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A visual study of the breakdown of emulsions in porous coalescers
c7,e,,,id
&,,ker&z
Science, Vol. 40, No.
12, PP. 2339-2350,
ooo9-2509/m s3.co+o.M) Pbxgamon Press Ltd.
1985.
Printed in Great Britain.
A VISUAL
STUDY OF THE BREAKDOWN IN POROUS COALESCERS S. F. MOSES
OF EMULSIONS
and K. M. NGt
Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003, U.S.A. (Received 30 July 1984) AIWmct-A two-dimensional photoetched glass flow cell was used to observe the process by which an emulsion was broken down by a granularporous coalescer.The effectsof wettability,emulsionand collector zeta potentials.and emulsiondroplet sizeon the coalescencephenomenonwerestudied.The possible use of a graded medium or a coalescer with mixed wettability to enhance the coalescence process was also examined. Based on these visual studies, a set of guidelines was developed to aid in the selection of a porous coalescer for a given separation duty.
REVIEW OF THE LITERATURE
INTRODUCTION
A number of techniques are available to break unwanted emulsions which are stable on settlingcentrifugal, electrical and magnetical separations, the use of chemicals and porous coalescers. There are some distinct advantages of porous coalescers over other existing methods; they have relatively low capital cost and allow continuous operation. In a complete coalescence process, the emulsion is first passed through a coarse filter to capture any solid particles in the stream, for otherwise the porous coalescer may get clogged. The particle-free liquids are then pumped through the coalescer, in which the micrometer-sized, suspen&d emulsion droplets are coalesced to form bigger drops. The size of the exit drops becomes sufficiently large that they are susceptible to gravity separation_ Reported applications of porous coalescers include the separation of water from aviation fuel (Bitten, 1970; Bitten and Fochtman, 1971), desalination of crude oil (Burtis and Kirkbride, 1946, Hayes et al., 1949), bilge water treatment (Douglas and Elliot, 1962), separation of emulsified oil (Sareen et al., 1966) and freon (Johnson, 1980) from water. Despite its attractive f&tures, the process is not completely understood as has bcen>repeatedly stressed by many researchers in this area (Spielman and Su, 1977; Sherony et al., 1978; Spielman and Goren, 1970). Considerable disagreement still exists concerning the mechanisms by which suspended emulsion droplets are coalesced and the movement of the resulting larger drops through the coalescer. In order to provide a clear picture of the coalescence phenomenon, the aim of this work was to perform direct visualization of a model porous coalescer in action. Specific questions concerning the effects of zeta potential, wettability and the geometrical characteristics of the coalescer on its performance were also investigated.
tTo whom correspondence should be addressed.
first stage of the development of porous coalescers involved mainly experimental testing and was concerned with spezific applications. Most of the contributions were documented in the patent literature. A review of some of the earlier patents has been presented by Sherony et al. (1978). A survey of recent patents is also available (Langdon and Wasan, 1979). Many materials have been suggested to be used as porous coalescers: glass fibres, glass spheres, Teflon, polyethylene, etc. In the second stage, systematic experiments were performed to investigate the effects of various factors on the performance of porous coalescers. The investigations were mainly qualitative in nature, some of which are described below. Voyutskii et al. (1953, 1958) investigated, among other things, the effect of wettability on coalescence and they claimed that intermediate wettability was the most effective in separating a water-in-oil emulsion. Gudesen (1964) used cotton and glass wool to break oil-in-water emulsions. He found that complete separation was achieved when the flow rate was below a certain critical value. Vinson and Churchill (1970) used fine mesh screens to study the coalescence of organicin-water emulsions. They varied the interfacial tension, and found that the separation efficiency dropped sharply at low interfacial tension values. Moreover, they reported that the performance was insensitive to the viscosity of the dispersed phase. In the work by Sarcen et al. (1966), among other things, bed depth was varied. It was concluded that a long bed enhanced the degree of coalescence. However, no suggestions were made as to what should be the optimal bed depth. Of course, a long bed requires a higher pumping cost. Bitten (1970), in his visualization study of the coalescence of water droplets on single fibres, observed the presence of chain-like structures consisting of individual water droplets piling one on top of another. Bitten and Fochtman (1971) were the first to measure the distribution of the dispersed phase in a porous
2339
The
2340
S. F. Moses
coalescer. Results indicated that the highest retention occurred at the inlet face of the coalescer, followed by a sharp decrease. This was confirmed later by Spielman and Su (1977), who used ‘X-ray absorption techniques to measure the dispersed phase saturation. Hazlett (1969a, b) examined a number of parameters in the breakdown of water-in-aviation fuel emulsions. Included were surfactant concentration, fibre size and material, bed depth, packing density, water content and the flow velocity of the continuous phase. He found that efficiency was not raised by increasing the bed depth beyond a certain length. In beds composed of two different-sized fibres, the small fibres should precede the large ones for good separation. It was concluded that the downstream fibre size has a marked control on separation efficiency. Besides, the surfactants were found to interfere with drop release from the outlet face of the coalescer by limiting droplet growth prior to detachment from the bed. More recently, Jeater et al. (1979) and Davies (1980) reported that phase inversion might occur at the coalescer outlet, forming a liquid membrane structure which broke down to produce a swarm of very small droplets of the dispersed phase. This, of course, has an adverse effect on the subsequent separation by gravity. In Table 1, the qualitative conclusions concerning the various factors that control the performance of a coalescer are summarized. It should be emphasized that these conclusions are based on some specific packing materials and emulsions. Whether or not they are universally true is highly questionable. The aim of this paper is to provide a basic understanding of the entire coalescence phenomenon. As will be seen below, the underlying physical reasons for some of the previous experimental observations can be rationalized. Furthermore, the present results also led to some suggestions for the selection of a coalescer for breaking a given emulsion. EXPERIMENTAL CHMtACTERlZATION
SYSTEM: DESIGN, AND PROCEDURES
A schematic diagram of the experimental setup for flow visualization is shown in Fig. 1. It has the following components: a two-dimensional glass flow
Flow __, .
and K. M. No
Table 1. A summary of the qualitativeconclusionsconcerning the various factors that control the performance of porous CO&SUXS
(I)
The velocity of the continuous (Gudesen, 1964)
phase should be low
(2) High temperature is preferred (Burtis and Kirkbride, 1946) (3) Intermediate wettability appears to be the best (Voyutskii et al.. 1953, 1958). On the contrary, Madia et al. (1976) reported that separation efficiency increased with increasing oil wettabitity of the bed (4) The effect of the viscosity of the dispersed phase on the bed performance is small (Vinson and Churchill, 1970) (5) The ratio of surface to volume of the bed should be high (Voyutskii et al., 1953, 1958) (6) Low interfacial tension is deleterious to bed performance (Vinson and Churchill, 1970) (7) Increased bed depth promotes coalescence but there is a
critical length beyond which the bed performance is not improved any further (Hazlett, 1969a, b)
(8) In a composite bed consistingof fibres of different sizes, the small fibres should precede the larger ones (Hazlett, lQ69a,b)
monitor and video camera, Cell, microscopz, videocassette recorder. A similar setup was used by Payatakes et al. (1981) to observe deposition of solid particles in deep-bed filters. cell (2.54 x 11 cm) was made up of a photoetched glass plate covered from below by a thin, flat glass panel (Fig. 2). The photoetching procedure was that of McKeller and Wardlaw (1982). Any pattern can be inscribed, allowing convenient simulation of the various features of a coalescer such as interconnectivity, the size and shape of flow channels and of the solid matrix. In this study, a centred hexagonal pattern of cylinders was used to stimulate a granular porous coalescer. These cylinders, onto which adhesion of Design
The
of the jbw flow
cell
Two-axis
Videocossett
Fig. 1. Schematic diagram of experimental setup.
Breakdown Flow
characteristic cup shape (Fig 3), signifying the adsorption of the anions on the surface of the emulsion droplet at low electrolyte concentration and suppression of the diffuse layer as the electrolyte concentration increased.
Top view
cell
2341
of emulsions in porous coalescers
Surjbce characterization of the flow cell Characterization of the glass flow cell was more involved_ Since it is not possible to measure the collector zeta potential I& directly, it was estimated by a streaming potential experiment. Soda-lime glass spheres with mesh size of 35 x 45, representing the glass flow cell, were packed into a glass capillary tube of 4 mm in diameter. An electrolyte solution of specified concentration was forced through the packed tube under a constant pressure drop. The streaming potential was measured with two reversible calomel electrodes and an electrometer (General Radio Co.). The collector zeta potential is related to the applied pressure drop, Ap, by (see Heimenz, 1977; Petrie, 1980):
0~7
cutowoy
v,ew
cover
Fig. 2. Top and cutaway views of the flow cell. (Not drawn to scale, and only a fraction of all the cylinders are shown.)
emulsion particles takes place, will also be referred to as collectors. The diameter of each cylindrical collector was 1030 pm and the size of the throat, i.e. the shortest distance of the space between two collectors, was 29Opm in most of the visualization experiments reported below. The height of each collector was controlled by the etching time and was approximately 100 firn in all cases. This model porous medium has a porosity of 0.45. It should also be noted that the circular surface of each cylinder was not perfectly even but tapered somewhat towards the edge. Hence, associated with each collector, there is a central area where the collector is in excellent contact with the flat glass plate followed by a small gap near the edge. This imperfection in etching turns out to be a blessing, as it simulates a contact point between the packings in an actual granular porous medium.
Emulsion preparation and characterization Emulsions of 0.5 0A by volume of silicon oil (specific gravity = 0.91; viscosity = 5 cs) in water were used in the experiments, with either sodium chloride, potassium sulfate or sodium pyrophosphate as the electrolyte. The emulsification was carried out in a blender or a Gaulin homogenizer. Photomicrographs showed that, by adjusting the blending time and speed of the blender, or the running time and pressure of the homogenizer, both are capable of producing relatively uniform-sized emulsion droplets in the micrometer range. The zeta potential of the emulsion, c,, was readily measured with a zeta meter (Laser Zee 501). The entire range of zeta potentials, from delicately stable to very stable, was covered. All zeta potential curves for the three different electrolytes displayed a
where p and k are the viscosity and specific conductivity of the solution, 0 is the streaming potential and E is the dielectric constant. Figure 4 shows the collector zeta potentials for both the water-wet and oil-wet media used in the visualization experiments at various sodium chloride concentrations. Direct determination of the wettability of the glass flow cell is difficult. Instead, water was allowed to intrude into a glass capillary tube filled with silicon oil. It was found that (as was evidenced by the photomicrographs) water preferentially wets the glass. The exact value of the contact angle, measured through the water phase, was difficult to obtain because of the
-50
-
-7o-
v A NoCl q K2 SO,
0 NoqPeO,
-SC0
I I
I 2 -log
I 3
I 4
I s
moLor ity
Fig. 3. Emulsion zeta potential vs. electrolyte concentration for various electrolytes.
2342
S. F. M~~E.s and K. M. NG
-60
I
Oil nonwetting
medium
I
l-l t 2
-I
-a0 0
droplets were clearly visible under an inverted microscope, to which a video camera and a video monitor were connected to allow easy, continuous viewing. In addition, a videocassette recorder was available to document the observations. An x-y gliding stage attached to the inverted microscope permitted convenient observation of any location inside the flow cell. A more detailed description of the experimental procedures can be found in Moses (1984).
-100 Ln -._’
07
I 00012
0
I 00025 Molority
I 0.005
(N&l)
Fig. 4. Collector zeta potential vs. sodium chloride concentration for the water-wet and oil-wet media.
uncertainty in locating the line tangent to the water-oil interface through the contact point. Nonetheless, it is about 57 f 2’ according to our estimation. The flow cell can be changed from water-wet to oil-wet by coating the surf&es with a surface modifier (3M FC723). In that case, the contact angle, again measured through the water phase, is 154 f 2”.
Experimental
PLOW CELL EXPERIMENTS
Q
procedures
In each experiment, after characterization, the emulsion was pumped through the flow cell at a specified volumetric flow rate with a gear pump (Zenith-Nichols). The micrometer-sized emulsion
The experiments were designed to isolate the effects of the various factors-emulsion and -%ollector zeta potentials, collector size, wettability and initial emulsion droplet sizeon the coalescence process. A summary of the experiments is given in Table 2; they can be classi6ed into four different categories. The first three categories are concerned with the effxt of The classifications emulsion zeta potential. delicately stable, stable and very stable-refer to the stability of the emulsion in flow through the coalescer and not to the traditional classification of emulsion stability on standing alone. No appreciable coalescence occurred to any of the emulsions used in the visualization experiments when they were simply left in the emulsion tank for several days. The effect of wettability was investigated in each of these three classes. Three different flow cells, all of size 2.54 x 11 cm, were used in this study. The regular one had collectors of 1030 pm in diameter; the small-size one had collectors of 431 pm in diameter. The third flow cell was a graded medium, consisting of three sections of equal length. The collector diameter in the first section at the inlet was 431 pm, and 731 and 1240 pm in the
Table 2. A s ummary of the experiments performed Experiment No. Delicately stable
Stable
Very
1
C, (mV)
r, (mv)
Medium characteristics
Observations
-10
-62
Water-wet, regular size
Adhesion and coalescence of emulsion
2A
-10
-144
Oil-wet, regular size
2B
-6
-150
Oil-wet, small size
3
-17
-67
4
-17
- 122
-20
-62
stable
Special purpose experiments
9
-20
-122
-17
-68
-5.5
-68
-6
Water-wet, regular size Oil-wet, regular size Water-wet,
regular size
Oil-wet, regular size Water-wet,
regular size
Water-wet, graded medium - 150/ - 68 Graded medium with changing wettability
droplets;
blob
formation,
motion, coalescence and breakup Adhesion and coalescence of emulsion droplets Formation, re-entrainment and redeposition of clusters Flowing rivulets consisting of filaments, tongues and pockets Formation, re-entrainment and redeposition of clusters No blob formation A lower rate of adhesion than in experiment 3 A significantly lower rate of adhesion than in experiment 3 Practically no adhesion Larger emulsion droplet sizes enhanced the rate of adhesion Small pore chambers should precede larger ones in a graded medium Rivulets changed to blobs as wettability changed along the coalescer
Breakdown of emulsions in porous coalescers
second and third regions. Since all flow cells were etched with masks based on the same original pattern, they are geometrically similar. The fourth category consisted of three experiments. Experiments 8 and 9 had the objective of investigating the advantages of using a graded medium, either with uniform or mixed wettability. In experiment 9, the first section was oil-wet while the rest of the graded flow cell was water-wet. Experiments 1 through 6 and 9 used emulsions with a droplet size of about 7 pm. A larger droplet size (14 pm) was used in experiments 7 and 8. All experiments had a volumetric flow rate of 0.7 cn?/min at the beginning, except experiment 1 which was at 0.29 cm3/min. In all cases, the Reynolds number, defined as ds u/v(Ia) (see, for example, Bird ef al., 1960) was less than 10, indicating a laminar flow regime. Although the gear pump was designed to deliver a constant vohrmetric flow rate, the flow rate dropped about O-14.2 cm3/min when there was significant clogging by the coalesced emulsion droplets inside the coalescer. The key observations in the visualization experiments are described below. Experiment 1 Adhesion became observable in about 10 min, with a decreasing level of adhesion along the length of the cell.
2343
Figure 5 shows how it occurred; the flow in these and other pictures in this paper is from left to right. Suspended emulsion droplets collided and adhered to the region near the stagnation point of the collector on the right [Fig. S(a)]_ As time progressed, more and more adhesion took place and the adhered droplets coalesced among themselves. Although most of the adhered droplets on the vertical cylindrical surface were not easy to observe, a few larger drops were clearly visible [Fig. S(b)]. The pictures of Fig. 5 were taken at the third row of the coalescer. Before more adhesion could take place, coalesced drops from the first two rows had already begun migrating downstream. Indeed, at a later time, some large drops could be seen at a throat in the second row [Fig. S(c)]. Figure 6 is a time sequence of pictures showing the fate of the coalesced drops. Further coalescence took place [Figs 6(a) and 6(b)]; finally, a blob was formed [Fig. 6(c)). (In this paper, the original emulsion oil particles are referred to as droplets. Drops are coalesced droplets, either suspended or attached to a collector, whose sixes are small compared to that of the throats. Large nonwetting drops filling completely a throat or one or more pore chambers are called blobs.) The coalesced oil phase continued the journey to the outlet of the coalescer through drop detachment and
Fig. 5. Time sequence showing adhesion in experiment 1. (a) The collector on the right-hand side of the picture was relatively free of adhered droplets. (b) At a later time, an increase in the amount of adhered droplets was clearly visible, and the adhered droplets began to coalesce among themselves. (c) Coalesced drops from upstream collectors were migrating through the pore throat on the left-hand side of the picture.
2344
S. F. MOSES and K. M. No
Fig. 6. Time sequence showing blob formation. (a) Adhered drops at a pore throat. (b) Coalescence took place. (c) Further coalescence resulted in a blob.
redeposition, as well as blob flow. Figure 7 illustrates the former event. A small drop at the top left-hand corner of Fig. 7(a) was pulled out from a group of adhered drops. It first became elongated [Fig. 7(b)] and then followed by detachment, migration through a throat and coalescence with a blob downstream [at the middle top of Fig. 7(c) J. Breakup prevents blobs from growing indefinitely, as is illustrated in Fig. 8. Figure 8(a) shows the initial shape of a blob on the lefthand side of the picture. As the blob was set in motion by the viscous pressure of the continuous emulsion phase, a water collar began to appear and grow at the pore throat [Figs 8(b) and 8(c)]. This blob then broke up into two daughter blobs [Fig. 8(d)]. The time duration for the entire episode was about 1 s. The various aspects of blob mechanics, namely, the motion, breakup and coalscence of oil blobs, have been the subject of extensive research in the area of enhanced oil recovery (Roof, 1970, Ng et al., 1978; Oh and Slattery, 1979; Payatakes et al., 1980; Ng and Payatakes, 1980; Mohanty et al., 1980). interested readers are referred to the references for further discussions. The region occupied by blobs expanded down the coalescer as the experiment progressed, which was stopped after 2 h. Experiment 2A Similar adhesion was observed in the first 10 min of the experiment. However, in contrast to experiment 1,
on coalescence, the adhered droplets migrated towards the gap next to the contact area under each cylindrical collector, due to capillary effect. Figure 9 shows the increase in the amount of oil trapped in the gaps of two collectors.
Experiment
2B
The small-size flow cell was used in this experiment. Similar to experiments 1 and 2A, considerable adhesion took place in the first several minutes of running time. A major difference was that the adhered droplets, instead of coalescing, formed clusters, which appeared to consist of a large number of oil droplets separated by water films. Such clusters at a relatively clean spot in the flow cell are shown in Fig. 10(a). The photograph was taken at about 10 min into the experiment. Within the next couple of minutes, the throat on the left-hand side of the picture was clogged [Fig. 10(b)] and then reopened by re-entrainment of the clusters [Fig. 10(c)]. At 20 min, most of the clusters disappeared and were replaced by coalesced oil in the gaps, similar to that in experiment 2A. A plausible explanation for this observation is as follows. There are two time scales in the coalescence process: one for the capture of flowing emulsion droplets by the collectors and one for the coalescence among the adhered droplets. At the same running time, the number of captured droplets was greater in experiment 2B than in
Breakdown of emulsions in porous coalesccrs
2345
Fig. 7. Time sequence showing entrainment and redeposition of an adhered drop in experiment 1. (a) The adhered drop was initially located at the top left-hand comer of the photograph. (b) It gradually elongated due to the flowing continuous phase. (c) Detachment occurred and the drop migrated through a throat and coalesced with a blob at the middle top of the picture.
experiment 2A, due to the larger collector surface area. The coalescence time scale .was, however, approximately the same in both experiments as it is mainly governed by the stability of the emulsion. In experiment 2B, the rate of increase of adhered droplets was faster than the rate of assimilating them and thus the formation of clusters. Unlike experiment 1 in which the coalesced oil phase was in the form of blobs, a wide variety of wetting structures were observed in this experiment: oil filaments connecting two collectors, and oil tongues protruding from the contact area of the collector into the throat area [Fig. 11(a)]. The oil tongues usually grew and moved downstream. A filament would form if the oil tongue spread to another collector. Figure 1l(b) shows a triangular wetting structure connecting three collectors; an oil pocket occupying two pore chambers is shown in Fig. 1 l(c). These. wetting structures were not in isolation, as can be seen in a low-magnification view of the coalescer. Many wetting structures were connected with one another through the collectors to form oil rivulets, which were approximately in alignment with the axial direction of the flow cell (Fig. 12). Instead of moving only as long, snake-like masses of oil, the rivulet merged into much wider oil pockets
occupying a number of pore chambers. The migration of an oil pocket is shown in Fig. 13, which was observed at 2 h of running time. The downstream part of the pocket continued to surround more and more collectors and the upstream end receded from other posts. Review of the videotape revealed that the receding interfaces were concave while the advancing interfaces were convex and somewhat irregular in shape, probably indicating an advancing contact angle different from the equilibrium value of 26”. Experiment 3 Clusters similar to those observed in the first 10 min of experiment 2B were the dominant feature of this experiment. Like the other experiments, there was a decreasing level of adhesion with distance down the coalescer. Throughout the entire experiment, formation, re-entrainment and redeposition of clusters were observed. After 3.5 h, tracks which were aligned with the direction of flow and devoid of clogged throats became apparent. The emulsion followed these relatively straight and unobstructed tracks through the flow cell. Most clusters resided at pore throats between the flow tracks, where the fluid was relatively stagnant. Observations after almost 6 h still showed no sign of coalescence among the droplets within the clusters.
2344
S. F. Moses
and K. M. No
Fig. 9. Time sequence showing accumulation of the coalesced oil phase in the gap between the collector and the llat glass panel in experiment 2A. (a) At the beginning. (h) At a later time.
Experiment 4 The outcome was almost identical to that of experiment 3. The major and interesting difference was that the rate of adhesion in this experiment was noticeably lower than that of the companion nonwetting experiment. The possible explanation is that the collector zeta potential was more negative in experiment 4, - 122 mV compared to - 67 mV, thus resulting in a lower coliection efficiency in this experiment. In addition, contrary to experiment 3, some coalesced oil phase was observed despite the fact that less clusters were present. It seems that the accumulation of adhered droplets at the gaps of the collectors enhanced the probability of coalescence.
Fig. 8. Time sequence showing blob breakup in experiment 1. (a) Initial shape on the left-hand side of the picture. (b, c) As the blob was set in motion by the viscous pressure of the continuous phase, a water collar began to appear and grow at the pore throat. (d) Snap-off occurred and two daughter blobs were formed.
Experiment 5 This flow cell showed a significantly lower rate of adhesion than that of experiment 3. The lower collection efficiency was probably due to the more negative emulsion zeta potential, -20 mV compared to - 17 mV. This was somewhat negated by the less negative collector zeta potential. The overall effect, however, resulted in less adhesion. In addition, similar experiments were performed with emulsion zeta potentials at - 30, - 50 and - 88 mV, absolutely no adhesion was observed for a prolonged period of time.
Breakdown
2347
of emulsions in porous coalescers
Fig. 10. Time sequence showing formation, re-entrainment and redeposition of the clusters in experiment 28. (a) Clusters were found at a relatively clean spot in the Row cell. (b) A throat on the left-hand side of the picture was clogged by clusters. (c) Then it was reopened by entrainmentof the clusters.
Experiment 6 This flow cell ran for 5 h with essentially no adhesion. Combination of experiments 5 and 6 confirmed the conjecture put forward in experiments 3 and 4 that, in an identical environment, the wetting flow cell had a Iower collection efficiency due to a more negative collector zeta potential. Furthermore, both experiments 5 and 6, in comparison with experiments 3 and 4, respectively, indicated the same conclusion, that there is a critical value of the emulsion zeta potential, about - 20 mV in this experimental system, beyond which the collection efficiency drops precipitously. size The purpose of experiment 7 was to investigate whether or not a larger droplet size at the inlet of the cell would result in enhanced coalescence. This experiment was identical to experiment 3 except for an increase of droplet size from 7 to 14 q. Comparison of the two experiments at the same running time showed that there was a higher level of adhesion in this experiment. The reasoning for this observation is that the larger droplets would enhance the collection efficiency through inertial impaction and interception.
Effect of emulsion droplet
CJnt@ormvs. graded media In all of the previous water-wet medium experiments it was observed that blobs usually did not exceed the size of one pore chamber because of the frequent breakup at the throats. We surmised, with hints from previous investigations (Hazlett, 1%9a, b), that larger pore chambers might allow further blob growth. This idea was tested out in experiment 8. After 1.5 h, the blob region extended to the sixteenth row out of about 100 rows of the first section of the flow cell. By 6 h running time, some drops had moved into the second section of the coal-r. The experiment was stopped after 47 h, with blobs in the second section visibly larger than those in the first. It should be noted that larger droplet size (14 m) and less negative emulsion zeta potential ( - 5.5 mV) were employed in this experiment, in comparison with those in experiment 1, simply to cut down on the run time.
Effect of wettability
change within the medium
Previous investigations (Jeater et al., 1979; Davies, 1980) indicated that the way in which the coalesced oil phase exits the coalescer might affect the performance of a porous coalescer significantly. A large amount of the coalesced phase accumulated at the outlet would
2348
S. F. MOSES and K. M. NG
Fig. 11. Various wetting structures of the coalesced oil phase were observed in experiment 2B. (a) Oil filament connecting two collectors and an oil tongue protruding from a collector into the throat area. (b) A triangular wetting structure connecting three collectors. (c) An oil pocket occupying two pore chambers.
nnnnn Fig. 13. Migration of an oil pocket by advancing interfaces at the front and receding interfaces at the back. The advancing contact angle was different from the equilibrium value. Fig. 12. Low-rnagni6cation view of the wetting structures which were connected with one another through the collectors to form oil rivulets, which were aligned with the direction of flow.
cause phase inversion. In practice, it was found that putting a sock, nonwetting with respect to the dispersed phase, at the outlet of the coalescer helps. This empirical conclusion was investigated in experiment 9 with a graded medium, the first section of which was oil-wet while the rest was water-wet. During this experiment, when the coalesced oil phase reached the second section, it changed from rivulets to blobs
(Fig. 14). The transition was very smooth. The rivulets protruded from the smaller pore spaces into the larger oil-nonwetting pore chambers. Snap-off took place and the blob was formed. This experiment seems to indicate that the blobs are likely to exit the coalescer whole, whereas rivulets tend to hold on to the collectors and form drops with sixes smaller than that of a pore chamber. DISCUSSION The visual studies have provided
a porelevel physical picture to understand some of the more important
Breakdown of emulsions in porous coalescers
2349 NOTATION
%l k
collector
diameter
specific conductivity
supefficial velocity porosity of coalescer pressure drop dielectric constant collector zeta potential emulsion zeta potential dynamic viscoSity kinematic viscosity streaming potential
Fig. 14. The effect of changing wettability was observed in a graded medium in experiment 9. The section with small collectors was oil-wet while the section with large collectors was water-wet. The coalesced oil phase changed from oil rivulets to oil blobs at the junction.
in Fig. 1 which are based on experimental testing. In addition, some suggestions can be made for the selection of a porous coalescer for separating a given emulsion:
conclusions
A less negative, or even positive, collector zeta potential is preferred in order to obtain a higher rate of adhesion (experiments 3 and 4). (2) A higher surface-to-volume ratio would also enhance the rate of adhesion (experiments 2A and 2B). This is in agreement with other investigators (Voyutskii et al., 1953, 1958). (3) The observation that blob size did not grow much beyond the size of a pore chamber lends support to the conclusion (Hazlett, 1969a, b) that increased bed length might not lead to better separation. Hence, the coalescer should be just long enough to form blobs; any extra length simply incurs a higher pumping cost and does not result in any improvement in separation efficiency. (4) In case a graded medium is used, small pore chambers should precede the larger ones (experiment 8; Hazlett, 1969a, b). (5) The wettability at the outlet of the coalescer should be nonwetting to the dispersed phase (experiment 9; Jeater et al., 1979).
(1)
Although these visual studies have advanced our understanding of the coalescence process and would serve as the basis for quantitative theoretical work, some key questions remain unanswered. For example, which is better, a fibrous bed or a granular medium, for the breakdown of an emulsion? Are similar blob rivulets present in a fibrous bed? These questions and quantitative analysis should be included in future studies. work was performed under Science Foundation Grant National CPE-8112824. Availability of a zeta meter from Professor R. L. Rowe11 is gratefully acknowledged.
Acknowledgements-This
REFERENCES Bird, R. B.. Stewart, W.
E. and Lightfoot,
E. N.,
1960,
Transport Phenomena. Wiley, New York. Bitten. J. F., 1970,Coalescence of water drodets on sinale f&e&. J. &lloid Inter&e Sci. 33,265-271: Bitten, J. F. and Fochtman, E. G., 1971, Water distribution in fiber-bed coalescers. J. Colloid Interfac Sci. 37, 312-3 17. Burtis, T. A. and Kirkbride, C. G.. 1946, Desalting of petroleum by use of fiberglass packing. Trans. A.X.Ch.E. 42, 413439. Davies, G. A., 1980, Coalescence of dismzrsions in rrsrticulate and fibre l&s-&ate of the art. Pa@ presented& the 73rd A.1.Ch.E. Annual Meeting, Chicago, IL. Douglas, E. and Elliot. I. G., 1962. Developments in oilywater separator design. Trans. Inst. Mar. %ngrs 74, 164:
Gudesen. R. C.. 1964,Coalescenceof ueetroleum comDounds in m&d fibrous beds. MS. Thesis, Illinois Ins&ute of Technology, Chicago, IL. Hayes, J. G., Hays, L. A. and Wood, H. S., 1949, Commercial fiberglass as contacting agent. desalting unit employing Chem. Erylm _ - Prw. 45.235-240. Hazlett, R. N., 19698; Fibrous bed coalescence of water. Ind. Engng C&m. Fundaw~ & 625-632. Hazlett, R. N., 1%9b, Fibrous bed coalescence of water. Ind.
Engng Gem. Hiemenz,
Fundam. a633-640.
P. C., 1977, Principles of Colloid and Surjbce Chemistry. Marcel Dekker, New York. Jeater, P., Rushton, E. and Davies, G. A., 1979, Coalescence of secondary dispersions in fibre bed coalescers. Proc. 2nd World Filtr. Congr. 605615. Johnson, R. A., 1980, Use of coalescers and settling tubes to remove dispersed liquid. Xnd. Engrtg Chem. Fundam. 19, 71-7.5. Langdon, W. M. and Wasan, D. T., 1979, Separation of organic dispersions from aqueous medium by fibrous bed coalescence. Recent Developments in Separation Science, Vol. v, pp. 159-184. McKelier, M. and Wardlaw, N. C., 1982, A method of making two-dimensional glass micromodels of pore systems. J. Can. Pet. Tech. 21, 3941. Madia, J. R., Fruh, S. M., Miller, C. A. and Beerbower, A., 1976, Granular packed bed coalescer: inlluence of packing wettability on coalescence. Environ. Sci. Tech. 10, 1044-1046. Mohanty, K. K., Davis, H. T. and Striven, L. E., 1980, Physics of oil entrapment in water-wet rock. Paper presented at S.P.E. 55th Annual Meeting, Dallas, TX. Moses, S. F., 1984, A visual study of emulsion coalescence in porous media. M.S. Thesis, University of Massachusetts, Amherst, MA. Ng, K. M., Davis, H. T. and &riven L. E., 1978, Visualization of blob mechanics in flow through porous media. Chem. Engng Sci. 33,100+1017. Ng, K. M. and Payatakes, A. C., 1980, Stochastic simulation of the motion, breakup and stranding of oil ganglia in water-wet granular porous media during immiscible dis-
2350
S. F. Moses and K. M. NG
placement. A.1.Ch.E. J. 26.419429. Oh, S. G. and Slattery, J. C., 1979,Interfacial tension required for sign&ant displacement of residual oil. S.P.E. J. 19, 83-96. Payatakes. A. C., Ng, K. M. and Flumerfelt, R. W., 1980, Gil ganglion dynamics during immiscible displacement: model formulation, A.1.CA.E. J. 26.43-3. Payatakes, A. C., Park, H. Y. and Petrie, J.. 1981, A visual study of particle deposition and reentrainment during depth filtration of hydrosols with a polyelectrolyte. Chem. Engng Sci. 36. 1319-1335. Petrie, J., 1984 Enhancement of depth filtration efficiency by polyelectrolytes. MS. Thesis, University of Houston, Houston, TX. Roof, J. G.. 1970. Snap-off of dronlets in water wet nores. S.P.E. J. 10,SS-90.Sareen, S. S., Rose, P. M., Gudesen, R. C. and Kintner, R. C., 1966, Coalescence in fibrous beds. A.I.Ch.E. J. 12, 1045-1050.
Sherony, D. F., Kintner, R. C. and Wasan, D. T., 1978, Coalescence of secondary emulsions in fibrous beds. Surfme Colloid Sci. 10, 99-161. Spielman. L. A. and Goren. S. L., 1970, Proaress in induced ‘coalesc&ce and a new -theoretical framework for coalescence by porous media Ind. Engng Chem. 62, 10-24. Spielman, L. A and Su, Y. P., 1977. Coalescence of oil-inwater suspensions by flow through porous media, Ind. Engng Chem. Fundam. 16.272-282. Vinson, C. G. and Churchill, S. W., 1970, Removal of drops from liquid-liquid dispersions upon flow through screens. Chem Engng J. 1. 110-119. Voyutskii, S. S., Fodiman, N. M. and Panich, R. M., 1958, Filtration of emulsions, Izu. Yyssh. Ucheb. &wed. Khim. Khim. Tekhnol. 2, 170-179. Voyutskii, S. S., Kalyanova, K. A., Panich, R. M. and Fodiman, N. M., 1953, Mechanism of separation of the disperse phase of emulsions during filtration. Dokl. Akd. Nauk SSSR 91, 1155-l 158.
&,,ker&z
Science, Vol. 40, No.
12, PP. 2339-2350,
ooo9-2509/m s3.co+o.M) Pbxgamon Press Ltd.
1985.
Printed in Great Britain.
A VISUAL
STUDY OF THE BREAKDOWN IN POROUS COALESCERS S. F. MOSES
OF EMULSIONS
and K. M. NGt
Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003, U.S.A. (Received 30 July 1984) AIWmct-A two-dimensional photoetched glass flow cell was used to observe the process by which an emulsion was broken down by a granularporous coalescer.The effectsof wettability,emulsionand collector zeta potentials.and emulsiondroplet sizeon the coalescencephenomenonwerestudied.The possible use of a graded medium or a coalescer with mixed wettability to enhance the coalescence process was also examined. Based on these visual studies, a set of guidelines was developed to aid in the selection of a porous coalescer for a given separation duty.
REVIEW OF THE LITERATURE
INTRODUCTION
A number of techniques are available to break unwanted emulsions which are stable on settlingcentrifugal, electrical and magnetical separations, the use of chemicals and porous coalescers. There are some distinct advantages of porous coalescers over other existing methods; they have relatively low capital cost and allow continuous operation. In a complete coalescence process, the emulsion is first passed through a coarse filter to capture any solid particles in the stream, for otherwise the porous coalescer may get clogged. The particle-free liquids are then pumped through the coalescer, in which the micrometer-sized, suspen&d emulsion droplets are coalesced to form bigger drops. The size of the exit drops becomes sufficiently large that they are susceptible to gravity separation_ Reported applications of porous coalescers include the separation of water from aviation fuel (Bitten, 1970; Bitten and Fochtman, 1971), desalination of crude oil (Burtis and Kirkbride, 1946, Hayes et al., 1949), bilge water treatment (Douglas and Elliot, 1962), separation of emulsified oil (Sareen et al., 1966) and freon (Johnson, 1980) from water. Despite its attractive f&tures, the process is not completely understood as has bcen>repeatedly stressed by many researchers in this area (Spielman and Su, 1977; Sherony et al., 1978; Spielman and Goren, 1970). Considerable disagreement still exists concerning the mechanisms by which suspended emulsion droplets are coalesced and the movement of the resulting larger drops through the coalescer. In order to provide a clear picture of the coalescence phenomenon, the aim of this work was to perform direct visualization of a model porous coalescer in action. Specific questions concerning the effects of zeta potential, wettability and the geometrical characteristics of the coalescer on its performance were also investigated.
tTo whom correspondence should be addressed.
first stage of the development of porous coalescers involved mainly experimental testing and was concerned with spezific applications. Most of the contributions were documented in the patent literature. A review of some of the earlier patents has been presented by Sherony et al. (1978). A survey of recent patents is also available (Langdon and Wasan, 1979). Many materials have been suggested to be used as porous coalescers: glass fibres, glass spheres, Teflon, polyethylene, etc. In the second stage, systematic experiments were performed to investigate the effects of various factors on the performance of porous coalescers. The investigations were mainly qualitative in nature, some of which are described below. Voyutskii et al. (1953, 1958) investigated, among other things, the effect of wettability on coalescence and they claimed that intermediate wettability was the most effective in separating a water-in-oil emulsion. Gudesen (1964) used cotton and glass wool to break oil-in-water emulsions. He found that complete separation was achieved when the flow rate was below a certain critical value. Vinson and Churchill (1970) used fine mesh screens to study the coalescence of organicin-water emulsions. They varied the interfacial tension, and found that the separation efficiency dropped sharply at low interfacial tension values. Moreover, they reported that the performance was insensitive to the viscosity of the dispersed phase. In the work by Sarcen et al. (1966), among other things, bed depth was varied. It was concluded that a long bed enhanced the degree of coalescence. However, no suggestions were made as to what should be the optimal bed depth. Of course, a long bed requires a higher pumping cost. Bitten (1970), in his visualization study of the coalescence of water droplets on single fibres, observed the presence of chain-like structures consisting of individual water droplets piling one on top of another. Bitten and Fochtman (1971) were the first to measure the distribution of the dispersed phase in a porous
2339
The
2340
S. F. Moses
coalescer. Results indicated that the highest retention occurred at the inlet face of the coalescer, followed by a sharp decrease. This was confirmed later by Spielman and Su (1977), who used ‘X-ray absorption techniques to measure the dispersed phase saturation. Hazlett (1969a, b) examined a number of parameters in the breakdown of water-in-aviation fuel emulsions. Included were surfactant concentration, fibre size and material, bed depth, packing density, water content and the flow velocity of the continuous phase. He found that efficiency was not raised by increasing the bed depth beyond a certain length. In beds composed of two different-sized fibres, the small fibres should precede the large ones for good separation. It was concluded that the downstream fibre size has a marked control on separation efficiency. Besides, the surfactants were found to interfere with drop release from the outlet face of the coalescer by limiting droplet growth prior to detachment from the bed. More recently, Jeater et al. (1979) and Davies (1980) reported that phase inversion might occur at the coalescer outlet, forming a liquid membrane structure which broke down to produce a swarm of very small droplets of the dispersed phase. This, of course, has an adverse effect on the subsequent separation by gravity. In Table 1, the qualitative conclusions concerning the various factors that control the performance of a coalescer are summarized. It should be emphasized that these conclusions are based on some specific packing materials and emulsions. Whether or not they are universally true is highly questionable. The aim of this paper is to provide a basic understanding of the entire coalescence phenomenon. As will be seen below, the underlying physical reasons for some of the previous experimental observations can be rationalized. Furthermore, the present results also led to some suggestions for the selection of a coalescer for breaking a given emulsion. EXPERIMENTAL CHMtACTERlZATION
SYSTEM: DESIGN, AND PROCEDURES
A schematic diagram of the experimental setup for flow visualization is shown in Fig. 1. It has the following components: a two-dimensional glass flow
Flow __, .
and K. M. No
Table 1. A summary of the qualitativeconclusionsconcerning the various factors that control the performance of porous CO&SUXS
(I)
The velocity of the continuous (Gudesen, 1964)
phase should be low
(2) High temperature is preferred (Burtis and Kirkbride, 1946) (3) Intermediate wettability appears to be the best (Voyutskii et al.. 1953, 1958). On the contrary, Madia et al. (1976) reported that separation efficiency increased with increasing oil wettabitity of the bed (4) The effect of the viscosity of the dispersed phase on the bed performance is small (Vinson and Churchill, 1970) (5) The ratio of surface to volume of the bed should be high (Voyutskii et al., 1953, 1958) (6) Low interfacial tension is deleterious to bed performance (Vinson and Churchill, 1970) (7) Increased bed depth promotes coalescence but there is a
critical length beyond which the bed performance is not improved any further (Hazlett, 1969a, b)
(8) In a composite bed consistingof fibres of different sizes, the small fibres should precede the larger ones (Hazlett, lQ69a,b)
monitor and video camera, Cell, microscopz, videocassette recorder. A similar setup was used by Payatakes et al. (1981) to observe deposition of solid particles in deep-bed filters. cell (2.54 x 11 cm) was made up of a photoetched glass plate covered from below by a thin, flat glass panel (Fig. 2). The photoetching procedure was that of McKeller and Wardlaw (1982). Any pattern can be inscribed, allowing convenient simulation of the various features of a coalescer such as interconnectivity, the size and shape of flow channels and of the solid matrix. In this study, a centred hexagonal pattern of cylinders was used to stimulate a granular porous coalescer. These cylinders, onto which adhesion of Design
The
of the jbw flow
cell
Two-axis
Videocossett
Fig. 1. Schematic diagram of experimental setup.
Breakdown Flow
characteristic cup shape (Fig 3), signifying the adsorption of the anions on the surface of the emulsion droplet at low electrolyte concentration and suppression of the diffuse layer as the electrolyte concentration increased.
Top view
cell
2341
of emulsions in porous coalescers
Surjbce characterization of the flow cell Characterization of the glass flow cell was more involved_ Since it is not possible to measure the collector zeta potential I& directly, it was estimated by a streaming potential experiment. Soda-lime glass spheres with mesh size of 35 x 45, representing the glass flow cell, were packed into a glass capillary tube of 4 mm in diameter. An electrolyte solution of specified concentration was forced through the packed tube under a constant pressure drop. The streaming potential was measured with two reversible calomel electrodes and an electrometer (General Radio Co.). The collector zeta potential is related to the applied pressure drop, Ap, by (see Heimenz, 1977; Petrie, 1980):
0~7
cutowoy
v,ew
cover
Fig. 2. Top and cutaway views of the flow cell. (Not drawn to scale, and only a fraction of all the cylinders are shown.)
emulsion particles takes place, will also be referred to as collectors. The diameter of each cylindrical collector was 1030 pm and the size of the throat, i.e. the shortest distance of the space between two collectors, was 29Opm in most of the visualization experiments reported below. The height of each collector was controlled by the etching time and was approximately 100 firn in all cases. This model porous medium has a porosity of 0.45. It should also be noted that the circular surface of each cylinder was not perfectly even but tapered somewhat towards the edge. Hence, associated with each collector, there is a central area where the collector is in excellent contact with the flat glass plate followed by a small gap near the edge. This imperfection in etching turns out to be a blessing, as it simulates a contact point between the packings in an actual granular porous medium.
Emulsion preparation and characterization Emulsions of 0.5 0A by volume of silicon oil (specific gravity = 0.91; viscosity = 5 cs) in water were used in the experiments, with either sodium chloride, potassium sulfate or sodium pyrophosphate as the electrolyte. The emulsification was carried out in a blender or a Gaulin homogenizer. Photomicrographs showed that, by adjusting the blending time and speed of the blender, or the running time and pressure of the homogenizer, both are capable of producing relatively uniform-sized emulsion droplets in the micrometer range. The zeta potential of the emulsion, c,, was readily measured with a zeta meter (Laser Zee 501). The entire range of zeta potentials, from delicately stable to very stable, was covered. All zeta potential curves for the three different electrolytes displayed a
where p and k are the viscosity and specific conductivity of the solution, 0 is the streaming potential and E is the dielectric constant. Figure 4 shows the collector zeta potentials for both the water-wet and oil-wet media used in the visualization experiments at various sodium chloride concentrations. Direct determination of the wettability of the glass flow cell is difficult. Instead, water was allowed to intrude into a glass capillary tube filled with silicon oil. It was found that (as was evidenced by the photomicrographs) water preferentially wets the glass. The exact value of the contact angle, measured through the water phase, was difficult to obtain because of the
-50
-
-7o-
v A NoCl q K2 SO,
0 NoqPeO,
-SC0
I I
I 2 -log
I 3
I 4
I s
moLor ity
Fig. 3. Emulsion zeta potential vs. electrolyte concentration for various electrolytes.
2342
S. F. M~~E.s and K. M. NG
-60
I
Oil nonwetting
medium
I
l-l t 2
-I
-a0 0
droplets were clearly visible under an inverted microscope, to which a video camera and a video monitor were connected to allow easy, continuous viewing. In addition, a videocassette recorder was available to document the observations. An x-y gliding stage attached to the inverted microscope permitted convenient observation of any location inside the flow cell. A more detailed description of the experimental procedures can be found in Moses (1984).
-100 Ln -._’
07
I 00012
0
I 00025 Molority
I 0.005
(N&l)
Fig. 4. Collector zeta potential vs. sodium chloride concentration for the water-wet and oil-wet media.
uncertainty in locating the line tangent to the water-oil interface through the contact point. Nonetheless, it is about 57 f 2’ according to our estimation. The flow cell can be changed from water-wet to oil-wet by coating the surf&es with a surface modifier (3M FC723). In that case, the contact angle, again measured through the water phase, is 154 f 2”.
Experimental
PLOW CELL EXPERIMENTS
Q
procedures
In each experiment, after characterization, the emulsion was pumped through the flow cell at a specified volumetric flow rate with a gear pump (Zenith-Nichols). The micrometer-sized emulsion
The experiments were designed to isolate the effects of the various factors-emulsion and -%ollector zeta potentials, collector size, wettability and initial emulsion droplet sizeon the coalescence process. A summary of the experiments is given in Table 2; they can be classi6ed into four different categories. The first three categories are concerned with the effxt of The classifications emulsion zeta potential. delicately stable, stable and very stable-refer to the stability of the emulsion in flow through the coalescer and not to the traditional classification of emulsion stability on standing alone. No appreciable coalescence occurred to any of the emulsions used in the visualization experiments when they were simply left in the emulsion tank for several days. The effect of wettability was investigated in each of these three classes. Three different flow cells, all of size 2.54 x 11 cm, were used in this study. The regular one had collectors of 1030 pm in diameter; the small-size one had collectors of 431 pm in diameter. The third flow cell was a graded medium, consisting of three sections of equal length. The collector diameter in the first section at the inlet was 431 pm, and 731 and 1240 pm in the
Table 2. A s ummary of the experiments performed Experiment No. Delicately stable
Stable
Very
1
C, (mV)
r, (mv)
Medium characteristics
Observations
-10
-62
Water-wet, regular size
Adhesion and coalescence of emulsion
2A
-10
-144
Oil-wet, regular size
2B
-6
-150
Oil-wet, small size
3
-17
-67
4
-17
- 122
-20
-62
stable
Special purpose experiments
9
-20
-122
-17
-68
-5.5
-68
-6
Water-wet, regular size Oil-wet, regular size Water-wet,
regular size
Oil-wet, regular size Water-wet,
regular size
Water-wet, graded medium - 150/ - 68 Graded medium with changing wettability
droplets;
blob
formation,
motion, coalescence and breakup Adhesion and coalescence of emulsion droplets Formation, re-entrainment and redeposition of clusters Flowing rivulets consisting of filaments, tongues and pockets Formation, re-entrainment and redeposition of clusters No blob formation A lower rate of adhesion than in experiment 3 A significantly lower rate of adhesion than in experiment 3 Practically no adhesion Larger emulsion droplet sizes enhanced the rate of adhesion Small pore chambers should precede larger ones in a graded medium Rivulets changed to blobs as wettability changed along the coalescer
Breakdown of emulsions in porous coalescers
second and third regions. Since all flow cells were etched with masks based on the same original pattern, they are geometrically similar. The fourth category consisted of three experiments. Experiments 8 and 9 had the objective of investigating the advantages of using a graded medium, either with uniform or mixed wettability. In experiment 9, the first section was oil-wet while the rest of the graded flow cell was water-wet. Experiments 1 through 6 and 9 used emulsions with a droplet size of about 7 pm. A larger droplet size (14 pm) was used in experiments 7 and 8. All experiments had a volumetric flow rate of 0.7 cn?/min at the beginning, except experiment 1 which was at 0.29 cm3/min. In all cases, the Reynolds number, defined as ds u/v(Ia) (see, for example, Bird ef al., 1960) was less than 10, indicating a laminar flow regime. Although the gear pump was designed to deliver a constant vohrmetric flow rate, the flow rate dropped about O-14.2 cm3/min when there was significant clogging by the coalesced emulsion droplets inside the coalescer. The key observations in the visualization experiments are described below. Experiment 1 Adhesion became observable in about 10 min, with a decreasing level of adhesion along the length of the cell.
2343
Figure 5 shows how it occurred; the flow in these and other pictures in this paper is from left to right. Suspended emulsion droplets collided and adhered to the region near the stagnation point of the collector on the right [Fig. S(a)]_ As time progressed, more and more adhesion took place and the adhered droplets coalesced among themselves. Although most of the adhered droplets on the vertical cylindrical surface were not easy to observe, a few larger drops were clearly visible [Fig. S(b)]. The pictures of Fig. 5 were taken at the third row of the coalescer. Before more adhesion could take place, coalesced drops from the first two rows had already begun migrating downstream. Indeed, at a later time, some large drops could be seen at a throat in the second row [Fig. S(c)]. Figure 6 is a time sequence of pictures showing the fate of the coalesced drops. Further coalescence took place [Figs 6(a) and 6(b)]; finally, a blob was formed [Fig. 6(c)). (In this paper, the original emulsion oil particles are referred to as droplets. Drops are coalesced droplets, either suspended or attached to a collector, whose sixes are small compared to that of the throats. Large nonwetting drops filling completely a throat or one or more pore chambers are called blobs.) The coalesced oil phase continued the journey to the outlet of the coalescer through drop detachment and
Fig. 5. Time sequence showing adhesion in experiment 1. (a) The collector on the right-hand side of the picture was relatively free of adhered droplets. (b) At a later time, an increase in the amount of adhered droplets was clearly visible, and the adhered droplets began to coalesce among themselves. (c) Coalesced drops from upstream collectors were migrating through the pore throat on the left-hand side of the picture.
2344
S. F. MOSES and K. M. No
Fig. 6. Time sequence showing blob formation. (a) Adhered drops at a pore throat. (b) Coalescence took place. (c) Further coalescence resulted in a blob.
redeposition, as well as blob flow. Figure 7 illustrates the former event. A small drop at the top left-hand corner of Fig. 7(a) was pulled out from a group of adhered drops. It first became elongated [Fig. 7(b)] and then followed by detachment, migration through a throat and coalescence with a blob downstream [at the middle top of Fig. 7(c) J. Breakup prevents blobs from growing indefinitely, as is illustrated in Fig. 8. Figure 8(a) shows the initial shape of a blob on the lefthand side of the picture. As the blob was set in motion by the viscous pressure of the continuous emulsion phase, a water collar began to appear and grow at the pore throat [Figs 8(b) and 8(c)]. This blob then broke up into two daughter blobs [Fig. 8(d)]. The time duration for the entire episode was about 1 s. The various aspects of blob mechanics, namely, the motion, breakup and coalscence of oil blobs, have been the subject of extensive research in the area of enhanced oil recovery (Roof, 1970, Ng et al., 1978; Oh and Slattery, 1979; Payatakes et al., 1980; Ng and Payatakes, 1980; Mohanty et al., 1980). interested readers are referred to the references for further discussions. The region occupied by blobs expanded down the coalescer as the experiment progressed, which was stopped after 2 h. Experiment 2A Similar adhesion was observed in the first 10 min of the experiment. However, in contrast to experiment 1,
on coalescence, the adhered droplets migrated towards the gap next to the contact area under each cylindrical collector, due to capillary effect. Figure 9 shows the increase in the amount of oil trapped in the gaps of two collectors.
Experiment
2B
The small-size flow cell was used in this experiment. Similar to experiments 1 and 2A, considerable adhesion took place in the first several minutes of running time. A major difference was that the adhered droplets, instead of coalescing, formed clusters, which appeared to consist of a large number of oil droplets separated by water films. Such clusters at a relatively clean spot in the flow cell are shown in Fig. 10(a). The photograph was taken at about 10 min into the experiment. Within the next couple of minutes, the throat on the left-hand side of the picture was clogged [Fig. 10(b)] and then reopened by re-entrainment of the clusters [Fig. 10(c)]. At 20 min, most of the clusters disappeared and were replaced by coalesced oil in the gaps, similar to that in experiment 2A. A plausible explanation for this observation is as follows. There are two time scales in the coalescence process: one for the capture of flowing emulsion droplets by the collectors and one for the coalescence among the adhered droplets. At the same running time, the number of captured droplets was greater in experiment 2B than in
Breakdown of emulsions in porous coalesccrs
2345
Fig. 7. Time sequence showing entrainment and redeposition of an adhered drop in experiment 1. (a) The adhered drop was initially located at the top left-hand comer of the photograph. (b) It gradually elongated due to the flowing continuous phase. (c) Detachment occurred and the drop migrated through a throat and coalesced with a blob at the middle top of the picture.
experiment 2A, due to the larger collector surface area. The coalescence time scale .was, however, approximately the same in both experiments as it is mainly governed by the stability of the emulsion. In experiment 2B, the rate of increase of adhered droplets was faster than the rate of assimilating them and thus the formation of clusters. Unlike experiment 1 in which the coalesced oil phase was in the form of blobs, a wide variety of wetting structures were observed in this experiment: oil filaments connecting two collectors, and oil tongues protruding from the contact area of the collector into the throat area [Fig. 11(a)]. The oil tongues usually grew and moved downstream. A filament would form if the oil tongue spread to another collector. Figure 1l(b) shows a triangular wetting structure connecting three collectors; an oil pocket occupying two pore chambers is shown in Fig. 1 l(c). These. wetting structures were not in isolation, as can be seen in a low-magnification view of the coalescer. Many wetting structures were connected with one another through the collectors to form oil rivulets, which were approximately in alignment with the axial direction of the flow cell (Fig. 12). Instead of moving only as long, snake-like masses of oil, the rivulet merged into much wider oil pockets
occupying a number of pore chambers. The migration of an oil pocket is shown in Fig. 13, which was observed at 2 h of running time. The downstream part of the pocket continued to surround more and more collectors and the upstream end receded from other posts. Review of the videotape revealed that the receding interfaces were concave while the advancing interfaces were convex and somewhat irregular in shape, probably indicating an advancing contact angle different from the equilibrium value of 26”. Experiment 3 Clusters similar to those observed in the first 10 min of experiment 2B were the dominant feature of this experiment. Like the other experiments, there was a decreasing level of adhesion with distance down the coalescer. Throughout the entire experiment, formation, re-entrainment and redeposition of clusters were observed. After 3.5 h, tracks which were aligned with the direction of flow and devoid of clogged throats became apparent. The emulsion followed these relatively straight and unobstructed tracks through the flow cell. Most clusters resided at pore throats between the flow tracks, where the fluid was relatively stagnant. Observations after almost 6 h still showed no sign of coalescence among the droplets within the clusters.
2344
S. F. Moses
and K. M. No
Fig. 9. Time sequence showing accumulation of the coalesced oil phase in the gap between the collector and the llat glass panel in experiment 2A. (a) At the beginning. (h) At a later time.
Experiment 4 The outcome was almost identical to that of experiment 3. The major and interesting difference was that the rate of adhesion in this experiment was noticeably lower than that of the companion nonwetting experiment. The possible explanation is that the collector zeta potential was more negative in experiment 4, - 122 mV compared to - 67 mV, thus resulting in a lower coliection efficiency in this experiment. In addition, contrary to experiment 3, some coalesced oil phase was observed despite the fact that less clusters were present. It seems that the accumulation of adhered droplets at the gaps of the collectors enhanced the probability of coalescence.
Fig. 8. Time sequence showing blob breakup in experiment 1. (a) Initial shape on the left-hand side of the picture. (b, c) As the blob was set in motion by the viscous pressure of the continuous phase, a water collar began to appear and grow at the pore throat. (d) Snap-off occurred and two daughter blobs were formed.
Experiment 5 This flow cell showed a significantly lower rate of adhesion than that of experiment 3. The lower collection efficiency was probably due to the more negative emulsion zeta potential, -20 mV compared to - 17 mV. This was somewhat negated by the less negative collector zeta potential. The overall effect, however, resulted in less adhesion. In addition, similar experiments were performed with emulsion zeta potentials at - 30, - 50 and - 88 mV, absolutely no adhesion was observed for a prolonged period of time.
Breakdown
2347
of emulsions in porous coalescers
Fig. 10. Time sequence showing formation, re-entrainment and redeposition of the clusters in experiment 28. (a) Clusters were found at a relatively clean spot in the Row cell. (b) A throat on the left-hand side of the picture was clogged by clusters. (c) Then it was reopened by entrainmentof the clusters.
Experiment 6 This flow cell ran for 5 h with essentially no adhesion. Combination of experiments 5 and 6 confirmed the conjecture put forward in experiments 3 and 4 that, in an identical environment, the wetting flow cell had a Iower collection efficiency due to a more negative collector zeta potential. Furthermore, both experiments 5 and 6, in comparison with experiments 3 and 4, respectively, indicated the same conclusion, that there is a critical value of the emulsion zeta potential, about - 20 mV in this experimental system, beyond which the collection efficiency drops precipitously. size The purpose of experiment 7 was to investigate whether or not a larger droplet size at the inlet of the cell would result in enhanced coalescence. This experiment was identical to experiment 3 except for an increase of droplet size from 7 to 14 q. Comparison of the two experiments at the same running time showed that there was a higher level of adhesion in this experiment. The reasoning for this observation is that the larger droplets would enhance the collection efficiency through inertial impaction and interception.
Effect of emulsion droplet
CJnt@ormvs. graded media In all of the previous water-wet medium experiments it was observed that blobs usually did not exceed the size of one pore chamber because of the frequent breakup at the throats. We surmised, with hints from previous investigations (Hazlett, 1%9a, b), that larger pore chambers might allow further blob growth. This idea was tested out in experiment 8. After 1.5 h, the blob region extended to the sixteenth row out of about 100 rows of the first section of the flow cell. By 6 h running time, some drops had moved into the second section of the coal-r. The experiment was stopped after 47 h, with blobs in the second section visibly larger than those in the first. It should be noted that larger droplet size (14 m) and less negative emulsion zeta potential ( - 5.5 mV) were employed in this experiment, in comparison with those in experiment 1, simply to cut down on the run time.
Effect of wettability
change within the medium
Previous investigations (Jeater et al., 1979; Davies, 1980) indicated that the way in which the coalesced oil phase exits the coalescer might affect the performance of a porous coalescer significantly. A large amount of the coalesced phase accumulated at the outlet would
2348
S. F. MOSES and K. M. NG
Fig. 11. Various wetting structures of the coalesced oil phase were observed in experiment 2B. (a) Oil filament connecting two collectors and an oil tongue protruding from a collector into the throat area. (b) A triangular wetting structure connecting three collectors. (c) An oil pocket occupying two pore chambers.
nnnnn Fig. 13. Migration of an oil pocket by advancing interfaces at the front and receding interfaces at the back. The advancing contact angle was different from the equilibrium value. Fig. 12. Low-rnagni6cation view of the wetting structures which were connected with one another through the collectors to form oil rivulets, which were aligned with the direction of flow.
cause phase inversion. In practice, it was found that putting a sock, nonwetting with respect to the dispersed phase, at the outlet of the coalescer helps. This empirical conclusion was investigated in experiment 9 with a graded medium, the first section of which was oil-wet while the rest was water-wet. During this experiment, when the coalesced oil phase reached the second section, it changed from rivulets to blobs
(Fig. 14). The transition was very smooth. The rivulets protruded from the smaller pore spaces into the larger oil-nonwetting pore chambers. Snap-off took place and the blob was formed. This experiment seems to indicate that the blobs are likely to exit the coalescer whole, whereas rivulets tend to hold on to the collectors and form drops with sixes smaller than that of a pore chamber. DISCUSSION The visual studies have provided
a porelevel physical picture to understand some of the more important
Breakdown of emulsions in porous coalescers
2349 NOTATION
%l k
collector
diameter
specific conductivity
supefficial velocity porosity of coalescer pressure drop dielectric constant collector zeta potential emulsion zeta potential dynamic viscoSity kinematic viscosity streaming potential
Fig. 14. The effect of changing wettability was observed in a graded medium in experiment 9. The section with small collectors was oil-wet while the section with large collectors was water-wet. The coalesced oil phase changed from oil rivulets to oil blobs at the junction.
in Fig. 1 which are based on experimental testing. In addition, some suggestions can be made for the selection of a porous coalescer for separating a given emulsion:
conclusions
A less negative, or even positive, collector zeta potential is preferred in order to obtain a higher rate of adhesion (experiments 3 and 4). (2) A higher surface-to-volume ratio would also enhance the rate of adhesion (experiments 2A and 2B). This is in agreement with other investigators (Voyutskii et al., 1953, 1958). (3) The observation that blob size did not grow much beyond the size of a pore chamber lends support to the conclusion (Hazlett, 1969a, b) that increased bed length might not lead to better separation. Hence, the coalescer should be just long enough to form blobs; any extra length simply incurs a higher pumping cost and does not result in any improvement in separation efficiency. (4) In case a graded medium is used, small pore chambers should precede the larger ones (experiment 8; Hazlett, 1969a, b). (5) The wettability at the outlet of the coalescer should be nonwetting to the dispersed phase (experiment 9; Jeater et al., 1979).
(1)
Although these visual studies have advanced our understanding of the coalescence process and would serve as the basis for quantitative theoretical work, some key questions remain unanswered. For example, which is better, a fibrous bed or a granular medium, for the breakdown of an emulsion? Are similar blob rivulets present in a fibrous bed? These questions and quantitative analysis should be included in future studies. work was performed under Science Foundation Grant National CPE-8112824. Availability of a zeta meter from Professor R. L. Rowe11 is gratefully acknowledged.
Acknowledgements-This
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