30
Ind. Eng. Chem. Process Des. Dev. 1905, 24, 38-4 1
Foam Fractionation with Synergism Shih Nan Hsu and Jer Ru Maa' Chemical Engineering Department, Cheng Kung University, Tainan, Taiwan, R. 0. C.
The application of the synergistic effect of mixed surfactant solutions to the improvement of the separation efficiency of foam fractionation is experimentally verified in this work. The surfactants used are anionic sodium dodecyl
benzenesulfonate (NaDBS) and cationic dodecyl trimethylammonium chloride (DTMACI). Foam fractionation experiments of cationic/anionic binary solution show that at low concentration the surface excess or adsorption of these solutes on the gas-liquid interface is unusually high, and the enrichment ratio is consequently much improved. This improvement diminishes with the increase of concentration, and the opposite effect may appear when the concentration is sufficiently high. Foam fractionation of a surfactant of poor foaming property can be carried out at low concentration with a good enrichment ratio by the addition of another surfactant of opposite ionic charge. In this work, enriching of dilute DTMACI was made possible by the addition of NaDBS.
Introduction The surface activity of mixed surfactant solutions is often greater than would be expected in the absence of any mutual influence between surfactants; for example, see Lucassen-Reynders et al. (1981), Goralczyk (1980), and Shpenzer et al. (1981). Such synergistic effects are largest in cationic/anionic surfactant mixtures and are of obvious importance for various operations involving the interface between phases. The fundamental mechanism of foam fractionation depends on the adsorption of surface-active solutes at the gas-solute interface; improvement of its separation efficiency is expected if synergistic effect occurs. This work is an experimental study of foam fractionation of cationic/anionic binary surfactant solution for the purpose of verifying this anticipated effect. Under equilibrium conditions, the adsorption of solute at the gas-liquid interface, or the surface excess, can be expressed by the Gibbs equation (Lemlich, 1968). When the solution is sufficientyl dilute, it can be simplified to
The operation of a foam fractionation column can be made batchwise as shown in Figure l(a). It can also be continuous in the simple, stripping, enriching, or combined mode as shown in Figures l(b), l(c), l(d), or l(e), respectively. The relationships between the concentrations of various streams in these figures can be obtained by simple computation of material balances. For example, the surface excess or adsorption at the bubble surface for the case of batchwise operation is
where d is the average bubble diameter and d / 6 is the volume to surface ratio of a spherical bubble. The concentration of the bottom product of a stripping column is (4) 6.59 is used instead of 6 in this equation to account for the fact that in foams of low liquid content, bubbles of uniform
size may be approximated as regular dodecahedra instead 0196-4305/85/1124-0038$01.50/0
of spheres (Lemlich, 1968) and CF is the feed concentration.
Experimental Section A schematic diagram of the experimental system is shown in Figure 2. Pure nitrogen from a cylinder is first saturated with moisture by bubbling through distilled water in two flasks. Its flow rate is controlled by a needle valve and measured by a rotameter before entering the foam column through a sintered glass distributor made of 100-Fm glass powder. The foam column is a Pyrex glass tube of 5.0 cm inside diameter and 125 cm tall. At every 15 cm interval of this column, there is a pair of side tubes of 8 mm outside diameter. These side tubes can serve as liquid inlets or outlets whenever desired. The foam breaker is a rotating basket made of 80-mesh stainless steel screen housed in a transparent tank. The reflux ratio of the liquid from the foam breaker is controlled by a valve and the speed of pumping. The feed rate is controlled by a metering pump, and the liquid level in the column is controlled by adjusting the height of the outlet of a tube leading from the lower end of the column. Bubble diameters are determined from photographs of the foam. The experimental system, with minimum changes, could be used for various modes of operation. During each experimental run, attention must be paid to the stability of operation and the necessary time of producing a reasonable amount of product for the determination of its composition. A nitrogen flow rate of 0.3 cm/s was found to be suitable and was used in most of the experimental runs. Before samples and data are taken, the system is allowed to run for 30 min after the flow rates of feed, products, and nitrogen gas and the liquid level in the column become stabilized. The surfactants used are sodium dodecyl benzenesulfonate (NaDBS) of CP grade supplied by Hayashi Pure Chemical Ind. Ltd. of Osaka, Japan, and dodecyl trimethylammonium chloride (DTMAC1) of CP grade supplied by the Tokyo Kasei Ind. Co. Ltd. of Tokyo, Japan. The concentration of NaDBS can be determined using an ultraviolet spectrophotometer at a wavelength of 224 nm. It can also be determined by the methylene blue method which involves an aqua-chloroform two-phase titration using Hyamine 1622 (Longman, 1975). The concentration of DTMAC1 can be determined by the Cross method which is an aqua-chloroform two-phase titration using sodium tetraphenylboron (Longman, 1975). In the case of solution containing both NaDBS and DTMACl, the UV spectro0 1984 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985
39
Q
IT-
Q
0
IF-
+-I
F
I
(e )
td)
Figure 3. Surface tensions of solutions of various NaDBS/DTMACl ratios as functions of total concentration.
Figure 1. Various modes of foam fractionation: (a) simple batchwise operation; (b) simple continuous operation; (c) continuous stripping; (d) continuous enriching; (e) combined continuous operation.
P9
1
OO
IO CF x io4 , mole/\
5
53
Figure 4. Surface excess of NaDBS solution as function of concentration. I
I
1 1
I
I
I
I
I
I
'
I
Figure 2. The foam fraction apparatus.
photometry is not affected by the reaction between these two surfactants, but the two-phase titration methods determine only the excess amount of NaDBS or DTMACl after the reaction. Hence, these methods have to be used complementally in order to find the reacted and excess amounts of these surfactants. Surface tension values of solutions of various composition are determined by the sessile drop method using a contact angle goniometer supplied by h e - H a r t Inc. The drop volume is made sufficiently large that the addition of more liquid causes only its diameter but not the height to increase. The equilibrium drop height, h, and the angle of contact between the drop body and the solid surface, 8, are determined and the surface tension is computed by (Boyes and Ponter, 1970)
0
1
I
I , .
29
IC
c,
x IO', mole/ 1
Figure 5. Surface excess of binary solution of NaDBS/DTMACl = 3 as a function of total concentration.
5c
l-----7
pgh2 = 2(1 - cos 8)
In order to increase the foam stability, analytic grade NaCl at a concentration of 0.01 M was added to the solution in all the foam fractionation experiments (Goldberg and Rubin, 1972). Results and Discussions (a) Surface Tension and Surface Excess of Surfactant Solutions. Surface tension values of surfactant solutions of various NADBS/DTMACl ratios are determined and the results are plotted in Figure 3 as functions
1 "
IC
1
C,X
1
,
lo3 mole./
1
I , , , / I 53 IOC
Figure 6. Surface excess of DTMACl solution as a function of concentration.
of total concentration. I t is shown in this figure that for this cationic/anionic surfactant pair, the values of surface tension of binary solutions are always lower than that of the solutions of single surfactant. The synergistic effect
40
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985 ?F
1
'7
4
i
Figure 7. Surface excess of binary solution of NaDBS/DTMACl = 0.1 as function of total concentration.
C, x l o 4 , mole/l Figure 9. Enrichment ratios as functions of feed concentration: curve A, NaDBS; curve B, NaDBS/DTMACl = 3 (other conditions the same as in Figure 8).
i
i ,
a2 C
I 20
I
1
1
t9
^ ^
BC
1i
4 , :v Figure 8. The CW/CF ratio as a function of stripping length: NaDBS concentration = 3.06 X lo4 M gas rate = 350 mL/min; feed rate = 20 mL/min; liquid level = 20 cm.
is clearly visible for all the compositions and concentrations studied. Foam fractionation experiments of simple mode, as shown in Figure l(b), were carried out for the purpose of determining the adsorption or surface excess of the surfactants, ,'I at the bubble surface. The I' values were computed by eq 2 and the results are shown in Figures 4, 5, 6, and 7. Figures 4 and 6 show that the values of adsorption on the surface of foam bubbles from solutions of individual surfactants increase with the concentration of the solutions. Figure 5 shows that when DTMACl is added to the solution of NaDBS, the I' vs. CF curve has a minimum. The adsorption on the surface of foam bubbles is not only high at the higher end but also surprisingly high at the lower end of the concentration scale of the solution. A similar phenomenon is also shown in Figure 7 for the case of small amounts of NaDBS added to the solutions of DTMACl. These results indicate that the synergistic effect is particularly significant when the concentration of the solution is sufficiently low. (b) Effect of Operating Conditions. Stripping Length. The effect of stripping length between the liquid surface and the feed point in the foam fractionation column was studied under fixed liquid level, gas and feed rates, and NaDBS concentration. One of the typical results is shown in Figure 8. The Cw/CF ratio decreases relatively rapidly within a stripping length of about 15 cm, and it changes little above this point. A similar relationship was obtained for the NaDBS/DTMACl binary solutions. These results are in agreement with the observations of Goldberg and Rubin (1975) that equilibrium can be reached within a stripping length of about 10 cm and that further increase of column height has little influence on the concentration of the bottom product. This also explains why in the experiments of Hass and Johnson (1965) there was little variation in HTU values with column length for countercurrent lengths of 10 to 28 cm, but the HTU values for 50-, 60-, and 85-cm countercurrent lengths scattered badly. The number of transfer units is difficult to measure when a large portion of the column is under
k
\
-1
1
7
5
IC
C z x io5 , mole/l
,
I
5C
,Mi 2:
Figure 10. Enrichment ratios as functions of feed concentration; curve A, DTMACl; curve B, NaDBS/DTMACl = 1:9 (other conditions the same as in Figure 8).
nearly equilibrium condition. The Cw/CFratio computed by using eq 4 is the equilibrium value which could be obtained experimentally by using a foam fractionation column of sufficiently long stripping length. Gas Rate. The experimental results shown that when the gas rate is slower the enrichment ratio, CD/CF, is higher, where CD is the concentration of top product. This is because less liquid between foam bubbles is carried to the foam breaker when the residence time of the h a m in the column or the available draining time is longer. It was also observed in this work that when the gas rate is close to the lowest operatable limit, its effect on the enriching ratio is the strongest. Feed Concentration. Simple mode foam fractionation of NaDBS of various feed concentrations were carried out under fixed operating conditions, and the results are shown in Figure 9. The enrichment ratio, CD/CF1is high at low feed concentration, and it is nearly unity when the feed concentration is above M. It was observed that when the concentration of surfactant is higher, the foaming property is better, the foam bubbles are smaller, and more liquid between bubbles is carried over to the foam breaker. Curve B in this figure shows the enriching ratio of the binary solutions of NaDBS/DTMACl = 3:l is significantly better than that containing NaDBA alone when the feed concentration is below M. Above this concentration very little enrichment effect was observed in either case. Curve A in Figure 10 shows the case of fractionation of solutions containing DTMACl alone. The enriching effect is insignificant when the feed concentration is higher than M. When the feed concentration is below this value, the foaming property of the solution is so poor that the column becomes inoperative. Curve B in this figure shows that the foam fractionation of very dilute DTMACl solution can be carried out smoothly with high enrichment
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985 41
7
L
LL
Y
t//
9
1
Te
/
0"
5i..i
4 L
5
3
u
I5
IO
C
R
Figure 11. Enrichment ratios as functions of reflux ratio: NaDBS/DTMACl = 2.39; Cp = 4.88 X lo-" M,curve A, NaDBS; curve B, DTMACl (other conditions the same as in Figure 8).
i
Zq-, 0
,
I
c-x
, 2
io5
3
mole/\
Figure 12. The effect of the addition of DTMACl on the enrichment ratio of NaDBS: C, = 1.13 X lo4 M (other conditione the same as in Figure 8).
ratio when 10% of NaDBS is added. Reflux Ratio. The effect of reflux ratio, R, which is the ratio of the portion of solution from the foam breaker pumped back to the top of the column to the portion withdrawn as product, was studied under various operating conditions. Figure 11 is one set of typical results. The enrichment ratio, c D / c F , increases rapidly with the increase of reflux ratio when the R value is less than 4 (Brunner and Lemlich, 1963). I t reaches an asymptotic value when the reflux ratio is higher. Further increase of R has no significant effect on the imprqvement of the enrichment ratio for ;total height of 125 pm. (cf The Effect of DTMACl on the Enrichment Ratio of NaDBS. A series of simple mode foam fractionation experiments were carried out for the purpose of testing the effect of the addition of DTMACl on the enrichment ratio of NaDBS. It is shown in the previous sections that in the case of binary solutions, the surface excess or the adsorption on the vapor-liquid interface is high at low concentration. Some improvement in enrichment ratio is therefore expected. One set of typical results at low NaDBS concentration is shown in Figure 12, where CT denote DTMACl concentration. This effect becomes less
I0
C+X
20
lo5 , mole/ I
Figure 13. The effect of the addition of DTMACl on the enrichment ratio of NaDBS: CF = 5.05 X lo-' M (other conditions the same as in Figure 8).
significant as the concentration of NaDBS is increased. The opposite effect appears when the concentration of NaDBS is increased to 5.05 X M. The enrichment ratio of NaDBS decreases as the added amount of DTMACl increases, as shown in Figure 13. It is interesting to note that the NaDBS concentration used in this figure is about the CF value which gives the minimum r in Figure 5. Conclusion Surface activity of mixed cationic/anionic surfactant solutions is often much greater than would be expected in the absence of any mutual influence between surfactants. It is expected that such synergistic effects can be used to improve the separation efficiency of foam fractionation. The experimental results of this work verified the applicability of this method. The enrichment ratio of a surface-active solute of low concentration may be increased considerably by the addition of another surfactant of opposite ionic property. Foam fractionation of ionic solute of poor foaming property can be made not only posaible but also with good enrichment ratio by use of this method. Registry No. NaDBS, 25155-30-0; DTMACl, 112-00-5. Literature Cited Boyes, A. P.; Ponter, A. 8. J . Chem. Eng. Date 1970, 15. 235-8. Brunner, C. A.; Lemlich, R. Ind. Eng. Chem. Fundem. 1983. 2 , 297-300. Wberg, M.; Rubln. E. Sep. Scl. 1972, 7,51-73. (kralczyk, D. J . Co//o&lInterface Sci. 1980, 77(1),68-75. Hass, P. A.; Johnson, H. F. AIChE J . 1985, 7 1 , 319-24. Lemlich. R. I d . Eng. Chem. 1988, 80(10), 16-29. Longman, (3. F. In "The Analysis of Detergents and Detergent Products"; Wiley: New York, 1975.
Lucassenaeynders. E. H.; Lucassen, J.; Glies, D. J . ColloM Interface Scl. 1981, Bl(l), 150-7.
Shpenzer, N. P.; Kovaleva, I. N.; Klm, L. A.; Talmud, S. L. Zh. Prikl. Khim. 1981, 54(7), 1500-4.
Received for review February 1, 1983 Accepted December 13, 1983
