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W emulsions stabilized by limiting amounts of tall oil
Colloids and Surfaces,
57 (1991) 99-114
Elsevier Science Publishers
99
B.V., Amsterdam
Ultrafiltration of O/W emulsions stabilized by limiting amounts of tall oil Marianne Nystriim Laboratory of Technical Polymer Chemistry, Department of Chemical Technology, Lappeenranta University of Technology, P.O. Box 20, Lappeenranta, Finland
(Received 18 August 1990; accepted
18 December
1990)
Ultrafiltration of oil-in-water (O/W) emulsions was investigated with tall oil as the emulsifier. Most of the experiments were carried out at pH z 11 in order to keep the tall oil in its surfactant mode. The permeate flux increased with increasing flow velocity, but decreased with increasing pressure (l-4 bar). The emulsions could be stable at very low emulsifier/oil (E/O) ratios, but permeate flux decreased with decreasing E/O ratio. The emulsions could be partially broken with acid in the pH range 6.5-8.5 and totally separated at pH 3.5. At alkaline pH the emulsions could be broken with salt. A higher concentration of salt was needed when the E/O ratio was high. Aniline could be continuously extracted into the oil droplets of the emulsion during ultrafiltration from an aqueous solution fed to the emulsion with the same rate as permeate was formed. The extraction of aniline could be carried out to a degree which exceeded the expectations based on its W/O distribution factor.
INTRODUCTION
Ultrafiltration (UF) is a separation process with which, by means of a semipermeable membrane under the influence of pressure (l-10bar), macromolecules or large particles are separated in the retentate from solvent or other smaller molecules in the permeate. UF is used on an industrial scale, e.g. in the dairy industry to concentrate milk and whey, or in juice processing for clarification. It is also an accepted method for the recycling of electrophoretic paint in the car industry. Ultrafiltration or microfiltration have also been used in the concentration of latex particles. Since emulsion droplets are usually more than ten times the size of an average pore in a UF membrane, it should be rather easy to use UF for the concentration of emulsions in oil effluent treatment. Ultrafiltration of oil emulsions has been used to clean different effluents containing oily bilge water, cutting oils [l] or soluble oils [2,3]. In some experiments this was done using rotating modules [ 41. In most experiments the
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Elsevier Science Publishers
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emulsifier used was contained in the system to be cleaned and the amount of emulsifier was more than sufficient. Permeate fluxes, J,, of 28.8-187.2 dm3 m-’ h- ’ were obtained for non-cellulosic membranes [ 51. A more fundamental study on droplet sizes at different stages of UF was undertaken by Lipp et al. [ 61, which showed that the droplets were about the same order of size independent of oil concentration in the sample as long as the ratio of emulsifier to oil was constant. Also in these experiments the concentration of emulsifier was kept rather high. Emulsions are usually stabilized by ionic surfactants in their dissociated form. Accordingly, the emulsions are stabilized by charge, which means that an electrostatic repulsion exists between the emulsion droplets [ 71. The total interaction energy can be calculated using the Derjaguin-Landau-Verwey-Overbeek (DLVO ) theory. The theory predicts a smaller repulsive energy if salt is added, as the double-layer is decreased. Also a pH change which causes a decrease in the absolute value of the charge density of the particle can result in separation of the emulsion. If the emulsifier also stabilizes sterically, a change in neither pH nor in salt concentration need destroy emulsion stability. In some cases the change in pH rather causes an inversion of the emulsion [ 81. The aim of the present work was to study the ultrafiltration of an oil-inwater (O/W ) emulsion and see how different parameters such as amount of oil, emulsifier, salt and pH, affected emulsion stability. Since the interest was focused on emulsion stability, extreme values for some of these parameters were tested. Another aspect of the work was to see if an additive in the water phase at very low concentration could be extracted and concentrated in the oil phase of the emulsion by means of ultrafiltration. MATERIALS
AND METHODS
Membranes
The membranes used were of the type FP 100 from Paterson Candy International (PC1 ) . They are made of poly (vinylidene fluoride ) (PVDF ) and have a nominal cut-off of 100 000 g mol-‘. Chemicals
The oil used was a commercial “petrol” fraction named EXXSOL D60 from Exxon Chemical International Inc. Its boiling point interval was 180-197°C the density (15°C) was 787 kg m-‘, the viscosity (25 “C) was 1.28 mPa s, the surface tension was 24.9 mN m-l and the refractive index (20°C) was 1.436. The fraction had an aromatic content of 0.6wt.%. Concentrations of l-35wt.% oil in water were used in the experiments. The emulsifier used was distilled tall oil (DTO 25,1011060-7) from the Enso
101
Gutzeit distillation plant in Imatra, Finland. The distilled tall oil contains mainly the fatty acids, oleic acid (9-octadecenoic acid) and linoleic acid (9,12octadecadienoic acid) and resin acids of which abietic acid is the most important [9]. The tall oil was titrated to pH 12 to give its sodium salt. Titration of the tall oil fraction verified that it had a satisfactory content of acidic groups, as its acid number was determined to be 175 mg KOH g-’ tall oil. It was noted from the potentiometric titrations that the fatty acids and the resin acids were titrated in different pH ranges. Therefore the detergent form of the tall oil will be only partly stable below pH 11-12, as the weak acids of tall oil dissociate over a wide pH range. Thus the experiments have to be run at pH z 11. The aniline used for the extraction experiments was pro analyse grade. The aniline contents in the UF permeates were determined by titration. Aniline has a pK value of 4.6 according to the literature [lo] and the standard titrations made were in accordance with this. Apparatus The ultrafiltration system used is represented schematically in Fig. 1. The UF module (1)) containing one membrane tube (PCI, FP 100) with a diameter of 12 mm, was inserted in a perforated stainless steel tube and fitted with rubber stoppers to the circulation system. The membrane area of the tube was 0.0452 m2. The liquid container (2)) made of stainless steel, could contain 24.7 dm3 of emulsion. The emulsion was vigorously stirred with a stirrer (Heidolph, RZR 50). The emulsion was pumped through the system by a seal-less sliding vane pump (3 ) (Pompe Caster, MPA 314316 ). Pressures up to 4 bar were used and regulation was achieved by a pressure valve (5) (Whitey 316, SS-5PDF8), to within 0.1 bar. The maximal flow velocity obtained was 6.25 1min-‘, which corresponds to an average linear velocity of 0.92 m s-l, which for the present system gives an approximate Reynolds number of 12400. Therefore the flow
Fig. 1. Schematic UF system used in the UF of emulsions.
102
in the tube can be considered turbulent. The flow velocity was measured with a Fischer-Porter rotameter (4) (MOD, DlO A1197A). The pressure was measured with a pressure meter (6) with a range of O-10 bar. Electronic equipment used for temperature measurement was based on the electronic temperature sensor AD590 (Analog Devices). In the extraction experiments the solution containing the extractable substance (aniline) was pumped into the system with a peristaltic pump (10) (Ismatic, IPS-12). The permeate was weighed on an electronic balance (7) (Alsep, EY6000A) before it was discarded or returned to the system. The flux of permeate (ti,) was calculated in g min- ’ as the density of the permeate could not always be measured and was not constant. When the emulsion was saturated with the extractable substance, the emulsion was tapped out from the system into an emulsion breakage tank, using the three-way valve (9 ). RESULTS
AND DISCUSSION
Stabilization of membranes The FP 100 (PCI) UF membrane was stabilized by ultrafiltration of pure water. The stabilization was carried out for 2 days. The pressure during stabilization was 3 bar, which was expected to be higher than the pressures used in the experiments. The stabilization of the FP membrane lasted for a very long time. When the pressure was released a new stabilization period was needed before the next experiment could take place. Even after 9 h some decrease in flux still took place. After a number of runs the stabilization periods became shorter. The PVDF membranes differ from the more extensively studied polysulfone membranes by being more elastic. This is the reason why pressure does not deform the membranes into “a final form” and the membranes recover from the deformation. Effect of salt on flux when no emulsion is present In order to study the effect of salt on flux without emulsion present, one experiment was performed where salt solutions regulated to a pH of 10.5 with NaOH were ultrafiltered through the membrane, and the flux was measured at different salt concentrations. The results are depicted in Fig. 2 and show a remarkable decrease in flux with increasing salt concentration. Part of the decrease could of course be due to the instability of the membrane but at concentrations > 0.04 M NaCl the normal time for membrane stabilization should have passed. This shows that the osmotic pressure of the salt solution affects the flux of permeate even though the membrane pore size is very large.
103
Fig. 2. Permeate flux (FP 100 membrane) as a function of salt concentration at pH 10.5, regulated with NaOH. Flux corrected to 25” C; p = 1.5 bar; flow velocity= 0.53 m S-‘; UJ=500 r.P.m. TABLE 1 Water flux (ti,; g min-‘)
before experiments, calculated to a pressure of 1 bar
No. of expt. m,
1 39
2 75
3 124
4 93
5 102
6 100
No. of expt. m,
8 81
9 78
10 90
11 66
12 61
13 70
I 82
Effect of cleaning Between the experiments with O/W emulsions, the membrane was cleaned by UF of NaOH solutions at pH> 11. No detergent was used. This procedure almost re-established the original water flux of the membrane, which means that no appreciable permanent fouling of the membrane by the emulsions took place even at low pH values (see Table 1). In the beginning there was even a threefold rise in flux after cleaning due to the high pH used. Cleaning with alkaline solution is known to increase the flux of a new membrane [111. Effect of pressure and flow velocity on flux in UF of emulsions The effect of pressure and flow velocity on flux was studied with an emulsion consisting of 2Owt.% oil and O.O2wt.% emulsifier. The stirring speed was 800 r.p.m. in both experiments.
104
The change in flux achieved by increase or decrease in linear flow velocity was studied at an applied pressure of 1.5 bar. The results are depicted in Fig. 3. It is seen that the retreating curve gives somewhat lower flux values than the advancing curve. This could partly depend on the fact that a stabilized condition was not achieved at the beginning of the experiment. The influence of linear flow velocity on flux seems to be a very important factor, which is
0.25
05
0 75
v
/ms?
Fig. 3. Ultrafiltration of an emulsion containing 2Owt.% oil and O.O2wt.% emulsifier; flux as a function of linear flow velocity. Flux corrected to 25’ C; o = 800 r.p.m.; p = 1.5 bar. I indicates the approximate point of stabilization of membrane.
a
b
5-
1
2
3
AP / bar
1
2
3
AP/
bar
Fig. 4. Ultrafiltration of an oil emulsion containing 20wt.% oil and O.O2wt.% emulsifier (E/ O=O.OOOl; ~~800 r.p.m.; flux corrected to 25°C). (a) Experiment 1; (A) advancingp, u=O.54 ms-’ and ( A ) retrieving p, u= 0.54 m s-r; ( A ) after flow rate adjustment, p = 1 bar; u = 0.71 m s-‘. (b) Experiment 2; (0) advancingp, u=O.71 m s-’ and (0) retrievingp, u=O.71 m s-‘; ( 0 ) after flow rate adjustment, p = 1 bar, u = 0.93 m s-‘.
105
quite natural as an increase in flow velocity decreases the concentration polarization layer. In order to avoid interference from the stabilization time on the flux results, when investigating the effect of pressure on flux, a stabilization period of at least 3 h at a pressure of 1 bar was used before the pressure was increased. The experiment was carried out at two different flow velocities. The results are depicted in Fig. 4. Measurements of flux were made both at increasing and decreasing pressure. At the end of the experiment the flow velocity was increased and a final flux value was measured. The curves clearly show that an increase in pressure decreases flux and that the flux is restored when pressure is released. It is probably pressure, since it increases concentration polarization, that makes the emulsion particles coalesce at the membrane surface and therefore gives a higher flow resistance to the membrane. At higher pressures some oil droplets could also be seen in the permeate, which again point.s to the fact that some kind of oil layer is formed at the membrane surface, and when pressure increases, the oil droplets can penetrate the pores into the permeate. Since the emulsion droplets have diameters which are lo-100 t,imes larger than the diameters of the pores, no penetration of emulsion droplets through the pores should occur. The pressure effects are probably more prominent at low flow velocities but could not be overcome even at the maximum flow velocity achieved in our system. Bhattacharyya [5] has also shown that for pressures above 1.5 bar the flux does not increase in the UF of emulsions. Effect of pH on emulsion stability The ultrafiltration of emulsions was studied at different pH values. The percentage by weight of oil (0) in water (W) was varied, but the percentage of emulsifier (E) was kept approximately constant. This means that both the E/ 0 ratio and the O/W ratio were varied. The emulsions were at first mixed, stirred and stabilized in the UF apparatus and the pH was adjusted to > 12. UF was carried out for some time until a stable flux was achieved. The pH was then decreased with 1 M HCl about half a pH unit at a time and the flux was measured. The results from three different runs are depicted in Fig. 5. One can see that at most pH values the flux is higher when the E/O ratios are higher and the O/W ratios are lower. Flux diminished in all experiments when pH was below 11, i.e. when the emulsifier had lost some of its detergent effect. At pH values around 7 the flux suddenly increased, and at the same time oil droplets separated from the emulsion. The real separation of the emulsion into oil and water phases did not take place until the pH dropped down to about 3.5. The free emulsifier (tall oil) at low pH did not seem to foul the membranes very heavily, and since the tall oil concentration was the same in all experiments, the fouling phenomenon must have been influenced by the total oil
106
3 91’
a
3
5
7
n
9
PH
I
, 3
I
5
. 7
.
1
, 9
tl
PH
Fig. 5. Emulsion stability at different pH in ultrafiltration; flux vs pH (p = 1.5 bar; u = 0.53 m s-l, co=800 r.p.m.; T=19.5-21°C). (a) (0) l.Owt.% oil and O.O5wt.% emulsifier; E/0=0.05; (b) (0) 5.0wt.% oil and O.O5wt.%emulsifier; E/0=0.01; (c) ( X ) 19.8wt.% oil and O.O37wt.%emulsifier; E/0=0.0019.
content: more oil-more fouling. When the detergent effect of tall oil diminished, larger emulsion droplets could be expected and as these cause more concentration polarization the flux decrease could also be due to this. Fouling was
107
not permanent as the membrane water flux returned to normal after cleaning the membrane at pH > 11. The pH range 6.5-8.5 possibly illustrates the state of an unstable emulsion containing three phases; an oil phase, a water phase and residual emulsion. The partial breakage of the emulsion makes the flux rise. Stable emulsions could also be obtained in the pH range 6.5-8.5 if much more tall oil was added to the emulsion. Possibly the change in pH caused an inversion of the emulsion to a W/O emulsion [8] when the emulsifying tall oil lost most of its ionic character. The fact that the flux is higher at pH 11 for the lower O/W ratios could depend on the fact that at low concentrations of oil there is more free emulsifier, which results in the emulsifier concentration being above the critical micelle concentration (CMC) and the micelles do not foul the membrane as much as the monomers and the dimers of the emulsifier. Effect of salt on emulsion stability in UF Emulsions with a low content of emulsifier were studied when salt was added to the emulsions and when the concentration of oil was 20-35wt.%. At some salt concentration the emulsions broke in the same way as at pH 6.5-8.5 in the experiments above. An increase in flux often accompanied this collapse, and it could be interpreted as an inversion from a one-phase system to a three-phase system, as above, when pH was lowered. When the inversion had taken place, the emulsion phase rose on top of the water phase, and at this point an oil phase also existed. Before the change took place the flux decreased with an increase in salt concentration. This decrease in water flux by the addition of salt also took place when there was no emulsion present, as can be seen from Fig. 2. The critical NaCl concentration for the breakage of the emulsion seems to depend on the E/O ratio as can be deduced from Fig. 6. Extrapolating to zero salt concentration ( CNaCl= 0)) would imply that the lowest E/O ratio at which O/W emulsions can exist is about 0.0002. UF of O/W emulsions; flux as a function of oil concentration and E/O ratio This experiment was carried out in such a way that at first only emulsifier was added to the system. Then the oil was added in portions to the emulsifierwater system. This means that with increasing oil concentration the E/O ratio decreased. Flux was plotted as a function of oil concentration and also as a function of E/O ratio (Fig. 7). It is seen that the flux diminished with an increase in oil concentration, diminishing faster in the beginning but then levelling off somewhat. It is interesting to note that at low E/O ratios there seems to exist a linear relation-
Fig. 6. Concentration of NaCl for the inversion of emulsions from O/W emulsion to W/O emulsion in ultrafiltration; E/O ratio vs concentration of NaCl (p = 1.5 bar; IJ= 0.53 m s-‘; w= 500 (800) r.p.m.; T=24.5-29.1”C, and pH-11). (*) 20wt.% 0; (0) 25wt.% 0; (x) 30wt.% 0 and (0) 35wt.% 0.
ship between E/O ratio and flux at constant pressure. One can imagine that the E/O ratio determines the drop size, when the concentration of emulsifier is limited. If this is true, the drop size should be smaller at high E/O ratios, which means that flux increases with decreasing drop size. This is in accordance with the experiments described above concerning the relationship between flux and pressure. If the emulsion droplets are small, the pressure influence on concentration polarization at the membrane is lower than with large droplets. Then the pressure does not coalesce the emulsion droplets at the membrane surface so easily and free pathways for permeate are present. No budding of oil into the permeate takes place either. The effect of oil concentration in the emulsions on flux was also tested with another kind of experiment. This experiment started out with 15 kg of an emulsion containing 2Owt.% oil and O.O2wt.% emulsifier. Then the emulsion was concentrated by discarding the permeate during the experiment. The real E/ 0 ratio during the experiment could not be evaluated as the amount of the
109
2s
0.01
0.03
0.05
0.07
E/O
g/min ‘---I 30.
lx
. E/O \
Fig. 7. Ultrafiltration of O/W emulsions: flux as a function of oil concentration ( X ) and of E/O (0) (p=2.0bar;u=‘0.70ms-‘,pfi~l~).
ratio
L
20
25
xl
co/ wt-70
Fig. 8. Concentration of an emulsion by ultrafiltration; flux vs oil concentration (p=1.5 bar; 0=500 r.p.m.; E/0=0.001 at start; u=O.53 m s-l; pHz 11; flux corrected to 25°C.
discarded emulsifier present in the permeate could not be measured. The results of this experiment are depicted in Fig. 8. It can be seen that flux decreases with increase in oil content. As the experiment was continued overnight some of the flux was restored until stabilization took place. The increase in flux probably arose from the fact that the emulsion partly separated during the night and the stable concentration polarization gel layer at the membrane had not yet developed.
110
Emulsion droplet sizes Emulsion droplet sizes were determined with a Malvern particle analyzer. The emulsions were difficult to analyze because they were too dense. When diluted with pure water the oil separated on the cuvette walls. Dilution seemed to be successful if it was carried out by adding permeate so that the equilibrium between free and bound emulsifier was maintained. Anyway one cannot be quite sure that the droplet sizes will not change upon dilution. For an emulsion containing 2Owt.% oil and O.O2wt.% emulsifier the mean droplet size was about 3.5 pm, which was larger than the droplet sizes ( z 100 nm) reported by Lipp et al. [ 61, but in their experiments the emulsifier concentration was not limiting. The size of the droplets in our study did not change much when the emulsifier content was increased to O.&vt.%. In some of the results droplets of about 70 pm were also observed, which could indicate the start of coalescence of droplets, when stirring is not continued during the Malvern measurements. Extraction experiments performed with aniline The solubility (S) of aniline in water and oil phases was tested at different temperatures. The results are depicted in Fig. 9. As expected the solubility is better in the oil phase at all temperatures but the solubility dependence of aniline in oil on temperature is more pronounced. The distribution factor for aniline between water and oil at 23 ‘C for aniline concentrations <6wt.% was independent of aniline concentration and approximately 0.89 (kg aniline per kg water : kg aniline per kg oil). At the limit of aniline solubility the water phase contained 3.46wt.% aniline and the oil phase 3.9Owt.% aniline. The ultrafiltration extraction experiment was carried out with an emulsion consisting of 13.3wt.% oil and O.Ol&vt.% emulsifier: E/O =0.00115. A 2.2wt.% aniline solution was fed into the emulsion at the same rate as permeate was collected. Table 2 shows the flux variation with aniline concentration in the
20
30
Fig. 9. Solubility
40
5o
t /“c
(S) in wt.% of aniline in oil ( A ) and in water ( 0
) at various temperatures.
111
TABLE 2 Flux of permeate
(tiL,) of an emulsion with an E/O ratio of 0.00115 containing
after adding different well as the distribution
amounts of aniline. The aniline percentage factor, K=aniline
in water phase/aniline
13.3wt.% oil
in the permeate
is reported as
in oil phase
m,
Aniline
Aniline in permeate
(g min-‘)
(wt.%)
(wt.%)
14.6
0.036
0.014
14.4
0.109
13.9
0.179
13.3
0.291
13.0
0.363
12.2
0.469
11.7 11.4
K
18.2
0.133
0.760
0.276
0.726
0.541
0.414
0.723
0.652
0.482
0.736
solution. The permeate concentration of aniline was titrated and for some values distribution coefficients (as above ) were calculated. It is seen that aniline was collected preferentially in the oil phase even more strongly than the distribution data above would have suggested. This could of course depend on the possibility that aniline was retained to some extent by the membrane. Another explanation is that aniline can partly interact with the emulsifier. It can be retained by free micelles in the water phase or by the surfactant at the interfacial layer of the emulsion droplet. At low pH values the aniline would be positively charged and more hydrophilic and then adhere to the outer border line of the micelle or emulsion droplet. At the high pH of the experiment the aniline is more hydrophobic and can be expected to interact in the border line between the hydrophilic and hydrophobic regions of micelles or emulsion droplets. Finally the aniline-containing emulsion (0.652wt.% aniline; 13.3wt.% oil) was concentrated by ultrafiltration. The results are depicted in Fig. 10 as flux, the concentration factor ( = original mass of emulsion/original mass of emulsion - accumulated collected permeate) or the natural logarithm of the concentration factor, all as functions of time. It can be seen that until the concentration factor was 1.5, flux was almost constant. At this point oil and emulsion droplets started to appear in the permeate and the flux decreased. At a concentration factor of 2.3, the flux no longer decreased so quickly. The natural logarithm of the concentration factor
a
--I
I
10
15
t/h cf
:.f.
In
25
1.5
1.0
0.5
1.5
I
15
t/h
Fig. 10. Concentration with UF of an emulsion of oil in water and tall oil as the emulsifier containing 0.652wt.% aniline and 13.3wt.% oil and with an E/O ratio of 0.00115, pH zz 11 (p = 2.5 bar; ~~500 r.p.m.; u=O.71 m s-l). The experiment went on for 4 days and overnight flux increased for some time. (a) ti, as a function of time ( X ). (b) The concentration factor (cf.) (0 ) and the natural logarithm of the concentration factor (A ) as functions of time.
113
vs UF time was linear. The aniline did not seem to disturb the concentration of the emulsion by ultrafiltration. CONCLUSIONS
The above results show that the ultrafiltration of emulsions is possible over a large range of O/W and E/O ratios. The pressures should be kept rather low, -C1.5 bar, and the flow velocities as high as possible to achieve maximal fluxes. The emulsions can be broken by acid because in acidic solution the emulsifier loses its detergent mode. Emulsions with a low content of emulsifier can be broken by salt and in this way a sudden increase in flux is achieved, when the O/W emulsion breaks to a system with an oil and a water phase and residual emulsion, which rises to the top of the vessel and cleans the membrane tube. The extraction and concentration of aniline from an aqueous solution into an emulsion was achieved by feeding the same amount of aniline-containing solution to the emulsion syst,em as was obtained as permeate during ultrafiltration. The aniline was concentrated into the oil phase of the emulsion to a degree which exceeded the expectations based on the distribution factor experiments. This better retention could be explained by the interaction of aniline with the surfactant. This type of extraction in an emulsion with subsequent ultrafiltration seems to be a possible way to clean effluent waters containing minor amounts of harmful small organic molecules. As the emulsions, when filled with the extracted substance, can easily be broken, the concentration can be carried out on a larger scale as a continuous process. Since tall oil seems to be a good emulsifying agent, even at small concentrations, and because it is rather inexpensive and produced on a large scale as a byproduct in the pulp and paper industry, it is economical to use in larger scale processes. ACKNOWLEDGEMENTS
The author thanks Associate Professor Matti Lindstrijm for reading and commenting on the manuscript, and the students Pia Jarvinen, Vesa-Pekka Kangas and Tero Salo for their technical assistance.
REFERENCES 1 2 3 4
F. Rickli, Umweltschutz/Gesundheitstechnik, 4 (1985) 100. S. Lee, Y. Aurelle and H. Roques, J. Membrane Sci., 19 (1984) 23. R.L. Goldsmith, D.A. Roberts and D.L. Burre, J. Water Pollut. Control Fed., 46(9) (1974) 2183. F. Vigo, C. Uliana and P. Lupino, Sep. Sci. Technol., 20(2/3) (1985) 213.
114 5 6 7 8 9 10 11
D. Bhattacharyya, A.B. Jumawan, R.B. Grieves and L.R. Harris, Sep. Sci. Technol., 14(6) (1979) 529. P. Lipp, C.H. Lee, A.G. Fane and C.J.D. Fell, J. Membrane Sci., 36 (1988) 161. H. Schubert and H. Armbruster, Chem.-Ing.-Tech., 61 (1989) 701. S. Qutubuddin, C.A. Miller and T. Fort, Jr., J. Colloid Interface Sci., 101 (1984) 46. R.K. Sharma and N.N. Bakhshi, Tappi, 73(9) (1990) 175. R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, 67th edn, CRC Press, Boca Raton, FL., 1986. G. Jonsson and P.L. Johansen, Proc. 5th World Filtration Congress, Nice, 1990, p. 203.
57 (1991) 99-114
Elsevier Science Publishers
99
B.V., Amsterdam
Ultrafiltration of O/W emulsions stabilized by limiting amounts of tall oil Marianne Nystriim Laboratory of Technical Polymer Chemistry, Department of Chemical Technology, Lappeenranta University of Technology, P.O. Box 20, Lappeenranta, Finland
(Received 18 August 1990; accepted
18 December
1990)
Ultrafiltration of oil-in-water (O/W) emulsions was investigated with tall oil as the emulsifier. Most of the experiments were carried out at pH z 11 in order to keep the tall oil in its surfactant mode. The permeate flux increased with increasing flow velocity, but decreased with increasing pressure (l-4 bar). The emulsions could be stable at very low emulsifier/oil (E/O) ratios, but permeate flux decreased with decreasing E/O ratio. The emulsions could be partially broken with acid in the pH range 6.5-8.5 and totally separated at pH 3.5. At alkaline pH the emulsions could be broken with salt. A higher concentration of salt was needed when the E/O ratio was high. Aniline could be continuously extracted into the oil droplets of the emulsion during ultrafiltration from an aqueous solution fed to the emulsion with the same rate as permeate was formed. The extraction of aniline could be carried out to a degree which exceeded the expectations based on its W/O distribution factor.
INTRODUCTION
Ultrafiltration (UF) is a separation process with which, by means of a semipermeable membrane under the influence of pressure (l-10bar), macromolecules or large particles are separated in the retentate from solvent or other smaller molecules in the permeate. UF is used on an industrial scale, e.g. in the dairy industry to concentrate milk and whey, or in juice processing for clarification. It is also an accepted method for the recycling of electrophoretic paint in the car industry. Ultrafiltration or microfiltration have also been used in the concentration of latex particles. Since emulsion droplets are usually more than ten times the size of an average pore in a UF membrane, it should be rather easy to use UF for the concentration of emulsions in oil effluent treatment. Ultrafiltration of oil emulsions has been used to clean different effluents containing oily bilge water, cutting oils [l] or soluble oils [2,3]. In some experiments this was done using rotating modules [ 41. In most experiments the
0166-6622/91/$03.50
0 1991-
Elsevier Science Publishers
B.V.
100
emulsifier used was contained in the system to be cleaned and the amount of emulsifier was more than sufficient. Permeate fluxes, J,, of 28.8-187.2 dm3 m-’ h- ’ were obtained for non-cellulosic membranes [ 51. A more fundamental study on droplet sizes at different stages of UF was undertaken by Lipp et al. [ 61, which showed that the droplets were about the same order of size independent of oil concentration in the sample as long as the ratio of emulsifier to oil was constant. Also in these experiments the concentration of emulsifier was kept rather high. Emulsions are usually stabilized by ionic surfactants in their dissociated form. Accordingly, the emulsions are stabilized by charge, which means that an electrostatic repulsion exists between the emulsion droplets [ 71. The total interaction energy can be calculated using the Derjaguin-Landau-Verwey-Overbeek (DLVO ) theory. The theory predicts a smaller repulsive energy if salt is added, as the double-layer is decreased. Also a pH change which causes a decrease in the absolute value of the charge density of the particle can result in separation of the emulsion. If the emulsifier also stabilizes sterically, a change in neither pH nor in salt concentration need destroy emulsion stability. In some cases the change in pH rather causes an inversion of the emulsion [ 81. The aim of the present work was to study the ultrafiltration of an oil-inwater (O/W ) emulsion and see how different parameters such as amount of oil, emulsifier, salt and pH, affected emulsion stability. Since the interest was focused on emulsion stability, extreme values for some of these parameters were tested. Another aspect of the work was to see if an additive in the water phase at very low concentration could be extracted and concentrated in the oil phase of the emulsion by means of ultrafiltration. MATERIALS
AND METHODS
Membranes
The membranes used were of the type FP 100 from Paterson Candy International (PC1 ) . They are made of poly (vinylidene fluoride ) (PVDF ) and have a nominal cut-off of 100 000 g mol-‘. Chemicals
The oil used was a commercial “petrol” fraction named EXXSOL D60 from Exxon Chemical International Inc. Its boiling point interval was 180-197°C the density (15°C) was 787 kg m-‘, the viscosity (25 “C) was 1.28 mPa s, the surface tension was 24.9 mN m-l and the refractive index (20°C) was 1.436. The fraction had an aromatic content of 0.6wt.%. Concentrations of l-35wt.% oil in water were used in the experiments. The emulsifier used was distilled tall oil (DTO 25,1011060-7) from the Enso
101
Gutzeit distillation plant in Imatra, Finland. The distilled tall oil contains mainly the fatty acids, oleic acid (9-octadecenoic acid) and linoleic acid (9,12octadecadienoic acid) and resin acids of which abietic acid is the most important [9]. The tall oil was titrated to pH 12 to give its sodium salt. Titration of the tall oil fraction verified that it had a satisfactory content of acidic groups, as its acid number was determined to be 175 mg KOH g-’ tall oil. It was noted from the potentiometric titrations that the fatty acids and the resin acids were titrated in different pH ranges. Therefore the detergent form of the tall oil will be only partly stable below pH 11-12, as the weak acids of tall oil dissociate over a wide pH range. Thus the experiments have to be run at pH z 11. The aniline used for the extraction experiments was pro analyse grade. The aniline contents in the UF permeates were determined by titration. Aniline has a pK value of 4.6 according to the literature [lo] and the standard titrations made were in accordance with this. Apparatus The ultrafiltration system used is represented schematically in Fig. 1. The UF module (1)) containing one membrane tube (PCI, FP 100) with a diameter of 12 mm, was inserted in a perforated stainless steel tube and fitted with rubber stoppers to the circulation system. The membrane area of the tube was 0.0452 m2. The liquid container (2)) made of stainless steel, could contain 24.7 dm3 of emulsion. The emulsion was vigorously stirred with a stirrer (Heidolph, RZR 50). The emulsion was pumped through the system by a seal-less sliding vane pump (3 ) (Pompe Caster, MPA 314316 ). Pressures up to 4 bar were used and regulation was achieved by a pressure valve (5) (Whitey 316, SS-5PDF8), to within 0.1 bar. The maximal flow velocity obtained was 6.25 1min-‘, which corresponds to an average linear velocity of 0.92 m s-l, which for the present system gives an approximate Reynolds number of 12400. Therefore the flow
Fig. 1. Schematic UF system used in the UF of emulsions.
102
in the tube can be considered turbulent. The flow velocity was measured with a Fischer-Porter rotameter (4) (MOD, DlO A1197A). The pressure was measured with a pressure meter (6) with a range of O-10 bar. Electronic equipment used for temperature measurement was based on the electronic temperature sensor AD590 (Analog Devices). In the extraction experiments the solution containing the extractable substance (aniline) was pumped into the system with a peristaltic pump (10) (Ismatic, IPS-12). The permeate was weighed on an electronic balance (7) (Alsep, EY6000A) before it was discarded or returned to the system. The flux of permeate (ti,) was calculated in g min- ’ as the density of the permeate could not always be measured and was not constant. When the emulsion was saturated with the extractable substance, the emulsion was tapped out from the system into an emulsion breakage tank, using the three-way valve (9 ). RESULTS
AND DISCUSSION
Stabilization of membranes The FP 100 (PCI) UF membrane was stabilized by ultrafiltration of pure water. The stabilization was carried out for 2 days. The pressure during stabilization was 3 bar, which was expected to be higher than the pressures used in the experiments. The stabilization of the FP membrane lasted for a very long time. When the pressure was released a new stabilization period was needed before the next experiment could take place. Even after 9 h some decrease in flux still took place. After a number of runs the stabilization periods became shorter. The PVDF membranes differ from the more extensively studied polysulfone membranes by being more elastic. This is the reason why pressure does not deform the membranes into “a final form” and the membranes recover from the deformation. Effect of salt on flux when no emulsion is present In order to study the effect of salt on flux without emulsion present, one experiment was performed where salt solutions regulated to a pH of 10.5 with NaOH were ultrafiltered through the membrane, and the flux was measured at different salt concentrations. The results are depicted in Fig. 2 and show a remarkable decrease in flux with increasing salt concentration. Part of the decrease could of course be due to the instability of the membrane but at concentrations > 0.04 M NaCl the normal time for membrane stabilization should have passed. This shows that the osmotic pressure of the salt solution affects the flux of permeate even though the membrane pore size is very large.
103
Fig. 2. Permeate flux (FP 100 membrane) as a function of salt concentration at pH 10.5, regulated with NaOH. Flux corrected to 25” C; p = 1.5 bar; flow velocity= 0.53 m S-‘; UJ=500 r.P.m. TABLE 1 Water flux (ti,; g min-‘)
before experiments, calculated to a pressure of 1 bar
No. of expt. m,
1 39
2 75
3 124
4 93
5 102
6 100
No. of expt. m,
8 81
9 78
10 90
11 66
12 61
13 70
I 82
Effect of cleaning Between the experiments with O/W emulsions, the membrane was cleaned by UF of NaOH solutions at pH> 11. No detergent was used. This procedure almost re-established the original water flux of the membrane, which means that no appreciable permanent fouling of the membrane by the emulsions took place even at low pH values (see Table 1). In the beginning there was even a threefold rise in flux after cleaning due to the high pH used. Cleaning with alkaline solution is known to increase the flux of a new membrane [111. Effect of pressure and flow velocity on flux in UF of emulsions The effect of pressure and flow velocity on flux was studied with an emulsion consisting of 2Owt.% oil and O.O2wt.% emulsifier. The stirring speed was 800 r.p.m. in both experiments.
104
The change in flux achieved by increase or decrease in linear flow velocity was studied at an applied pressure of 1.5 bar. The results are depicted in Fig. 3. It is seen that the retreating curve gives somewhat lower flux values than the advancing curve. This could partly depend on the fact that a stabilized condition was not achieved at the beginning of the experiment. The influence of linear flow velocity on flux seems to be a very important factor, which is
0.25
05
0 75
v
/ms?
Fig. 3. Ultrafiltration of an emulsion containing 2Owt.% oil and O.O2wt.% emulsifier; flux as a function of linear flow velocity. Flux corrected to 25’ C; o = 800 r.p.m.; p = 1.5 bar. I indicates the approximate point of stabilization of membrane.
a
b
5-
1
2
3
AP / bar
1
2
3
AP/
bar
Fig. 4. Ultrafiltration of an oil emulsion containing 20wt.% oil and O.O2wt.% emulsifier (E/ O=O.OOOl; ~~800 r.p.m.; flux corrected to 25°C). (a) Experiment 1; (A) advancingp, u=O.54 ms-’ and ( A ) retrieving p, u= 0.54 m s-r; ( A ) after flow rate adjustment, p = 1 bar; u = 0.71 m s-‘. (b) Experiment 2; (0) advancingp, u=O.71 m s-’ and (0) retrievingp, u=O.71 m s-‘; ( 0 ) after flow rate adjustment, p = 1 bar, u = 0.93 m s-‘.
105
quite natural as an increase in flow velocity decreases the concentration polarization layer. In order to avoid interference from the stabilization time on the flux results, when investigating the effect of pressure on flux, a stabilization period of at least 3 h at a pressure of 1 bar was used before the pressure was increased. The experiment was carried out at two different flow velocities. The results are depicted in Fig. 4. Measurements of flux were made both at increasing and decreasing pressure. At the end of the experiment the flow velocity was increased and a final flux value was measured. The curves clearly show that an increase in pressure decreases flux and that the flux is restored when pressure is released. It is probably pressure, since it increases concentration polarization, that makes the emulsion particles coalesce at the membrane surface and therefore gives a higher flow resistance to the membrane. At higher pressures some oil droplets could also be seen in the permeate, which again point.s to the fact that some kind of oil layer is formed at the membrane surface, and when pressure increases, the oil droplets can penetrate the pores into the permeate. Since the emulsion droplets have diameters which are lo-100 t,imes larger than the diameters of the pores, no penetration of emulsion droplets through the pores should occur. The pressure effects are probably more prominent at low flow velocities but could not be overcome even at the maximum flow velocity achieved in our system. Bhattacharyya [5] has also shown that for pressures above 1.5 bar the flux does not increase in the UF of emulsions. Effect of pH on emulsion stability The ultrafiltration of emulsions was studied at different pH values. The percentage by weight of oil (0) in water (W) was varied, but the percentage of emulsifier (E) was kept approximately constant. This means that both the E/ 0 ratio and the O/W ratio were varied. The emulsions were at first mixed, stirred and stabilized in the UF apparatus and the pH was adjusted to > 12. UF was carried out for some time until a stable flux was achieved. The pH was then decreased with 1 M HCl about half a pH unit at a time and the flux was measured. The results from three different runs are depicted in Fig. 5. One can see that at most pH values the flux is higher when the E/O ratios are higher and the O/W ratios are lower. Flux diminished in all experiments when pH was below 11, i.e. when the emulsifier had lost some of its detergent effect. At pH values around 7 the flux suddenly increased, and at the same time oil droplets separated from the emulsion. The real separation of the emulsion into oil and water phases did not take place until the pH dropped down to about 3.5. The free emulsifier (tall oil) at low pH did not seem to foul the membranes very heavily, and since the tall oil concentration was the same in all experiments, the fouling phenomenon must have been influenced by the total oil
106
3 91’
a
3
5
7
n
9
PH
I
, 3
I
5
. 7
.
1
, 9
tl
PH
Fig. 5. Emulsion stability at different pH in ultrafiltration; flux vs pH (p = 1.5 bar; u = 0.53 m s-l, co=800 r.p.m.; T=19.5-21°C). (a) (0) l.Owt.% oil and O.O5wt.% emulsifier; E/0=0.05; (b) (0) 5.0wt.% oil and O.O5wt.%emulsifier; E/0=0.01; (c) ( X ) 19.8wt.% oil and O.O37wt.%emulsifier; E/0=0.0019.
content: more oil-more fouling. When the detergent effect of tall oil diminished, larger emulsion droplets could be expected and as these cause more concentration polarization the flux decrease could also be due to this. Fouling was
107
not permanent as the membrane water flux returned to normal after cleaning the membrane at pH > 11. The pH range 6.5-8.5 possibly illustrates the state of an unstable emulsion containing three phases; an oil phase, a water phase and residual emulsion. The partial breakage of the emulsion makes the flux rise. Stable emulsions could also be obtained in the pH range 6.5-8.5 if much more tall oil was added to the emulsion. Possibly the change in pH caused an inversion of the emulsion to a W/O emulsion [8] when the emulsifying tall oil lost most of its ionic character. The fact that the flux is higher at pH 11 for the lower O/W ratios could depend on the fact that at low concentrations of oil there is more free emulsifier, which results in the emulsifier concentration being above the critical micelle concentration (CMC) and the micelles do not foul the membrane as much as the monomers and the dimers of the emulsifier. Effect of salt on emulsion stability in UF Emulsions with a low content of emulsifier were studied when salt was added to the emulsions and when the concentration of oil was 20-35wt.%. At some salt concentration the emulsions broke in the same way as at pH 6.5-8.5 in the experiments above. An increase in flux often accompanied this collapse, and it could be interpreted as an inversion from a one-phase system to a three-phase system, as above, when pH was lowered. When the inversion had taken place, the emulsion phase rose on top of the water phase, and at this point an oil phase also existed. Before the change took place the flux decreased with an increase in salt concentration. This decrease in water flux by the addition of salt also took place when there was no emulsion present, as can be seen from Fig. 2. The critical NaCl concentration for the breakage of the emulsion seems to depend on the E/O ratio as can be deduced from Fig. 6. Extrapolating to zero salt concentration ( CNaCl= 0)) would imply that the lowest E/O ratio at which O/W emulsions can exist is about 0.0002. UF of O/W emulsions; flux as a function of oil concentration and E/O ratio This experiment was carried out in such a way that at first only emulsifier was added to the system. Then the oil was added in portions to the emulsifierwater system. This means that with increasing oil concentration the E/O ratio decreased. Flux was plotted as a function of oil concentration and also as a function of E/O ratio (Fig. 7). It is seen that the flux diminished with an increase in oil concentration, diminishing faster in the beginning but then levelling off somewhat. It is interesting to note that at low E/O ratios there seems to exist a linear relation-
Fig. 6. Concentration of NaCl for the inversion of emulsions from O/W emulsion to W/O emulsion in ultrafiltration; E/O ratio vs concentration of NaCl (p = 1.5 bar; IJ= 0.53 m s-‘; w= 500 (800) r.p.m.; T=24.5-29.1”C, and pH-11). (*) 20wt.% 0; (0) 25wt.% 0; (x) 30wt.% 0 and (0) 35wt.% 0.
ship between E/O ratio and flux at constant pressure. One can imagine that the E/O ratio determines the drop size, when the concentration of emulsifier is limited. If this is true, the drop size should be smaller at high E/O ratios, which means that flux increases with decreasing drop size. This is in accordance with the experiments described above concerning the relationship between flux and pressure. If the emulsion droplets are small, the pressure influence on concentration polarization at the membrane is lower than with large droplets. Then the pressure does not coalesce the emulsion droplets at the membrane surface so easily and free pathways for permeate are present. No budding of oil into the permeate takes place either. The effect of oil concentration in the emulsions on flux was also tested with another kind of experiment. This experiment started out with 15 kg of an emulsion containing 2Owt.% oil and O.O2wt.% emulsifier. Then the emulsion was concentrated by discarding the permeate during the experiment. The real E/ 0 ratio during the experiment could not be evaluated as the amount of the
109
2s
0.01
0.03
0.05
0.07
E/O
g/min ‘---I 30.
lx
. E/O \
Fig. 7. Ultrafiltration of O/W emulsions: flux as a function of oil concentration ( X ) and of E/O (0) (p=2.0bar;u=‘0.70ms-‘,pfi~l~).
ratio
L
20
25
xl
co/ wt-70
Fig. 8. Concentration of an emulsion by ultrafiltration; flux vs oil concentration (p=1.5 bar; 0=500 r.p.m.; E/0=0.001 at start; u=O.53 m s-l; pHz 11; flux corrected to 25°C.
discarded emulsifier present in the permeate could not be measured. The results of this experiment are depicted in Fig. 8. It can be seen that flux decreases with increase in oil content. As the experiment was continued overnight some of the flux was restored until stabilization took place. The increase in flux probably arose from the fact that the emulsion partly separated during the night and the stable concentration polarization gel layer at the membrane had not yet developed.
110
Emulsion droplet sizes Emulsion droplet sizes were determined with a Malvern particle analyzer. The emulsions were difficult to analyze because they were too dense. When diluted with pure water the oil separated on the cuvette walls. Dilution seemed to be successful if it was carried out by adding permeate so that the equilibrium between free and bound emulsifier was maintained. Anyway one cannot be quite sure that the droplet sizes will not change upon dilution. For an emulsion containing 2Owt.% oil and O.O2wt.% emulsifier the mean droplet size was about 3.5 pm, which was larger than the droplet sizes ( z 100 nm) reported by Lipp et al. [ 61, but in their experiments the emulsifier concentration was not limiting. The size of the droplets in our study did not change much when the emulsifier content was increased to O.&vt.%. In some of the results droplets of about 70 pm were also observed, which could indicate the start of coalescence of droplets, when stirring is not continued during the Malvern measurements. Extraction experiments performed with aniline The solubility (S) of aniline in water and oil phases was tested at different temperatures. The results are depicted in Fig. 9. As expected the solubility is better in the oil phase at all temperatures but the solubility dependence of aniline in oil on temperature is more pronounced. The distribution factor for aniline between water and oil at 23 ‘C for aniline concentrations <6wt.% was independent of aniline concentration and approximately 0.89 (kg aniline per kg water : kg aniline per kg oil). At the limit of aniline solubility the water phase contained 3.46wt.% aniline and the oil phase 3.9Owt.% aniline. The ultrafiltration extraction experiment was carried out with an emulsion consisting of 13.3wt.% oil and O.Ol&vt.% emulsifier: E/O =0.00115. A 2.2wt.% aniline solution was fed into the emulsion at the same rate as permeate was collected. Table 2 shows the flux variation with aniline concentration in the
20
30
Fig. 9. Solubility
40
5o
t /“c
(S) in wt.% of aniline in oil ( A ) and in water ( 0
) at various temperatures.
111
TABLE 2 Flux of permeate
(tiL,) of an emulsion with an E/O ratio of 0.00115 containing
after adding different well as the distribution
amounts of aniline. The aniline percentage factor, K=aniline
in water phase/aniline
13.3wt.% oil
in the permeate
is reported as
in oil phase
m,
Aniline
Aniline in permeate
(g min-‘)
(wt.%)
(wt.%)
14.6
0.036
0.014
14.4
0.109
13.9
0.179
13.3
0.291
13.0
0.363
12.2
0.469
11.7 11.4
K
18.2
0.133
0.760
0.276
0.726
0.541
0.414
0.723
0.652
0.482
0.736
solution. The permeate concentration of aniline was titrated and for some values distribution coefficients (as above ) were calculated. It is seen that aniline was collected preferentially in the oil phase even more strongly than the distribution data above would have suggested. This could of course depend on the possibility that aniline was retained to some extent by the membrane. Another explanation is that aniline can partly interact with the emulsifier. It can be retained by free micelles in the water phase or by the surfactant at the interfacial layer of the emulsion droplet. At low pH values the aniline would be positively charged and more hydrophilic and then adhere to the outer border line of the micelle or emulsion droplet. At the high pH of the experiment the aniline is more hydrophobic and can be expected to interact in the border line between the hydrophilic and hydrophobic regions of micelles or emulsion droplets. Finally the aniline-containing emulsion (0.652wt.% aniline; 13.3wt.% oil) was concentrated by ultrafiltration. The results are depicted in Fig. 10 as flux, the concentration factor ( = original mass of emulsion/original mass of emulsion - accumulated collected permeate) or the natural logarithm of the concentration factor, all as functions of time. It can be seen that until the concentration factor was 1.5, flux was almost constant. At this point oil and emulsion droplets started to appear in the permeate and the flux decreased. At a concentration factor of 2.3, the flux no longer decreased so quickly. The natural logarithm of the concentration factor
a
--I
I
10
15
t/h cf
:.f.
In
25
1.5
1.0
0.5
1.5
I
15
t/h
Fig. 10. Concentration with UF of an emulsion of oil in water and tall oil as the emulsifier containing 0.652wt.% aniline and 13.3wt.% oil and with an E/O ratio of 0.00115, pH zz 11 (p = 2.5 bar; ~~500 r.p.m.; u=O.71 m s-l). The experiment went on for 4 days and overnight flux increased for some time. (a) ti, as a function of time ( X ). (b) The concentration factor (cf.) (0 ) and the natural logarithm of the concentration factor (A ) as functions of time.
113
vs UF time was linear. The aniline did not seem to disturb the concentration of the emulsion by ultrafiltration. CONCLUSIONS
The above results show that the ultrafiltration of emulsions is possible over a large range of O/W and E/O ratios. The pressures should be kept rather low, -C1.5 bar, and the flow velocities as high as possible to achieve maximal fluxes. The emulsions can be broken by acid because in acidic solution the emulsifier loses its detergent mode. Emulsions with a low content of emulsifier can be broken by salt and in this way a sudden increase in flux is achieved, when the O/W emulsion breaks to a system with an oil and a water phase and residual emulsion, which rises to the top of the vessel and cleans the membrane tube. The extraction and concentration of aniline from an aqueous solution into an emulsion was achieved by feeding the same amount of aniline-containing solution to the emulsion syst,em as was obtained as permeate during ultrafiltration. The aniline was concentrated into the oil phase of the emulsion to a degree which exceeded the expectations based on the distribution factor experiments. This better retention could be explained by the interaction of aniline with the surfactant. This type of extraction in an emulsion with subsequent ultrafiltration seems to be a possible way to clean effluent waters containing minor amounts of harmful small organic molecules. As the emulsions, when filled with the extracted substance, can easily be broken, the concentration can be carried out on a larger scale as a continuous process. Since tall oil seems to be a good emulsifying agent, even at small concentrations, and because it is rather inexpensive and produced on a large scale as a byproduct in the pulp and paper industry, it is economical to use in larger scale processes. ACKNOWLEDGEMENTS
The author thanks Associate Professor Matti Lindstrijm for reading and commenting on the manuscript, and the students Pia Jarvinen, Vesa-Pekka Kangas and Tero Salo for their technical assistance.
REFERENCES 1 2 3 4
F. Rickli, Umweltschutz/Gesundheitstechnik, 4 (1985) 100. S. Lee, Y. Aurelle and H. Roques, J. Membrane Sci., 19 (1984) 23. R.L. Goldsmith, D.A. Roberts and D.L. Burre, J. Water Pollut. Control Fed., 46(9) (1974) 2183. F. Vigo, C. Uliana and P. Lupino, Sep. Sci. Technol., 20(2/3) (1985) 213.
114 5 6 7 8 9 10 11
D. Bhattacharyya, A.B. Jumawan, R.B. Grieves and L.R. Harris, Sep. Sci. Technol., 14(6) (1979) 529. P. Lipp, C.H. Lee, A.G. Fane and C.J.D. Fell, J. Membrane Sci., 36 (1988) 161. H. Schubert and H. Armbruster, Chem.-Ing.-Tech., 61 (1989) 701. S. Qutubuddin, C.A. Miller and T. Fort, Jr., J. Colloid Interface Sci., 101 (1984) 46. R.K. Sharma and N.N. Bakhshi, Tappi, 73(9) (1990) 175. R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, 67th edn, CRC Press, Boca Raton, FL., 1986. G. Jonsson and P.L. Johansen, Proc. 5th World Filtration Congress, Nice, 1990, p. 203.