OXIDATION OF SODIUM SULPHIDE IN A FOAM BED CONTACTOR

ASHOK BHASKARWAR and R. KUMAR* Department of chemical Engineering, Indian Institute of Science, Bangalore-560012, India

(Received 4 July 1983; accepted 14 December 1983)

Abstract-Oxidation of sodium sulphide to sodium thiosulphate has been experimentally investigated in a foam bed contactor using air as oxidizing medium. The variables studied are supertick air velocity, initial aulphide. wnwntration, height of the foam bed and the nature of the surfactant. The results indicate that the conversion increases with the increase in the superficial air velocity and initial sulphide concentration. The nature of the surfactant has considerable influence on the conversion, even when their surface resistance is negligible. When sodium dodecyl sulphate is used as a surfactant, the conversion decreases as the foam height increases. Opposite trend has heen observed when octyl phenoxy polyethoxyethanol is used. The apparently contradictory results are satisfactorily explained on the basis of a simplified single stage model of a foam bed cootactor.

INTRODUCTION S8 + 12 OH- + 4S-2 + 2S,0;2 + 6H,O. (4) Oxidation of sodium sulphide is an industrially im- portant reaction. It is encountered in the manufacture of sodium thiosulphate from sulphide and in coal gas chemical recovery process[l]. It is also used in Kraft pulp mills[2] where it is employed as a partial solu- tion to the problem of emission of malodourous compounds.

The reversion reaction (4) results in an incomplete oxidation of sodium sulphide. However, it is possible to avoid the formation of elemental sulphur almost completely by operating at temperatures above 70%

f31.

The mechanism of oxidation of sodium sulphide to tbiosulphate is available in literature. At tem- peratures above 7O”C, the sulphide can be converted directly to thiosulphate. Sharma et a1.[3,4] have reported the second order rate constant at 75°C for tbis reaction.

Recent work on foam bed contactors shows them to offer high gas-liquid interfacial area per unit volume of the liquid and large contact times apart from having moderate pressure drops[5]. Such con- tactors could be usefully employed for contacting relatively large quantities of gases with liquids.

The present investigation concerns itself with the oxidation of sodium sulphide with air at 75°C using a foam bed contactor and predicting the performance of the contacting device.

The oxidation of sodium sulphide is a second order reaction. It is first order each with respect to sodium sulphidc and oxygen. As the rate constant of this reaction is reported to depend upon the presence of metallic impurities, experiments were done to deter- mine its value for the sulphide solution used in this work, following the procedure used by Chan- drasekaran and Sharma[3]. The rate constant was found to be 4340 cm’/gmole . set and was used while making calculations through the model. This value, as can be seen, is close to the reported value of 4460 cm’/gmole . sec[4].

EXPERIMENTAL Experimental set-up

MECHANISM AND KINETICS The mechanism of oxidation of sodium sulphide

involves the following reaction steps [3,6].

16S2+70;+ 14H,O+ZS,*+28OH- (1)

The polysulphide ions can then yield either sulphur or thiosulphate.

The experimental set-up used in the present in- vestigation is shown schematically in Fig. 1. It con- sists of air supply and foam column sections. Clean air at constant pressure is metered and then saturated with water at 75°C through heating and passing through a heated bubble column. The saturated air at 75°C is then fed into the flow-stabilization chamber from where it is sparged in the foam column section. The details of tbe column and the distributor plate are given in Table 1 and those of the foam column in Fig. 2.

2S~2+02+2H~0-2S~+40H- (2)

2 S,* + 9 0, + 12 OH- -8 SrO,* + 6 H,O (3)

*Author to whom correspondence should be addressed.

Materials used Sodium dodecyl sulphate and octyl phenoxy poly-

ethoxyethanol (Briton X-100) were used as surfac- tants. Analar grade sodium sulphide in aqueous solution with excess sodium hydroxide was used as the reactant solute. Purity of the sulphide is im-

1393

1394 A. B~A~KARW~ and R. KVMAR

Table 1. portant because the oxidation reaction is known to be

Distribufor plate sensitive to the presence of metallic impurities[3].

Material of construction: Stainless steel Zinc sulphate heptahydrate, iodine, sodium thio-

Thickness of plate: 1.5875 mm sulphate pentahydrate and starch were used in the No. of holes: 15, with a triangular pitch chemical analysis of the samples. All the reagents Diameter of a hole: 0. IS cm used were of analytical grade. Free area: 0.3375%

l”

COlUWWl Material of construction: Glass Diameter of the column (i.d.): 1Ocm Height of the column: 120cm

Experimental procedure At the start of an experimental run, saturated air

at 75°C was passed through the column and the system was allowed to reach a steady state. When the

1. Air supply 2. Surge tank 3. CO2 Scrubber 4. Scrubber with glass WOOk

5. Rotameter 6. Preheater 7. Bubble column with heater 8. Flaw &abiL~satibn chamber 9. Reactor section

@l

Fig. 1. Experimental set-up.

l.Air Wet valve 2. liquid outlet valve

3. Flow slabilisation chamber

4. Distributor date

5. S.S.Sleeuo

6. Inlet port

7 Sampling port 6. Manometer tapping

9. Asbestos insulation

T, , T2,13 - Thermometer sockets

10. Glass column

Fig. 2. Details of foam CO~UIIUI.

temperature of air had been steady at 75°C for about half an hour, the solution of sodium sulphide (250 ml) at the desired concentration and already heated to 75°C was poured into the column through the inlet just above the distributor plate. A known quantity of surfactant was present in this solution. The solution foamed immediately. In all experiments carried out in this work, the solution of sulpbide was kept strongly alkaline by using excess sodium hydroxide to ensure that no hydrolysis would occur.

To maintain the height of foam at a desired level, a sieve-plate was suspended from the top of the column at the required height. As the foam attained this height, it collapsed and the released gas escaped through the holes in the plate, whereas the liquid drained down to the storage through the Plateu borders. The progress of the reaction was determined by withdrawing samples of the reaction mixture in the storage pool at suitable time intervals. At the end of the experimental run, the entire quantity of the reaction mixture was withdrawn.

Oxidation of sodium sulphide in a foam bed contactor

The samples were analysed for thiosulphate using the standard iodimetric method of chemical analysis given by Vogel[n.

Liquid holdup was measured experimentally using a manometer connected at a height of 10 cm from the distributor plate.

The superficial velocity of air was varied from

:__-___-_____-_

Foam section

1.7 cm/set to 5.1 cm/set. Under these conditions sta- ble foam was obtained and the bed worked in the semibatch mode. At higher air flowrates, the bed exhibited recirculation which was not investigated, during the present study. The conversion character-

Draining liquid 012 %l

istics of the foam bed contactor were studied for two initial concentrations of the sulphide solution, viz. 20.16 g/l and 11.74 g/l. These fall within the range of concentrations of interest in the oxidation of black liquor from pulp mills.

The effect of nature of surfactant and its influence on the conversion characteristics of foam bed was studied using two surfactants, viz. sodium dodecyl sulphate and octyl phonoxy polyethoxyethanol.

THEORY

A model to predict the conversions in a foam bed contactor has recently been reported by Biswas and Kumar[S]. Their model divides the contactor in two sections, viz. the storage section and the foam section. Liquid from the storage section is carried into the foam section, where it reacts with the gas and the partially converted solution continuously drains back to the storage through a network of Plateau borders. The extent of reaction in the foam is calculated by assuming a foam IYm to be surrounded by a limited quantity of gas.

To account for the variation of liquid holdup with height, the foam section is further divided into a number of sub-sections, each having a different but constant holdup. The number of sub-sections are to be decided by judgement; and as the number of sub-sections increases, the calculations become in- creasingly involved. Further, Desai and Kumar[B] have found that for low viscosity liquids like water containing surfactants the liquid holdup decreases very rapidly near the foam-liquid pool interface (within 2-3 cm). and then remains virtually constant. In view of this, the possibility of visualizing the entire foam bed as a single section with a uniform liquid holdup equal to the average value was explored in this work.

The idealization of the system is shown in Fig. 3. The material balance for component B becomes:

-2 =tQ,+Q>CB-QC,-Q,G,. (1)

To solve eqn (l), C,, has to be expressed in terms of C,. This is done by considering the reaction to occur in a f&n surrounded by limited amount of gas. The gaseous reactant A diffuses into the liquid film and reacts with the solute B. The pertinent differential equation for the present second order, irreversible

1395

1 Gas out

Fig. 3. One-stage model of B foam bed.

reaction is:

It was decided to verify whether the present reac- tion could be expressed as a pseudo-first order one, during the time r: for which the film is in contact with the gas. The condition for pseudo-first order kinetics[9]

is well satisfied for the present case and hence the reaction under study has been treated as a pseudo- first order one.

Equation (2) thus becomes:

where I = k, * C,. Assuming the gas phase to be well-mixed, the

boundary conditions for eqn (3) are as follows: At

t,=o,--aIxIa,C”=O (B.C.])

At

v, ac: ac, t,>o,x= f&---=+D,.s.- 12 at, ,3X

or

(B.C.2)

where

The solution of (3) with (B.C.l) and (B.C.2) is [lo],

1396

at i, = ry, and

M=M, (4)

where pn’s are the non-zero roots of

1 - P. -=k,tank,a and k,f=-‘+. (5) D.4 1

The concentration of unreacted reactant solute, Cs,, at the end of the foam section with a contact time r* is E

2Y,M c,, =c,----.

v,

From eqns (1) and (6) we have

dC, -pP,C,=P,

where

PI=: and PI=; 1 Equation (7) cannot be solved as it stands, because

the term P2 depends on M which in turn is a function of C,, through 1. Unfortunately, M cannot be ex- pressed in terms of C, in a straightforward fashion as C, appears through eqn (5), the roots of which have to be used in the solution. However, this problem can be overcome by dividing the total time of reaction into a number (N) of short time intervals and integrating eqn (7) over the successive time intervals. During a short interval Af, M does not change appreciably, and can be treated as a constant and eqn (7) can be integrated using the value of C, obtained from the previous interval as the initial value. For the next interval M is obtained using the value of C, at the end of this interval. The procedure is repeated for the next short time interval and so on till C, is found at the desired time f. The time interval in the present case was chosen as one minute which corresponds to approximately P/0 of variation in A. Solving the eqn (7) with constant M in the ith time interval (between nodes I; and t;,,) with the condition that at

t = ti, c, = c,*,,

we obtain

p2 p2 p f c, = c,, eP,o,-r) + _ _ _ e I( i - 0

PI PI (8)

where

i = 0, 1,2,3, . N.

Expansion of e Pl(r,~‘) in the series form and further simplification gives,

P1zP2(ti - I)’ - 3! . (9)

Equation (9) gives the concentration of liquid phase reactant B at any instant t (between the nodes fi and ti + ,) in a continuous foam bed contactor. For a semi-batch operation,

Q = 0, p, = 0, pr = - ?!z$. 1

Using these in eqn (9), we obtain

c,= C&, -qg(t -t,). (10) 1

Equation (10) is applicable for short time intervals (r, to t,,,) and M has to be modified after every such interval.

The calculation of other parameters is based on the bubble radius (rb) and the average liquid holdup (Z). The bubble radius is estimated using the simplified model of bubble formation[ll]. The value of Z is determined experimentally. The expressions for the other parameters involved are[5]:

C.QC Q,=- 1-F

V = V.-F 1 6(1--E)

(11)

n . Q a = 7.537. (1 - c) tan 54”

I= Vb 10.05 . k . rb2 tan 54”

I c

= H - xr,2(1 - E)

Q,

(13)

(14)

(15)

The concentration of sodium sulphide in the storage section (Cs) at any instant and the percentage con- version based on the initial concentration of sulphide is computed from eqn (lo), used in the way explanted in the text.

RESULTS AND DISCUSSION

Efict of air pow rate on conversion Figure 4 shows the effect of the superficial air

velocity on the conversion of sodium sulphide in the

Oxidation of sodium sulpbide in a foam bed contactor 1397

Experimental 0 A 0

c 0.013 0.033 0.06

uc (cmlsec) 1.7 3.4 5.1

H (cm) 34.0 34-o 34.0

C&(gm/lit) 20.16 20.16 20.16

Thewetical - - -

I I I I I I

0 25 50 TIME (min.)

Fig. 4. Effect of superS&l velocity of air on the conversion of sodium sulphide in the storage.

foam bed contactor. The height of the bed was maintained constant at 34cm. It is Seen that the conversion attained in a given time is higher for a higher supcriicial air velocity for the same foam height and the same initial concentration of sulphide. One should expect a mild increase due to the overall higher oxygen concentration in the gas or a mild decrease because of the higher bubble size and

thereby lower interfacial area. However, in the present case the effect is quite significant and cannot be explained on the basis of the above rationale alone. The higher effect is really the manifestation of the effect of the average liquid holdup in the bed. It has been observed that the average liquid holdup, for the same height of the foam bed, is higher for a higher superficial air velocity. This is also reported and explained earIier[l]. The diffusion and reaction of oxygen in the libn of a limited thickness gives rise to the development of a concentration profile of the unreacted dissolved oxygen in the film of a foam bubble. The concentration of the unreacted oxygen which is initially zero at the centre of the tilm builds up with time as oxygen goes on accumulating in the tilm while the foam bubble rises through the bed. This results in the lower concentration gradients on both sides of the film. For a higher superficial air velocity, the liquid lilm is thicker and the effective concen- tration gradient on each side of the film is higher. This results in high mass fluxes into the film, which in turn implies higher conversion. It is evident from Fig. 4 that the model takes this factor into account and predicts this behaviour fairly well. Figure 5 shows the simulated conversions through the model for three superficial air velocities while maintaining a constant E. It is clear that the effect is very small, which further reinforces the tinding that the real change in behaviour is due to the variation of Z only.

Efecr of initial sulphide concentration Figure 6 shows the effect of initial concentration on

the conversion of sulphide. The points are experi mental and the lines represent the theoretical ones corresponding to eqn (10). The height of foam bed and the superficial air velocity were kept constant. It is evident from Fig. 6 that the rate of conversion is lower for the lower initial concentration of sulphide.

w- Theorctyal prediction

uc (cm/set) 1.7 A 5:

H(cm) 31.0 31.0 31.0

Cg,(gmllit) 11.m 11.739 11.739

B 0.023 0.023 0.023

5 oo-

z LJ

5 Y

E xl-

Fig. 5.

OV I I I

20 40 60 TIME (min 1

Simulated effect of superlicial air velocity on conversion at constant L

1398 A. BHASKARWAR and R. K~XAR

Experimental * *

CD0 (gmllit) 20.16 11.74

UC lcmL%=cc) 3.4 3 -4

H (cm) 34.0 34.0

o-033 0.025

Theoretical - -

0’ I I I

25 50 ?5 TIME (min.)

Fig. 6. Effect of initial concentration of sodium sulphide on conversion.

Experimental 0 A 0

H (cm) 34.0 45.0 62.0

Cgo [gmflit) 20.16 20.16 2u.16

uc (cmisec) 3.4 3.‘ 3.4

E 0.04 0.016 0.007 Theoretical ---

i

TIME (mln)

Fig. 7. Effect of height of foam bed on conversion (surfactant-sodium dodecyl sulphate.).

Experimental A 0 0

H km) 34.0 45.0 62.0

Cg,(gmllit) 11.74 Il.74 11.7‘

uc (cmlsrc) 3.4 3.4 3.4

P 0.025 0.023 0.019 TheOretIcal - - -

15 30 45 TIME (min)

h

Fig. 8. Effect of height of foam bed on com%?r~ion (surfactant-octyl phenoxy phenethoxyethanol).

This is in agreement with the order of the reaction with respect to sodium sulphide. The model also closely predicts this effect.

E@ct of foam height and nature of the surfactanr Figures 7 and 8 represent the effect of two surfac-

tants on the conversion of sulphide. Results for two different surfactants, each at three bed heights of 34 cm, 45 cm and 62 cm are shown. The superficial air velocity was kept the same at 3.4cm/sec.

Figure 7, corresponding to the use of sodium dodecyl sulphate as the foaming agent, shows that the conversion attained in a given time decreases as the height of the bed is increased. Figure 8, for octyl phenoxy polyethoxyethanol, on the other hand, indi- cates just the opposite trend. The conversion here increases as the height is increased.

These trends are also explained in terms of the average liquid holdup in the bed. For sodium dodecyl sulphate, the average holdup decreased sharply as the height was increased. The conversion decreases actu- ally because of the considerably lower E for a higher height, in spite of the larger contact time. In the other case, the increase in height of bed resulted in only a slight decrease in the average liquid holdup. Here the slight effect of Z on conversion is suppressed by the larger contact time corresponding to a higher height. Thus, the conversion increases as the height of foam bed is increased. The model also predicts these trends.

It is interesting to observe that though Biswas and Kumar[S] did not find any effect of the variation in liquid holdup on the conversion, the present system (k, lower by a factor of more than 2000) is found to be significantly sensitive to variation of Z.

Oxidation of sodium sulphide in a foam bed contactor 1399

The surfactants used do not seem to o&r any surface resistance but influence the conversion by changing the surface viscosity and thereby the liquid holdup.

In all the cases discussed above the one-step model for a foam bed contactor predicts the conversions fairly well and the average deviation in prediction of the experimental values is found to be of the order of 10%.

It would appear from the present investigation that a single step model can be used with advantage for systems involving liquids of low viscosity.

NOTATION

thickness of a liquid film, cm concentration of reactant A in liquid film,

g mole/cm’ concentration of reactant A at the liquid

film-gas interface, g mole/cm3 concentration of reactant A in the gas bub-

ble, g mole/cm’ concentration of liquid phase reactant B in

the storage section at time r, gmole/cm3 inlet concentration of liquid phase reactant

B, g mole/cm3 concentration of liquid phase reactant E in

the liquid draining into the storage section from the foam section, gmole/cm’

diffusion coefficient of reactant A in liquid phase, cm’/sec

height of foam section, cm equilibrium distribution factor second order reaction rate constant,

cm3/(g mole . set) defined in eqn (5) equals V,/12ks, cm total amount of reactant A, both free to

diffuse and immobilized. in half the liquid film of surface area s at contact time rz g mole.

total amount of reactant A, both free to diffuse and immobilized, in half the liquid film of surface area s after infinite time, g mole.

number of intervals in the total time of reaction

integer number 1,2, 3, _ . equals Q/v defined in text defined in eqn (5)

flow rate of liquid entering the foam bed reactor, cm’/sec

flow rate of the liquid draining into storage section from the foam section, cn?/sec

average radius of a bubble, cm average radius of the contactor column, cm surface area of a liquid tilm, cm2 time of operation of the foam bed contactor,

set time-nodes corresponding to the ith interval,

SeC

contact time between the liquid film and the gas bubble in the foam section, sec.

total contact time between the liquid film and the gas bubble in the foam section, set

ith time interval after which I from (i - 1)th interval is corrected, set

storage section liquid volume, cm’ volume of a liquid film in foam section, cm3 volume of the bubble, cm3 distance from the centre of a liquid film

towards the gas-liquid interface, cm stoichiometric factor, i.e. moles of liquid

phase reactant B consumed per mole of gaseous reactant A

average value of the liquid holdup in the foam bed

pseudo-first order kinetic rate constant for the reaction, set-’

REFERENCES [l] Kirk-Othmer, Encyclopedia of Chemical Technology,

2nd Edn, Vol. 20, p. 236. Interscience, New York 1969. [2] Kirk-Othmer, Encyclopedia of Chemical Technology,

3rd Fdn, Vol. 16, p. 770. Wiley-Interscience, New York 1981.

[3] Chandrasekaran K. and Sharma M. M., C/u-m. Engng Sci. 1974 29 2130.

[4] Chandrasekaran K. and Sharma M. M., Chem. Engng Sci. 1976 31 861.

[5] Biswas J. and Kumar R., Chem. Engng Sci. 1981 36 1547.

[6] Murray F. E., Pulp Paper Mug. Can. 1968 69 3. [7] Vogel A, I., A Texr Book of Quantitative Inorganic

Analvsis. 3rd Edn, D. 352. ELBS and Longman, Loncion 1962. .

[S] Desai D. and Kumar R., Chem. Engng Sci. (In Press). [9] Danckwerts P. V., Gas-Liquid Reactions, p. 89.

McGraw-Hill, New York 1970. [IO] Crank J., The Mathematics of Difliiom Clsrendon

Press, Oxford 1956. [ll] Kumar R. and Kuloor N. R., Advances in Chemical

Engineering, Vol. 8, p. 286. Academic Press, New York 1970.