Chemical Reaction Engineering-Houston

27 Aromatic Sulfonation in a Cyclone Reactor, a Stirred Cell, and a Cocurrent Tube Reactor; Influence of Mass Transfer on Selectivity

Downloaded by FUDAN UNIV on April 25, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch027

A N T O N I E A. C. M . B E E N A C K E R S Department of Chemical Engineering, Twente University of Technology, Enschede, P.O. Box 217, The Netherlands For mass transfer, followed by a fast chemical reaction of type

Van de Vusse [1] pointed out that selectivity with respect to I increases with an increase of the mass transfer coefficient (k ). In light of this observation, we have developed a new reactor of cyclonic type in which, due to strong centripetal forces on the gas bubbles, a very high k is realized [2]. This paper deals with the selectivities obtained in sulfonation of benzene with sulfur trioxide. Both neat benzene and benzene diluted with 1,2-dichloroethane were used. This re action was selected as a model reaction for industrially important aromatic sulfation (e.g. deter gents). We studied the reaction in three reactor types that greatly differ in mass transfer charac teristics, i.e. in a stirred cell reactor (low k ), a co-current gas-liquid tube reactor (intermediate k ) and in the cyclone reactor(highk ). L

L

L

L

L

Reaction Kinetics; Regime of Mass Transfer with Chemical Reaction We have discussed reaction mechanism and kinetics of sulfonation of benzene (B) with SO (A) in aprotic media [3] and have concluded that the reaction proceeds according to Van de Vusse kinetics (1-3), with k (25°C) >9.4 m /kmol s and z = / . Pyrosuifonic acid (I) and Ar S O H (I') are both unstable and react with benzene to give the desired product benzenesulfonic acid (P) and the unwanted product diphenyl sulfone (X), respectively 3

3

1

x

2

3

9

Reaction (4) is slow with respect to mass transfer and thus of negligible influence on absorption rate. Reaction (5) consumes only minor amounts of benzene (all experimentally observed selec tivities are (often much) above 70%based on benzene). Therefore this reaction also does not influence absorption rate appreciably. For reaction sequence (1-3), the relation between mass transfer parameters and conversion rate is — in general — complex. However, as long as observed selectivity η is high, the influence of reaction (3) on SO absorption rate is an effect that may be neglected in the first, rough estimation of the regime of absorption with reaction [4] which characterizes the system. Which regime occurs, mainly depends on the numerical values of the dimensionless groups Ha, E^and mk E/k . Due to uncertainties in the kinetic rate constant, in local liquid viscosity in the interface diffusion zone (to be discussed later), and in S0 -solubility, a prediction of the regime characterizing sulfonation of benzene, solved in 1,2-dichloroethane, is not free of speculation. In case of no liquid viscosity increase at the interface during reaction, Table I gives numerical estimations of the relevant parameters for atmospheric sulfonation at 3

L

Q

3

©

0-8412-0401-2/78/47-065-327$05.00/0

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

CHEMICAL REACTION ENGINEERING—HOUSTON

328

2 0 ° C with a mixture of S 0 % benzene

k

by

Ha

G

1 + zD m(c )

volume

A

A

mk

G

L

taining 10 mol % S 0

3

3

and nitrogen con

(typical for our experi

ments in cyclone and tube reactor) in a conven tional bubble c o l u m n . In our stirred cell sulfona

> 4

5.6

0.7

30

> 2.2

2.4

0.7

tion experiments, k

10

> 1.3

1.4

0.7

10 lower than the value 1.2 10"

5

> 0.9

1.2

0.7

calculating Table I. Therefore Ha »

100

/zD m(c ) A

T a b l e I.

Estimated mass transfer parameters for

sulfonation of

benzene, solved in

1,2-dichloroet

hane with gaseous sulfur trioxide (10 m o l

A

Q

L

was found to be a factor of 4

[m/s] used in 1 +

D c B

B

in the stirred cell. This means that

the reaction is instantaneous with respect to mass transfer in that reactor.

% in ni

r

trogen) at 2 0 C and 1 0 Pa in a conventional bub0.7 10

ble c o n t a c t o r with μ

s Pa and k i t h e m i -

Influence of Mass Transfer on Selectivity


n i m u m value of 9.4 m ^ / k m o l s. F o r h y d r o d y n a m i cal

and physical constants applied, see reference

A s first pointed out by V a n de Vusse [1 ], the ob

No

[3].

served selectivity η (1-3)

plies Ha > 0.5 and not much smaller than

for fast reactions of

type

is influenced by the value of k . 'Fast' im L

[5]. Some experimental results are available for

chlorination [1,6-9]. A numerical analysis [10], an analog simulation [1 ] and trial and error pro cedures [6,8,10] by which approximate solutions can be obtained, have been presented. A s in our sulfonation experiments, there is often much doubt about the exact values of the relevant parameters (c, m, ρ, μ , D, T) at the interface, mainly because local interface conditions differ from bulk conditions. Because of this difficulty, explicit rough approximate relations for 1? are sophisticated enough to discuss experimental results and are therefore very useful. Harriott [5] derived such a simple model for the intermediate regime between fast and instantaneous re action. Our experiments are mainly in the instantaneous regime. Based on film theory, we deriv ed for this regime [3]

k D 2

(1

[D c /D, +

A

B

c,]

(6) 2z

for (1

B

-η')=k

L

2 E

~

2

- τ ? ' ) « 1 .

Equation (6) shows the manner in which the selectivity is favoured in the instantaneous regime by a high value of k , provided that the selectivity is not much smaller than one. T h e latter con L

dition is always fulfilled in our experiments.

Experimental The stirred cell reactor was of the Danckwerts type [4, page 180]. The reactor was filled with de gassed (diluted) benzene and kept under its own vapour pressure. T h e experiment was then start ed by connecting the space above the liquid to a thermostrated ( 3 0 ° C ) container, filled with de gassed stabilized liquid sulfurtrioxide, which was also under its own vapour pressure. Due to the difference in partial pressure of reaction mixture and liquid S 0 , the latter evaporated and flow 3

ed via a flow controller and a rotameter to the cell reactor where it absorbed into the liquid. Figure 1 is a sketch of the cyclone reactor. The liquid is fed tangentially into it (A). A gas mixture of S 0 The

3

and N

2

is introduced into the reactor via a porous section of the cylindrical wall.

liquid phase is the continuous phase in the reactor, except near the cyclone-axis. Here, a

gaseous core is f o u n d , due to a strong centripetal field, generated by the rotating liquid. This field causes gas bubbles to spiral from the wall to the cyclone-axis. Gas leaves the reactor via the upper outlet which is known as the vortex. Liquid leaves the reactor via the bottom outlet which


27.

329

Aromatic Sulfonation and Mass Transfer

BEENACKERS

is referred t o as the apex. Cone Ε prevents gas entrainment with the liquid. Liquid entrainment through the vortex varied between 12 and 2 0 % depending o n gas and liquid velocities. L i q u i d conversion per pass through the reactor was small. Therefore the system was operated batch wise with respect to the liquid, b y recycling reaction mixture over the reactor. Absorption efficiency of S 0

was ^ 100%.

3

3

The diameter o f the cocurrent gas-liquid tube reactor was 8 1 C T m . Gas and liquid were in troduced via a T-piece of the same diameter.

Results and Discussion Mass Transfer in Absorption without Reaction. We measured k 0

2

- H

0 system (figure 2). Forced convection k

2

L

in the stirred cell with an

L

in the reaction mixtures was calculated from


this result according t o [11] n

Sh~

Re Sc

l / 3

(7)

with η = 1.1. Reaction effects were neglected in this procedure. However, in reaction experi ments, as will be explained in a later section, a local increase of viscosity at the interface caused by pyrosulfonic acid accumulation, produced a lower k We measured k

L

L

than calculated this way.

in a cyclone reactor with simultaneous absorption of C 0

droxide solution [2]. Figure 3 gives results. T h e figure shows that k

L

2

and 0

2

in a hy

reaches extremely high

4

values in this reactor ( k is o n the order of 10" m/s in conventional reactors as the stirred tank L

[13], the bubble column [14], the bubble cap plate [15] and the packed column [ 1 6 , 1 7 ] . Slugflow is obtained in the tube reactor in the range of gas and liquid velocities, we applied in sulfonation [18, Figure 10.3]. k

L

values realized in this flow regime have been reported b y

Gregory and Scott [19]. F r o m this reference we calculated [3] k

L

in our tube reactor (see Figure

3). Mass Transfer in Stirred Cell Reactor during Sulfonation. T h e actual k

L

during sulfonation

follows experimentally from

k

(8)

L=C

Ai

E

As derived earlier, sulfonation of benzene is instantaneous in a stirred cell reactor. Therefore [4]: 2D c B

c

E

A i

a

s

° A i

+

B

- S —

Because o f uncertainties in c

(

A

j

, only an approximate k

L

9

)

during sulfonation can be obtained this

way. Experiments were carried out with both neat and diluted benzene (5.3 and 3 0 vol% benzene initial

Τ

reaction

J

(1 -n)

[ ° C ] [10 " k m o l / m ^ 3

mixture

k

L

c

Ii

-0.5

=1

= 4.1

= 1.5

= 3.75

0 187

= 2.2

0 25

= 1.5

0.15

0 031

=2

30 vol%

25

0.092

0 107

35

0.13

0 137

45

0.23

28

0.35

T

i

t°c]

= 3.6

35

T a b l e II.

x

3

_5

5.3 vol%

100% Β

Ii"5I

C10 m/s] [kmol/m J

2

k 2DA 5 2 [m /kmol s ]

51

3.0 Ι Ο "

= 0.7

44

0.67 Ι Ο "

= 0 .6

53

1.9 1 0 " 1 1

= 4.25

= 0.8

67

4.2 1 0 " 1 1

= 8.75

= 1

78

14 1 0 " 1 1

1 1

1 1

E s t i m a t e d values f o r gas-liquid interface p y r o s u l f o n i c acid c o n c e n t r a t i o n rise ( C j j - C j ) , s u r f a c e tempera

ture rise (Tj-T) a n d reaction rate constant ^ D / ^ ) '

ns t

i

r r e

d

c

e

" reactor sulfonation experiments.


CHEMICAL REACTION

330

ENGINEERING—HOUSTON


26

33

Figure 1. Cyclone reactor. Unit of length: J 0 " m. ( A) liquid inlet (4 J O " m), (B) gas outlet (vortex) (3 JO" m), (C) liquid outlet (apex) (8.66 J O " m ), (D) gas inlet, (E) cone (120°C), (PI) pressure indicator, (a)8°C. 3

3

3

6

2

Figure 2. k in stirred cell reactor. (O), ( ) our measurements (O in H 0at25°C);( ) Jhaveri and Sharma [12]; (- · -) SOs in both 1,2dichloroethane and benzene, with Equation 7. L

s

2


27.

Aromatic Sulfonation and Mass Transfer

B E E NA C K E R S

in 1,2-dichloroethane

331

respectively). Stirrer speed (N) varied between 0 and 2 r e v / s . k ^ showed

to be independent of Ν and t o be appreciably lower than without reaction. T h e average values of k

L

are summarized in Table II. Table II also shows that both the interface pyrosulfonic acid

concentration ( c ) and the interface temperature M

(T.) are much higher than in the bulk of the

liquid. T h e first quantity (c .) has been approximated with (

1

c .-c = / r?J/k |

j

2

T h e second quantity

(10)

L

(T.) has been estimated from the simple film model according to Danck-

werts [4]

Τ. —Τ = J D


ι

R

Β

(ΔΗ

a

+ ΔΗ ) / r

ΧΛ.

(11)

S L

T h e influence o f pyrosulfonic acid concentration on viscosity is neither known nor measurable because o f the

instability

o f this intermediate

[3]. We measured the viscosity o f a reaction

mixture at 2 3 . 5 ° C as a function of benzenesulfonic acid concentration. T h e relationship obtained is:

Ιη(μ(χ )/μ(ο))=8.85χ ρ

Without

(12)

ρ

measurements, our best possible assumption is that pyrosylfonic acid has the same

influence on viscosity as sulfonic acid. Recalling from eqn. (7) that

k ~(D/M)

2 / 3

L

and applying the Stokes-Einstein equation results in

k

It

L

~M"

4

/

3

(13)

follows from x

( j

(Table

II) and eqns (12,13) that k

L

is appreciably lowered by viscosity

effects that occur during reaction. In practice this tendency is counteracted by both the inter face temperature

rise and free convection, driven by density and/or surface tension gradients.

Both effects lower the extent of interface viscosity increase. T h u s , a k

L

is obtained which is

independent of stirrer speed and lower than that for forced convection in the absence of inter face viscosity effects as given in Figure 2. Selectivity in Stirred Cell Reactor.

Observed 1 -

η is always «

1. Therefore eqn. (6)

is expected t o be applicable though its accuracy is probably low due to the discussed interface viscosity increase. F r o m eqn. (6)

1-T?~1/k

2

(14)

L

Figure 4 shows (1— η)

t o be nearly independent o f N . Even the abcense o f stirring does not

lower selectivity significantly. This fact is in agreement with, and additional argument for, our preceding conclusion that k

L

is independent of stirrer speed.

Figure 4 shows that by-product formation Taking as a first approximation D

Q

= D , c, « (

increases with initial benzene concentration. δ

β

and 1 - η = 1 - i? (allowed for f «

1)

we obtain from eqn. (6) with ζ = % :

1-T?^k D 2

A

c

B

/(k EJ

2

L


(15)

CHEMICAL REACTION ENGINEERING—HOUSTON


332

Figure 3. k as f(U ) for C0 in 2.07M NaOH solution in a cyclone reactor with Vi = 5.97 m/s (1) and 9.15 m/s (2) and in tube reactor for U = I m/s (3) and 1.75 m/s (4) L

s

2

L

30-

1-η[·/.] *

t

; 20-

υ-, 0

, 066

,

,

,

1

133

2

•

N[s-1]

Figure 4. By-product formation in sulfonation of benzene with gaseous sulfur trioxide in a stirred-cell reactor in rehtion to initial benzene concentration and stirrer speed. (A) 5.3 vol % benzene in dichloroethane, T ss 35°C, ζ = 0.8; (Ο) 30 vol % benzene in dichloroethane, T ss 25°C, ζ 0.09; (Ο) 30 vol % benzene in dichlo roethane, Τ ss 35°C, ζ Ξ* 0.1; (A) 100 vol % benzene in dichloroethane, Τ as 28°C, ζ ss 0.005.


27.

333

Aromatic Sulfomtion and Mass Transfer

BEENACKERS

Combining with equation (8) gives k D = * ( 1 - T?) J / ( c c 2

2

A

B

2 A j

)

(16)

ι/Vith this equation, the value of k D 2

II). Figure 5 shows log k D 2

k D 2

A

=k

0

0

D

A

e-

A

E

/

R

T

A

has been estimated from the experimental results (Table

as a function of 1/T.. Fitting the experimental data to

A

i

(17)

esults in


k

and

oo

D

5

=4.5 m /kmol s

A

2

6

Δ Ε = 3 0 . 7 10 J/kmol

The obtained value for Δ Ε is likely for this consecutive reaction because Bosscher [20] found the approximate same value for Δ Ε in a chemically, very similar reaction in sulfonation of chloroberizene. Assuming at the interface: D 2

l 0

A

2

s 1 0 " m / s , it follows from eqn. (17) that k

2

(25°C) =

3

1.7 10" m / k m o l s. Reaction rate constant for the first reaction (eqn. (2)) has been shown to be 3]: 3

k ( 2 5 ° C ) > 9 . 4 m /kmols 1

Hence, in homogeneous sulfonation, no diphenyl sulfone will be obtained. Selectivity in the Cyclone and in the Tube Reactor. Differential selectivity (τ?') was measued as a function o f f . A b o u t 25 experimental runs were carried o u t . Table III shows the range between τ

Initial

vol %

f

A

ς

benzene i n liquid

40

ved η

on

[m/s]

20

the operation

depended mainly o n ini

tial benzene concentration and

phase

[°cl

which

parameters were varied. Obser

10

0 02 - 0.04

3.5 - 6.8 0 09 - 0 1 1 0 1 3 - 0 46

30

0 01 - 0.38

3.3 - 7.9 0 06 - 0 13

0 07 - 0 42

100

0 01 - 0.21

2.8 - 6.6 0 08 - 0 13

0 01 - 0 05

30

0 01 - 0.08

2.7 - 7.9 0 09 - 0 12

0 04 - 0 51

100

0 01 - 0.02

2.4 - 7.6 0 11 - 0 12

0 01 - 0 05

are

s

summarized

Figure

6

in Figure 6.

also

shows

results

from tube reactor experiments (1 < U

Chemical Reaction Engineering-Houston

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