e f f i c i e n t

x

k

gas phase mass t r a n s f e r c o e f f i c i e n t

M

Hatta number a t i n t e r f a c e temperature

Ν

s t i r r e r speed

Τ

temperature

A sulphur t r i o x i d e

χ

position i n transfer film

Β dodecylbenzene

XJJ

t h i c k n e s s o f mass t r a n s f e r f i l m

b bulk value

^

t h i c k n e s s o f mass t r a n s f e r f i l m

Superscript

Ot

thermal d i f f u s i v i t y

Subscripts

* i n t e r f a c e value

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Danckwerts P . V . , Appl. S c i . Res. 1953, A3, 383 Van de Vusse, J.G., Chem. Engng. Sci 1966, 21, 631 Chiang, S.H. and Toor, H . L . , A . I . C h . E . J l . 1964, 10, 398 Clegg, G.T. and Mann, R., Chem. Engng. S c i . 1969, 24, 321 Mann, R., and Clegg, G . T . , Chem. Engng. S c i . 1975, 30, 97 Ikemizu, Κ., Int. Chem. Eng. 1979, 19(4), 611 Mann, R., and Moyes, H . , A . I . C h . E . J l . 1977, 23, 17 Beenackers, A.C.M. and Swaaji, W.P.N. Chem. Eng. Jl. 1978, 15, 25 S i l v i s , S . J . and Ballestra M . J . J. Am. O i l Chem. Soc. 1963, 40, 618 Knaggs, E.A. and Nussbaum, M.S. Soap Chem. Spec. 1962, 38, 145 Mutzenberg, A.B. and Giger, A. Trans. I . Chem. Engrs., 1968, 46, T187 Lohr, J.W., J. Am. O i l Chem. Soc. 1958, 35, 532 Allan, J.C., and Mann, R., Submitted to Can. Jl. Chem. Eng. Allan, J.C., M.Sc. Thesis University of Manchester, 1978 Brostrom, Α . , Trans. I . Chem. Engrs. 1975, 53, 26 Van Krevelen, D.W. and Hoftijzer, P . J . Rec. Trav. Chim. 1948, 67, 563 Wilke, C.R. and Chang, P. A . I . C h . E . J l . 1955, 1, 264 Shoji, H. and Majima, K. J. Am. O i l Chem. Soc. 1963, 40, 179

Received April 27,

1982.


Chemical Reaction EngineeringâBoston - American Chemical Society

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