Colloids and Surfaces, 50 (1990) 87-99 Elsevier Science Publishers B.V., Amsterdam

87

Effect of Dyes and Surfactants on the Dynamic Behaviour of Liquid-Liquid Gravity Settlers

AMIT PANDIT, SHAIK A.K. JEELANI, ROK GUNDE and STANLEY HARTLAND*

Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of

Technology, CH-8092 Zurich (Switzerland)

(Received 7 September 1989; accepted 16 February 1990)

ABSTRACT

The dynamic response of the steady-state dispersion height in gravity settlers due to the pulse injection of aqueous solutions of dyes and surfactants into the mixer or settler inlet is experimen- tally investigated.

Positive changes in dispersion height are observed when an anionic or nonionic surfactant solution is injected at the settler inlet. Ampholytic surfactants produce a small negative change and cationic surfactants have more complicated effects, first producing a positive change followed by a negative change, probably related to the decrease in interfacial tension with concentration combined with the variation in interfacial tension gradients within the dispersion as the surfac- tant passes through. In all cases an increase in surfactant concentration decreases the interfacial tension but does not significantly affect the pH.

When dyes are injected, positive and negative changes in dispersion height are observed de- pending on whether the pH increased or decreased; the dispersion height returning to its original value after a short time when the injected dye has passed through the system. The magnitude of the effect depends on the concentration and is lower when the dye is injected at the settler inlet rather than in the mixer. By adjusting the concentration of mixtures of dyes producing positive or negative changes it is possible for the dispersion height to be unaffected by the injection. Dyes have a pronounced effect on the pH but only a small effect on the interfacial tension. This is also true for mixtures of dyes and surfactants.

Batch experiments indicate that reducing the pH so the solution is acidic increases the rates of both binary and interfacial coalescence whereas increasing the pH, so the solution is alkaline, decreases the interfacial coalescence rate but has no effect on the binary coalescence. These ob- servations substantiate the effect of acidic and alkaline dyes on the dynamic behaviour of contin- uous dispersions.

INTRODUCTION

The dynamic behaviour of liquid-liquid gravity settlers depends both on changes in operating conditions and properties of the liquid-liquid system.

*Author to whom correspondence should be addressed.

0166-6622/90/$03.50 0 1990 - Elsevier Science Publishers B.V.

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Their response to changes in dispersion throughput was recently considered by Jeelani and Hartland (1988). The accidental contamination of the liquid inlet streams to the mixer or settler with surface active agents could result either in an increase or decrease in the dispersion height depending on the type of the contaminant. The former causes flooding while the latter results in un- der utilisation of settler capacity. A closer study of these effects is therefore necessary for efficient design and operation. In the present paper, the effect on the dynamic behaviour of the settler of the pulse injection of solutions of dyes and surfactants into a mixer-settler unit, operating initially at steady state, is experimentally investigated. The injection points were located in the mixer and at the entry to the settler. Anionic, cationic, nonionic and ampholytic surfactants were used and both acidic and alkaline dyes.

When a dispersion formed in a mixer enters a settler, sedimentation of the drops occurs to create a dense-packed zone. The sedimentation rate is en- hanced by binary coalescence and coalescence of drops with their homophase is affected by the height of the dense-packed zone. The total dispersion volume is the sum of those of the sedimentation and dense-packed zones. At steady state the dispersion volume for a given liquid-liquid system is constant at a fixed dispersion throughput. Additives which enhance the rates of binary and interfacial coalescence will reduce the total dispersion volume whereas impur- ities which reduce the coalescence rate will increase the dispersion volume. Since the volumes of the individual zones and the associated change in drop size are difficult to measure, dynamic analysis is complicated.

In a mechanically agitated mixer of a continuous mixer-settler unit oper- ating at steady state, a drop size distribution is established as a consequence of a dynamic equilibrium between the opposing mechanisms of droplet break- up and coalescence under constant conditions of agitation. The fact that only few semi-empirical correlations over a limited range of dispersed phase hold- up and physical properties exist (as also for a batch mixer without inlet and outlet streams) reflects the complexity of the problem. However, the available correlations (Gnanasundaram et al., 1979; Lee and Soong, 1985) suggest that the average drop diameter increases with interfacial tension. The injection of a pulse of known volume of additive into the mixer operating at steady state results in an initial decrease followed by slow recovery of the interfacial tension to its original value (corresponding to the pulse variation in concentration), the average drop diameter initially decreasing then increasing. As the average drop diameter in the mixer decreases, the dispersion height in the settler in- creases and then subsequently decreases to its original value before the injec- tion of the additive. If a pulse of the additive is injected at the entry to the settler, the additive must diffuse in the continuous phase of the turbulent sedi- mentation and calm dense-packed zones. The concentration of the additive is higher in the bulk solution compared to that in the films during diffusion. This results in interfacial tension gradients which create forces inhibiting the drain-

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age of the continuous phase film (between the drops, and drops and coalescing interface), hinder binary and interfacial coalescence and thereby increase the height of the dispersion. This is further complicated by the electrical effects (such as zeta potential) which are influenced by the pH of the aqueous phase.

EXPERIMENTAL

Liquid-liquid system

The dispersed phase was 50% by volume of n-heptane in paraffin oil while the continuous phase was demineralized water. The interfacial tension of the mutually saturated system was 50.5 mN m-l, and the density and viscosity of the organic phase were 779.3 kg rn-’ and 2.18 mPa s, while those of the aqueous phase were 997.7 kg rnp3 and 0.98 mPa s at 20°C. An Ubbelhode viscometer and a Paar (Type DMA 46) density meter were used to measure viscosities and densities, respectively, while interfacial tension and pH were measured by the drop volume method and by Palintest Model Microcomputer pH meter (PT 110).

The dyes investigated were indigo caramin, malachite green and methylene blue, the chemical structures are given in Fig. 1. The surfactants included an- ionic [sodium lauryl sulfate (SLS ) 1, cationic [ tetradecyltrimethylammonium bromide (TDTMAB) 1, nonionic [polyoxyethylene lauryl ether (Brij 35) ] and

Indigo caramin

H,C \+,C”, N-OOCCOOH

Malachite green

Fig. 1. Structure of dyes used in experiments.

90

6C

50

E 2 E 40

c 0 v/ 2 30

2 ; ; 20 aI

-: ;

IO

0 1 I I I I I I I I

200 400 600 800 1000 1200 1400 1600 1800 2( -pp-rI I I I

SO 100 150 200 250 31

Concclrlrntmn, ppm

n 0

0

)

Fig. 2. Effect of concentration (dyes and surfactants) on interfacial tension [upper scale refers to dyes (o-2000 ppm), lower scale to surfactants (O-300 ppm) 1.

‘I- 1

a

/ 2500 3000

Fig. 3. Effect of concentration of dyes and surfactants on pH.

ampholytic [ N,N-dimethyldodecyclamine N-oxide (LDAO ) 1. All materials were obtained from Fluka Chemical. The variations in the interfacial tension and pH as a function of the concentration of the dyes and surfactants are shown in Figs 2 and 3.

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Equipment

The single stage mechanically agitated QVF glass mixer-settler unit con- sisted of a 200 mm diameter and 300 mm tall mixer and a 200 mm diameter and 500 mm tall settler as used in earlier experiments (Jeelani and Hartland, 1988). The dispersion entered the settler through a pipe 27 mm in diameter located at 150 mm from the base. The mixer consisted of two 10 mm thick stainless steel end plates; the top end plate had four equally spaced vertical wall baffles (each 2 cm in width ), welded to it and a bottom end plate had two 15 mm diameter holes connected to continuous and dispersed phase inlet pipes. A 67 mm diameter stainless st.eel six flat bladed turbine impeller with standard geometry was driven by a 0.12 kW motor.

Procedure

The equipment cleaning, the method of mutual saturation, the setting of turbine agitator speed, the maintainance of constant temperature (20 2 0.2’ C ) and the settler start-up procedure were described in our previous work referred above. In all experiments the impeller speed was 11.8 SK’ and dispersed phase holdup 0.5.

The dispersion was allowed to grow until the steady-state height was at- tained. A pulse of 20 ml of the additive of known concentration was then in- jected at the entry to the settler and the variation in the dispersion height with time was recorded until it returned to its initial value. These experiments were repeated by using different dyes, surfactants, mixture of dyes and of dyes and surfactants. In some experiments dyes were injected directly into the mixer.

RESULTS

Injection into the mixer

Figure 4 shows the variation in dispersion height with time in the settler for the additive-free dispersion, and after injection of a pulse of 20 ml of 2000 ppm dye solution into the mixer. It can be seen that the initial steady-state height H=13.5 cm corresponding to the specific throughput q,=O.168 cm s-’ and holdup fraction cF = 0.5 of the dispersed phase is attained in about 15 min. This was maintained for a further 25 min before injecting a pulse of dye solutions. Indigo caramin, malachite green and methylene blue are represented by circles, triangles and crosses, respectively. For all three dyes the dispersion height re- covered its initial steady state value of 13.5 cm after about 35 min.

Injection at the entry to the settler

Figure 5 shows the effect of injecting a pulse of 20 ml of aqueous solutions of surfactants (anionic, cationic, nonionic and ampholytic) at the entry to the settler.

The effect of volume fraction of malachite green solution in a 20 ml aqueous

0 0 B

0 20 40 60 80

Tnne, mln

100 120

Fig. 4. Effect of different dyes (concentration 2000 ppm) injected into the mixer on the variation in dispersion height with time.

i, C

i 100 120

Fig. 5. Effect of different surfactants (concentration 500 ppm) injected at the entry to the settler on the variation in dispersion height with time.

mixture of malachite green and indigo caramin (total concentration 2000 ppm) injected at the entry to the settler is shown in Fig. 6. Figure 7 shows the effect of volume fraction of malachite green solution in a 20 ml aqueous mixture of malachite green and sodium lauryl sulfate (total concentration 100 ppm) in-

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l mmmm

m

m

100 1

Fig. 6. Effect of mixtures of malachite green and indigo caramin (total concentration 2000 ppm) at the entry to the settler on the variation in dispersion height with time.

I

mm n n n n n

;t;= +TD :=,h I . * + . + . +

. +?

I I

2b I

40 6C 51z 1 Of? 120

l‘lme. mm

Fig. 7. Effect of mixtures of malachite green and sodium lauryl sulfate (total concentration 100 ppm) injected at the entry to the settler on the variation in dispersion height with time.

jetted at the entry to the settler. These results are summarized in Table 1 which also includes the effect of different dyes (at a concentration of 2000 ppm) and of different concentrations of indigo caramin, injected at the entry to the settler.

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TABLE 1

Effect of concentration of dyes and surfactants (injected at the entry to the settler) on the amplitude and duration of the change in dispersion height

Additive Concentration Amplitude (cm) Duration (min )

Dye Indigo caramin (alkaline)

Malachite green (acidic)

Methylene blue (weakly acidic )

SWjUCtUnt

SLS (anionic)

Brij 35 (nonionic )

TDTMAB (cationic)

LDAO (ampholytic )

Mixed Indigo caramin + malachite green (total concentration 2000 ppm)

SLS +malachite green (total concentration 100 ppm )

(mm) 20

200 1000 2000 2000”

200 1000 2000 2000”

200 2000 2000”

3.0 10 7.5 12

11.5 15 15.0 20 29.0 30

-4.0 12.5 -6.5 20

-11.0 25 - 13.0 35

1.5, -2.5 135 5.5, -3.5 3, 12

13.5, -6.5 1.5, 22.5

20 9.5 10 50 16.5 12.5

100 22.5 16 200 25.5 20 500 35.5 25 50 3.0 3

100 4.5 4 500 8.0 6

1000 10.5 10 100 1.0, -2.5 1,6 500 3.0, -3.5 2,8

1000 4.0, -4.0 4,12 50 -3.5 5

100 -4.0 6 500 - 5.0 8

1000 -6.5 10

(Volume fraction) 0.000 0.020 0.040 0.055 0.062 0.200 0.500 1.000 0.00 0.50 0.80 0.87 0.89 0.94 1 .oo

15.0 20 7.5 10 2.5 7.5 0.5 4

-2.0 10 -6.0 15 -8.5 20

-11.0 25 22.5 22 14.0 20 7.5 15 4.5 7.5 1.0 4

-3.0 15 -4.0 20

“Directly injected into the mixer.

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DISCUSSION

Injection of dyes

The injection of indigo caramin solution in the mixer can be seen from Fig. 4 to give a positive pulse change in the dispersion height in the settler while malachite green solution gave a negative change. Curiously enough methylene blue solution resulted in a positive followed by a negative pulse change in dis- persion height. Aqueous indigo caramin solution is alkaline (pH range from 8.8-10.7) while malachite green and methylene blue are acidic (pH ranges from 3.4-4.3 and 4.7-6.2, respectively) as shown in Fig. 3. Although the inter- facial tension decreases with the concentration for all dyes and surfactants (Fig. 2), the pH increases for indigo caramin while it decreases for both mal- achite green and methylene blue, with concentration indicating the presence of strong electrical effects as shown in Fig. 3. The initial increase or decrease in the dispersion height in the settler for indigo caramin or malachite green in Fig. 4 can thus be attributed to the increase or decrease in the value of pH.

Table 1 shows that the effect of injection of dyes at the entry to the settler, rather than to the mixer, is to reduce the magnitude of the maximum (or min- imum) dispersion height as the time required for the dye to diffuse into the continuous phase films is higher in the former than in the latter case. Increas- ing the dye concentration always magnifies the change in the dispersion height. The amplitudes of both the positive and negative pulse changes shown in Fig. 4 for methylene blue are given.

However, it would appear that the variation in the interfacial tension has no direct effect on the dispersion height other than through the change in drop sizes irrespective of whether the dye solution is injected into the mixer or at the entry to the settler. The complicated variation in height observed for the different dyes cannot be explained in terms of the variation in the interfacial tension but would appear to be due to electrical effects.

Injection of surfactants

Figure 5 shows that the observed positive pulse change in the dispersion height for both anionic and nonionic surfactants can be attributed to the fact that not only the interfacial tension decreases with concentration (Fig. 2) but also that the concentration of the surfactant in the injected solution (500 ppm) is much more than the critical micelle concentration (0.43. lop3 and 0.28.10W3 kmol m-‘, respectively). There is negligible variation in pH with concentra- tion (Fig. 3) for all surfactants and there would be less possibility of changes in electrical effects. However, it would appear that the variation in pH has a negligible effect on the dispersion height. In the case of an ampholytic surfac- tant a negative pulse change in the dispersion height is observed while a posi-

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tive pulse change followed by a negative pulse change in the dispersion height is obtained for a cationic surfactant. The values of the maximum amplitudes are given in Table 1, which also shows that increasing the surfactant concen- tration injected at the entry to the settler always increases the magnitude of the observed change in the dispersion height as for the dyes.

Injection of mixtures

Figure 6 shows the effect of the mixtures of malachite green and indigo car- amin in the aqueous solution (of total concentration 2000 ppm) injected at the entry to the settler. Pure alkaline solutions of indigo caramin (with a pH of 10.7) give positive pulse changes in the dispersion height whereas pure acidic solutions of malachite green (with a pH of 3.4) result in negative changes. There is no change in the dispersion height when the volume fraction of mal- achite green is 0.055, the solution then being neutral (with a pH of 7.1). This corresponds to the aqueous continuous phase used in the start-up experiments before injection of the mixed dye solution. Figure 7 shows that the effect of mixtures of malachite green and sodium lauryl sulfate (of total concentration 100 ppm) in the aqueous solution injected at the entry to the settler is similar to that for the dyes shown in Fig. 6, except that the positive pulses are more predominant.

The effect of malachite green is thus more powerful than that of indigo car- amin since small amounts make the solution neutral as shown by the open symbols in Fig. 8. On the other hand, the effect of malachite green dye is much less than that of the surfactant sodium lauryl sulfate as shown in Fig. 7. When the dye concentration is less than 0.90 (volume fraction), the dispersion height is significantly affected by the surfactant even though the pH is close to 7 as shown by the solid symbols in Fig. 8.

Batch experiments

The effect of pH on the batch sedimentation and coalescence was experi- mentally investigated (Jeelani et al., 1989) in a 123 mm diameter glass baffled mixer-settler unit with standard six blade turbine stainless steel impeller (speed of agitation being 10 s-l). Equilibrated 50 vol.% n-heptane in paraffin oil dis- persed in an acidic ( HCl), an alkaline ( NaOH ) and a neutral (distilled water) aqueous solution were used, the corresponding values of pH being 3.25,10.85 and 6.95.

Figure 9 shows the experimental variation in the positions of the sediment- ing and coalescing interfaces x and y relative to the final undisturbed interface with time, t corresponding to equilibrated acidic, alkaline and neutral aqueous solutions, represented by squares, triangles and circles, respectively. The spe- cific volume rate of sedimentation -&/dt can be seen to be larger for the

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. v*

x

I / I

00 01 03 04 05 OG 07 08 09 ’

Vol fraction (Malach~le green)

Fig. 8. Effect of volume fraction of malachite green on pH (symbols correspond to those in Figs 6 and 7).

acidic solution than for the neutral solution. This indicates that the drop growth due to binary coalescence increases with acidity, giving faster sedimentation and a larger drop diameter in the dense-packed zone. However the sedimen- tation rate for the alkaline solution is identical with that for the neutral solu- tion indicating that the binary coalescence is unaffected.

The maximum value of the specific rate of interfacial coalescence -dy/dt (measured at the point of inflection) is significantly faster for the acidic case (0.05 cm s-l) and slower for the alkaline case (0.02 cm s-l) than for the neutral solution (0.04 cm s- ’ ), the corresponding values of the separation time being 110,380 and 240 s. Values of the sauter mean diameter (measured at the coalescing interface for a dispersion height of 4.0 cm) were about 3.0, 2.5 and 2.5 mm for the acidic, alkaline and neutral solutions after 60,120 and 180 s.

It would thus appear that the pH has a different effect on binary and inter- facial coalescence. An acidic solution (pH 3.25) increases the rates of both binary and interfacial coalescence whereas an alkaline solution (pH 10.85) appears to have no effect on the binary coalescence but to slow down the rate of interfacial coalescence. Acidic solutions thus increase the rate of drainage of the films between the drops and at the coalescing interface (reducing the batch separation time from 240 s for a neutral solution to 110 s), whereas alkaline solutions only decrease the drainage rate at the coalescing interface (increasing the batch separation time to 380 s). This may be explained by the fact that binary coalescence is slow in neutral solutions so can be increased by the acidity but not significantly decreased by the alkalinity.

Experiments on electrophoretic mobility using a Laser-Doppler velocity

98

64 7-

0 100 200 300 400

I, 5

Fig. 9. Effect of pH on batch sedimentation and coalescence profiles.

technique (Malvern Zetasizer 3) indicate that the zeta potential for 50% n- heptane in paraffin oil dispersed in the aqueous continuous phase is indepen- dent of the drop size between 25-105 pm. The zeta potential becomes more negative as the pH of the aqueous continuous phase increases from 2.4 to 12.0, indicating that the repulsive force acting on a drop is greater in alkaline than acidic solution corroborating the observed batch sedimentation and coales- cence decay profiles.

Identical results were obtained in the batch dispersion when the tracer was injected into the mixer just before stopping the impeller suggesting that chang- ing the pH in the mixer had no effect on the final drop size but affected the rates of binary and interfacial coalescence after agitation ceased as discussed above.

These conclusions confirm the observations made in the continuous exper- iments in which an acidic dye (malachite green) with a pH range of 3.4 to 4.3 decreased the steady-state dispersion height (due to enhanced binary and in- terfacial coalescence), whereas an alkaline dye (indigo caramin) with a pH range of 8.8 to 10.7 increased the steady-state dispersion height (due to the inhibited interfacial coalescence ) . Methylene blue which is weakly acidic (pH 4.7-6.2) had only a small (though admittedly complicated) effect on the dis- persion height.

In the continuous experiments injecting the dye into the mixer had a bigger effect on the steady-state height than when the dye was injected at the entry to the settler, since the dye was uniformly dispersed throughout the continuous phase in the mixer whereas the dye must diffuse throughout the continuous phase when the injection is at the entry to the settler.

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CONCLUSIONS

When a dye is injected into the mixer or at the entry to the settler operating at the steady state, positive and negative changes are observed in the disper- sion height. The magnitude of the change in dispersion height increases with the dye concentration, being smaller when the dye is injected at the entry to the settler. The changes are related to the pH being positive for alkaline solu- tions (e.g., indigo caramin) and negative for acidic solutions (e.g., malachite green). Certain mixtures of these dyes do not affect the dispersion height, mal- achite green being more powerful than indigo caramin in this respect, 5% by volume being sufficient.

Similar results are observed when surfactants are injected. The injection of an anionic surfactant (sodium lauryl sulfate ) gives a large positive pulse but the change in dispersion height for nonionic, cationic and ampholytic surfac- tants is small. The behavior is related to the decrease in interfacial tension, the change in pH being small. Certain mixtures of surfactants and dyes pro- ducing positive and negative pulses (for example 10% sodium lauryl sulfate and 90% malachite green by volume) have no effect on dispersion height.

Changing the pH in batch experiments shows that the electrical effects in- fluence binary and interfacial coalescence differently. Acidic solutions increase the rate of film drainage and hence the rates of both binary and interfacial coalescence. On the other hand alkaline solutions decrease the rate of film drainage and hence that of interfacial coalescence but have no significant ef- fect on binary coalescence since this is small in neutral solution.

These conclusions substantiate the observations made in the continuous ex- periments involving acidic and alkaline dyes. The effect of surfactants would appear to be due to their reduction in the interfacial tension, pH having a negligible effect.

ACKNOWLEDGEMENT

Thanks are due to Dr K. May for his cooperation and assistance in the zeta potential measurements.

REFERENCES

Gnanasundaram, S., Degaleesan, TX. and Laddha, G.S., 1979. Can. J. Chem. Eng., 57: 141. Jeelani, S.A.K. and Hartland, S., 1988. AIChE J., 34: 335.

Jeelani, S.A.K., Pandit, A. and Hartland, S., 1989. Can. J. Chem. Eng., in press.

Lee, J.M. and Soong, Y., 1985. Ind. Eng. Chem. Process Des. Dev., 24: 118.