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  • 标题:Phase inversion and associated phenomena in oil-water vertical pipeline flow.
  • 作者:Hu, Bin ; Angeli, Panagiota
  • 期刊名称:Canadian Journal of Chemical Engineering
  • 印刷版ISSN:0008-4034
  • 出版年度:2006
  • 期号:February
  • 语种:English
  • 出版社:Chemical Institute of Canada
  • 摘要:Keywords: phase inversion, vertical flow, two-phase flow, liquid-liquid flow, ambivalent region, pressure gradient
  • 关键词:Fluid dynamics;Oil separators;Phase transformations (Statistical physics);Phase transitions (Physics)

Phase inversion and associated phenomena in oil-water vertical pipeline flow.


Hu, Bin ; Angeli, Panagiota


Phase inversion and its associated phenomena are experimentally investigated in co-current upward and downward oil-water flow in a vertical stainless steel test section (38 mm I.D.). Oil ([rho].sub.o]= 828 kg/m3, [[micro].sub.o]= 5.5 mPa s) and tap water are used as test fluids. Two inversion routes (w/o to o/w and o/w to w/o) are followed in experiments where either the mixture velocity is kept constant and the dispersed phase fraction is increased (type I experiments), or the continuous phase low rate is kept constant and that of the dispersed phase is increased (type II experiments). By monitoring phase continuity at the pipe centre and at the wall it was found that phase inversion does not happen simultaneously at all locations in the pipe cross-section. In type I experiments, the velocity ratios ([U.sub.o]/[U.sub.w]) where complete inversion appeared acquired the same constant value in both low directions, although the phase inversion points, based on input phase fractions, were different. In contrast to previous results in horizontal lows, frictional pressure gradient was found to be minimum at the phase inversion point. The interfacial energies of the two dispersions before and after phase inversion, calculated from the measured drop sizes, were found to be different in contrast to the previously suggested criterion of equal energies for the appearance of the phenomenon. In type II experiments the phase inversion point was found to depend on mixture velocity for low and medium velocities but not for high ones. In all cases studied an ambivalent region, commonly reported for inversion in stirred vessels, was not observed.

L'inversion de phase et ses phenomenes associes sont etudies experimentalement dans un ecoulement huile-eau dans une section d'essai en acier inoxydable verticale (38 mm de diametre interieur). De l'huile ([rho].sub.o]= 828 kg/m3, [[micro].sub.o]= 5,5 mPa s) et de l'eau du robinet sont utilisees comme guides d'essai. Deux voies d'inversion (de w/o a o/w et de o/w a w/o) sont etudiees dans les experiences, dans lesquelles soit la vitesse de melange est gardee constante et la fraction de phase dispersee est augmentee (experiences de type I), ou soit le debit de phase continue est garde constant et la phase dispersee est augmentee (experiences de type II). En surveillant la continuite des phases au centre de la conduite et a la paroi, on a trouve que l'inversion de phase n'arrive pas simultanement a tous les endroits de section transversale de la conduite. Dans les experiences de type I, les rapports de vitesse ([U.sub.o]/[U.sub.w]) ou apparait l'inversion complete, acquierent la meme valeur constante dans les deux directions de l'ecoulement, meme si les points d'inversion de phase, bases sur les fractions de phase d'entree, sont differents. Contrairement aux resultats anterieurs dans les ecoulements horizontaux, on a trouve que le gradient de pression frictionnel etait minimum au point d'inversion de phase. Les energies interfaciales des deux dispersions avant et apres l'inversion de phase, calculees d'apres les tailles de gouttes mesurees, s'averent differentes que dans le cas du critere anterieurement suggere pour l'apparition des phenomenes. Dans les experiences de type II le point d'inversion de phase s'avere dependre de la vitesse du melange pour des vitesses faibles et moyennes mais non pour des vitesses elevees. Dans tous les cas etudies, on n'a pas observe la region ambivalente couramment signalee pour l'inversion en reservoir agite.

Keywords: phase inversion, vertical flow, two-phase flow, liquid-liquid flow, ambivalent region, pressure gradient

Immiscible liquid-liquid dispersions, where usually one phase is aqueous (e.g. water) and the other is organic (e.g. oil), are very common in a number of applications such as crude oil transportation and production of food and pharmaceuticals. Generally, for dispersions formed by two immiscible liquids one phase acts as the continuous medium entrapping the other phase in the form of drops. Phase inversion is then defined as a phenomenon of phase interchange, whereby the continuous phase changes to become dispersed and the dispersed phase becomes continuous. This phenomenon often occurs spontaneously at some critical operational condition, for example volumetric phase fraction or power input (e.g. velocity for pipeline systems or agitation speed for stirred vessels). Knowing when phase inversion appears is important for industries as the change in phase continuity will lead to a system with different properties (i.e., rheology). In many cases phase inversion is part of the process (e.g. production of margarine or polymerisation). Of significance to transportation of dispersions is the observed increase in pressure gradient accompanying phase inversion. In the petroleum industry, for example, where crude oil and water need to be transferred from seabed to offshore to plant, failure to predict this phenomenon can result in substantial decrease of oil productivity and pipeline capacity.

A large number of previous studies on phase inversion have been carried out in dispersions generated by mechanical agitation in batch or semi-batch systems (for a review see Yeo et al., 2000). The results clearly indicated that the critical volume fraction of dispersed (or organic) phase where inversion appears (phase inversion point) varies with system and is affected by a number of physical and operational parameters, such as liquids' properties, container geometry and initial conditions. In contrast to stirred vessels significantly less work has been carried out on phase inversion during pipeline flows.

A number of researchers have investigated experimentally the occurrence of phase inversion and its associated phenomena in oil-water horizontal pipe flow (Arirachakaran et al., 1989; Pal, 1993; Nadler and Mewes, 1997; Soleimani, 1999; Ioannou et al., 2004, 2005). The main aspects of these studies are summarized in Table 1. Generally, it is found that the phase inversion point is dependent on many system parameters, e.g. viscosity ratio, velocity, pipe diameter and material. Previous experiments suggested that the more viscous oil tends to form the dispersed phase while the less viscous water is likely to be continuous. The critical input water fraction required for inverting a water-in-oil dispersion decreased as the oil viscosity increased. For oil-water dispersed flow, the pressure drop in the pipeline could differ greatly between an oil-in-water and a water-in-oil system. Pal (1993) observed an abrupt increase of the mixture viscosity at phase inversion from experimental data in laminar pipeline flow. In turbulent dispersed flow, a large pressure drop during phase inversion has also been widely reported, which suggests a dramatic increase in mixture viscosity (Arirachakaran et al., 1989; Angeli and Hewitt, 1998; Soleimani, 1999; Ioannou et al., 2005). Based on the experimental data obtained from horizontal pipe flow, Arirachakaran et al. (1989) reported that the mixture velocity has little effect on the inversion point as long as there is no transition in the flow regime. Furthermore, Ioannou et al. (2005) investigated the effect of pipe diameter and wettability on phase inversion by using different sizes of stainless steel and acrylic pipes. Their experiments clearly showed an ambivalent range (within which either phase can be dispersed) in the large diameter (60 mm I.D.) stainless steel and acrylic pipes, which however, was not observed in the small acrylic pipe (32 mm I.D.). In addition, the pressure gradient peak at conditions where phase inversion occurs was much sharper and larger in the acrylic than in the steel pipe.

Compared to investigations of phase inversion in horizontal flows, only limited work has been reported for vertical pipe flows (Luo et al., 1997; Liu et al., 2004; Liu, 2005). Luo et al. (1997) experimentally studied the oil-water upward flows in a stainless steel pipe of 44 mm I.D., using oil with viscosity 452.6 mPa s. From the measured frictional pressure drops, they obtained the effective emulsion viscosity at pre- and post-inversion and correlated it to water fraction, temperature and pressure. They found that phase inversion is affected by the mixture velocity while the emulsion becomes unstable probably due to phase inversion when the mixture velocity exceeds 0.8 m/s. The phase inversion point was not measured directly but calculated from a correlation by Yeh et al. (1964) while it was also associated to the pressure drop change observed. Recently, Liu et al. (2004) used a laser-induced fluorescence (LIF) technique to directly visualize the dynamic evolution of the dispersion and the internal flow structure in concentrated liquid-liquid downward flows. It was found that there is an unstable region between the two possible dispersions (o/w and w/o), wherein complex flow structures (i.e., multiple dispersions) are often observed. Phase inversion for a given well-established dispersion is suggested as a transitional process of crossing this unstable region.

Information on phase distribution in a horizontal pipe cross-section by gamma densitometry (Soleimani, 1999; Hussain, 2004) and high frequency impedance probe (Angeli and Hewitt, 2000; Ioannou et al., 2004) indicated mixture inhomogeneity and suggested that phase inversion could occur at different cross-sectional locations even in fully dispersed flow. For example, the inversion of a water-in-oil dispersion could take place first at the bottom part of the pipe, where water tends to accumulate. Local rather than global appearance of phase inversion was also found by Liu et al. (2005) in agitated systems using the LIF technique.

Apart from the above studies of unstable dispersions (no added surfactants) in pipeline flow, Pal et al. (1986) and Pal (1993) also investigated phase inversion during stable emulsion pipe flow where non-ionic surfactants have been added. The viscosity of the emulsion (measured by a coaxial cylinder viscometer) was found to increase as the dispersed phase concentration increased and, similarly to unstable dispersions, there was a sudden increase in the viscosity when inversion occurred. Furthermore, secondary and multiple emulsions (continuous phase is entrapped in dispersed drops) were widely observed in the water-in-oil emulsion before inversion. Due to the large number of secondary continuous phase drops a dramatic increase in the dispersed phase drop size was also observed before inversion, which has not been reported for pipe flow of unstable dispersions.

A number of mechanisms for phase inversion have been suggested in the literature such as minimum system energy or equal surface energy of the two possible dispersions (oil-in-water and water-in-oil) at phase inversion (Tidhar et al., 1986; Brauner and Ullmann, 2002), the inverse of mixture relative viscosity minus 1 tending to 0 (Pal et al., 1986) and zero interfacial shear stress (Yeh et al., 1964; Nadler and Mewes, 1997). Nevertheless, there are large discrepancies among the predictions of these models.

Despite the amount of work on phase inversion, mainly experimental but also theoretical, the underlying mechanism of this phenomenon is still not well understood. The segregation of the phases and the resulting inhomogeneity in horizontal systems add an extra complication to the study of phase inversion in dispersed pipeline flows. In this paper, phase inversion and a number of associated phenomena (e.g. pressure drop, in situ hold-up and drop size distribution) are again addressed and further investigated in both upward and downward vertical pipeline flows where mixture spatial inhomogeneities are reduced.

EXPERIMENTAL

The experimental work is carried out in the WOLF (Water-Oil Liquid Flow) system in the Department of Chemical Engineering at University College London. Tap water and EXXSOL Oil D140, whose properties are given in Table 2, are used as test liquids. Oil and water are stored separately in 0.88 [m.sup.3] capacity tanks made of glass reinforced plastic and are pumped out by two 7.5 KW centrifugal pumps (Ingersoll-Dresser, 40-25CPX200) respectively, with 240 l/min (3.5 m/s superficial velocity) maximum output capacity for each liquid. Figures 1a and 1b illustrate the experimental rigs for upward and downward flow respectively used in this study. After leaving their respective storage tank and pump, the oil and water phases join together right before the entrance of the vertical test section. Following a 90[degrees] elbow, the oil-water mixture passes through a coarse mixer (see Figure 2), which is located just after the elbow. The purpose of the inlet mixer is to premix the fluids and shorten the flow developing length so that fully dispersed flow can be obtained at the downstream measuring point. The oil-water mixture after exiting from the test section is delivered into a 0.8 [m.sup.3] separator unit (containing a KnitMesh DC 9201/SS/PPL coalescer inside). The separator together with the liquids' storage tanks provide sufficiently long residence time (at least 10 min for the highest mixture velocities used) that allows efficient separation of the small droplets expected at the highest mixture velocities in this study (2.5 m/s). Separated oil and water are then transported back to their respective storage tanks to assure the flow is running continuously. Two armoured variable area flow meters (ABB, 10A5400) one for each phase are mounted at the beginning of the flow loop and the recorded flow rates are logged into a PC for analysis. Each test section has 38 mm I.D. (D) and is made of stainless steel except for a transparent part located at the end, designed for visualization and hold-up measurements. The vertical test sections for upward and downward flow measurements have a total length of 3.2 m and 2.3 m respectively. A heat exchanger immersed in the oil tank is used to remove excess heat from the oil. In this work the temperature of the oil-water mixture was kept at about 25[degrees]C.

[FIGURES 1-2 OMITTED]

Pressure gradient is measured with a differential pressure transducer connected to the test section between 42D and 81D from the inlet for upward flow and between 16D and 55D for downward flow. The type of dispersion (o/w or w/o) in the test section and the phase inversion point, where phase continuity changes, are identified by three different techniques, namely conductivity probe, glue-on hot-film probe and visual observation. The conductivity probe consists of two wire sensors 10 mm apart, with each sensor exposed by 10 mm in the axial direction into the flow. The probe is located at the pipe centreline at a distance from the inlet 81D for upward flow and 58D for downward flow. The data from the conductivity probe are logged into a PC at a sampling rate of 10 Hz; each measurement lasts 1 min, which, from comparison with longer sampling times, was found to be sufficient for obtaining representative time averaged values. To obtain the dispersion type near the pipe wall a glue-on hot-film anemometry (HFA) probe (Dantec Ltd., 55R47) is employed and linked to a computer controlled hot-film anemometer (Dantec Ltd., HFA 90P10/C10). The glue-on hot-film probe is flush mounted onto the inner wall and is positioned at the same distance from the inlet as the conductivity probe. The hot film sensor is very sensitive to the cooling effect of the ambient liquid that depends on its nature, and thus the HFA system can produce a fast and precise indication of the continuous phase that covers it. For example, for fully dispersed oil-in-water flows where water wets the wall surface and the probe, a high voltage signal is expected due to the fast cooling effect of the water phase. Alternatively, in a water-in-oil dispersion the HFA system gives a low voltage output as oil has reduced cooling effect. Because of its location, however, it can only detect the continuous phase in the vicinity of the wall. Although to the authors' knowledge this technique has not been used before to identify phase inversion, the results from the present study show a good stability and accuracy of this instrument. Apart from the above techniques, the phase inversion point can also be identified by observing the dispersion through the transparent test sections using a Kodak digital camera. Water-in-oil dispersions appear more opaque than oil-in-water ones while there is also a major visual difference between water and oil drops.

The drop velocity profile and drop size distribution are measured by a dual impedance probe. The detailed description of this instrument and signal processing technique can be found elsewhere (Lovick and Angeli, 2004; Hu et al., 2006). In total, 11 different radial locations in the pipe cross-section are sampled. During drop size measurements the test section of the glue-on hot-film probe is replaced with that of the impedance probe. The average in situ hold-up of each phase is also measured by simultaneously shutting two quick-closing valves, installed at each end of the transparent acrylic pipe sections (see Figures 1a and 1b).

In the present studies, two types of experiments were carried out to observe phase inversion and its associated phenomena. In the first one (type I), phase inversion is achieved by varying the input superficial velocities of both oil and water phases, while keeping the total mixture velocity constant at each run (e.g. 1.5, 2.0 and 2.5 m/s). The velocity of the initial continuous phase is reduced while that of the dispersed phase is increased until phase inversion is achieved and beyond it. Within type I experiments more detailed studies on pressure drop, in situ hold-up and drop size are carried out. In the second type (type II) of experiments phase inversion is obtained by increasing the input superficial velocity of the initial dispersed phase, either oil or water, while maintaining that of the other, continuous, phase unchanged until phase inversion occurs and beyond it.

During both types I and II of experiments, phase inversion has been approached from two different routes, from oil to water continuous (denoted by w/o [right arrow] o/w) and from water to oil continuous (denoted by o/w [right arrow] w/o) dispersions, respectively. In the o/w [right arrow] w/o experiments, for example, the test section is filled initially with either pure water or oil-in-water dispersion such that the inner wall is wetted only by water phase.

RESULTS AND DISCUSSION

Type I Experiments

Phase Inversion Point

Figure 3 illustrates the average dimensionless conductivity value of the mixture (normalized to the single-phase water value) at the pipe centre obtained by the conductivity probe in upward vertical flows at 2.0 and 2.5 m/s mixture velocity. The conductivity values are plotted as a function of input oil volume fraction and are compared for the two different routes used to approach phase inversion, namely o/w [right arrow] w/o and w/o [right arrow]o/w, respectively. Here, the input oil fraction is defined as [[epsilon].sub.o]=[Q.sub.o]/([Q.sub.o]+[Q.sub.w]) where [Q.sub.o] and [Q.sub.w] are the volumetric flow rates of oil and water phases respectively. It can be seen that for both inversion routes there is a high conductivity value for water-continuous dispersions until a critical input oil fraction [epsilon].sub.1.sub.0], while there is a low conductivity value for oil-continuous dispersions beyond a second critical input oil fraction [[epsilon].sup.2.sub.0]. The conductivity value in the water continuous dispersion decreases with increasing dispersed phase fraction as more and larger drops pass through the two probe needles and bridge them. The results indicate that between the two types of dispersions there exists a transitional region since the two critical oil fractions are not the same, i.e. ][epsilon].sup.1.sub.0]. =74% and [[epsilon].sup.2.sub.0] =84% for 2.5 m/s (Figure 3b). In this transitional region the two phases seem to compete to become continuous. This is similar to the transitional region between the two types of dispersions observed visually by Liu et al. (2004) and Liu (2005) with the LIF technique. Their work indicated that within the transitional region (also called unstable region) the flow structure is very complex, where secondary and multiple dispersions are frequently seen, and both o/w and w/o dispersions can co-exist in the pipe. The unstable characteristics of the transitional region can also be seen from the time history plot of the conductivity signals for volume fractions before and after phase inversion, as illustrated in Figures 4a to f for 2.5 m/s mixture velocity. High values can be seen for 70% and 74% oil fraction indicating a water continuous phase interrupted by the many oil drops at these dense dispersions, while low values are seen for an oil continuous dispersion at 84% oil fraction. In the intermediate volume fractions the signals are shifting in between these two limits reflecting that the dispersion is neither clearly water-continuous nor clearly oil-continuous but complex structures may exist.

[FIGURES 3-4 OMITTED]

The phase continuity was also identified with the glue-on hot-film probe and the results are illustrated in Figure 5 for all cases studied in this work, both upward and downward flows at 1.5, 2.0 and 2.5 m/s mixture velocities. High values indicate that water is in contact with the probe and the pipe wall while low values indicate that oil is now in contact with the wall. Since visual observations showed that the flow regime was fully dispersed, it can be assumed that the phase in contact with the wall is the continuous phase of the dispersion. As can be seen from Figure 5, the HFA output illustrates a clear and abrupt transition between o/w and w/o dispersions in between the two critical oil fractions given by the conductivity probe. A possible explanation for this difference is that even when the flow at the centre of the pipe has started to invert and create complex structures, as indicated by the changing conductivity values, the region close to the wall will preserve the continuous phase until phase inversion is complete and the new continuous phase establishes everywhere in the pipe and at the wall, which is the point detected by the HFA probe. Therefore, the phase inversion detected by the glue-on HFA probe is the complete phase inversion in pipeline flows. These complete phase inversion points are further found to match those identified visually from the difference in the optical characteristics of the two types of dispersions.

[FIGURE 5 OMITTED]

Both measurements of phase inversion in the middle of the pipe and at the wall showed very few differences between the o/w [right arrow] w/o and w/o [right arrow] o/w inversion routes (Figures 3 and 5), suggesting that there is no hysterisis effect (or ambivalent range) in the current pipeline system. This observation agrees with the experimental data obtained by Ioannou et al. (2005) in horizontal acrylic pipes with 32 mm I.D., where the phase inversion point was found to be the same for inversions from o/w [right arrow] w/o and from w/o [right arrow] o/w. In that work, however, ambivalent range was seen in the large diameter pipes (60 mm I.D.) used for the same range of flow rates, with width about 6% input oil fraction. This difference was attributed to flow pattern transitions in the large pipes where higher flow rates would have been required to get fully dispersed flow than in the small pipe. It should be noted here that in all the other works reported in Table 1 the existence of ambivalent range between the inversion points from oil continuous and from water continuous dispersions was not addressed.

From Figure 3 and Figure 5 it can also be seen that the critical input oil fraction [[epsilon].sub.o] for complete phase inversion, found by the HFA probe, is close to the upper oil fraction limit [[epsilon].sup.2.sub.0],of the transitional region, indicated by the conductivity probe, for both inversion routes. This could be due to the different coalescing characteristics of o/w and w/o dispersions (Chesters, 1991). Water drops coalesce more promptly and perhaps once some inversion has been initiated it spreads to the whole pipe cross-section with very little further increase in the dispersed phase volume fraction. On the other hand oil drops in water have low coalescence rate, perhaps because of the electric double-layer effect that repels drops from each other (Kumar, 1996), and they remain dispersed even when some initial inversion appears until the volume fraction of the oil increases significantly more.

Additional phenomenological investigations around the complete phase inversion point (which is abbreviated in the following to phase inversion point) were carried out in type I experiments.

Frictional Pressure Drop

Equation (1) is used to calculate the frictional pressure gradient in the vertical pipeline flows:

[(dp/dx).sub.f] = [(dp/dx).sub.g] [+ or -] [[rho].sub.w] - [[rho].sub.o]) [[phi].sub.0]g (1)

where (dp/dx)f is the frictional pressure gradient, [(dp/dx).sub.m] is the pressure gradient measured by the pressure transducer, [[rho].sub.w] and [[rho].sub.o] are the densities of water and oil phase, [[phi].sub.o] is the average in situ hold-up of the oil phase that is obtained by the quick-closing valves, g is the gravitational acceleration and the '[+ or -]' sign corresponds to upward or downward flow, respectively. In the current experiments, the maximum and average errors in obtaining the frictional pressure gradient are 6.0% and 4.6%, respectively.

In horizontal pipeline flow phase inversion is often accompanied by a peak in pressure drop, which suggests a maximum of the dispersion viscosity (Arirachakaran et al., 1989; Angeli and Hewitt, 1998; Soleimani, 1999; Ioannou et al., 2005). This increase in pressure drop is more noticeable at high mixture velocities. The frictional pressure gradients measured in the current work are shown in Figures 6a-c for vertical upward flow and in Figures 6d-f for downward flow. Interestingly, there is no peak in the pressure gradient data during phase inversion. In contrast, comparisons with the change in phase continuity shown in Figure 5 indicate that at the phase inversion point pressure gradient seems to have its lowest value. For both routes of approaching phase inversion starting from a high water fraction, pressure gradient decreases slightly with increasing oil fraction. At higher oil fractions, and especially in upward flow, pressure gradient seems to increase and then to sharply decrease before phase inversion. After the inversion point it increases again with further increase in the oil volume fraction towards the single-phase oil value. The region of sharp decrease and increase in pressure drop is within the transitional region given by the conductivity probe (see Figure 3) while the minimum in this region matches exactly the boundary identified by the glue-on hot-film probe (see Figure 5).

[FIGURE 6 OMITTED]

The reduction in pressure gradient from the single phase oil or water values with the addition of dispersed water or oil, respectively, is attributed to the drag reduction phenomenon. This is in agreement with previous findings (Pal, 1993; Nadler and Mewes, 1997; Angeli and Hewitt, 1998). Drag reduction has been found to be stronger in oil than in water continuous dispersions and to increase with dispersed phase fraction. Pal (1993) reported that the drag reduction behaviour can only be observed during the flow of unstable dispersions but not of stable emulsions with added surfactant. He further attributed the appearance of drag reduction to the turbulence modification of the continuous phase in the presence of dynamic drop breakage and coalescence processes, which would explain its absence in stable emulsions.

In addition, it has been suggested that drop size can also affect the pressure gradient of dispersions. Pal (1993) found experimentally that the effective viscosity of liquid-liquid mixtures was strongly dependent on the size of the dispersed phase. By comparing stable (with surfactant) and unstable (without surfactant) emulsions, he observed that there is a dramatic rise of the relative viscosity with volume fraction of dispersed phase in stable emulsions that have small droplets, but not in unstable emulsions that have large droplets in both laminar and turbulent flows. It was further suggested that for a given volume fraction the larger the drops are, the easier their deformation during flow and therefore the lower the viscosity of the mixture will be. As the mixture approaches phase inversion the drops will tend to grow even larger (which is more significant at low velocities) and, based on the above, cause a reduction on mixture viscosity and pressure gradient.

A combination of the drag reduction phenomenon and the effect of drop size (see Figure 12) on pressure gradient could explain the pressure drop behaviour found in the current work. Starting from pure oil the addition of the dispersed water would decrease pressure gradient because of drag reduction. As the mixture enters the transitional region and the drops grow larger they start deforming and further reduction in pressure gradient occurs. At the point of complete phase inversion a new continuous (water) phase establishes. With further decrease of the dispersed oil fraction, the oil drops decrease in size as well and the pressure gradient increases again until it reaches either a peak or a plateau at the limit of the transitional region. After the transitional region the dispersed drops are becoming small and less easy to deform. Combined with the reduced effect of drag reduction in water continuous dispersions the pressure gradient is much closer to the single phase water value and approaches it as the dispersed phase oil fraction further decreases. A lower drag reduction is observed in the water than in the oil continuous dispersions perhaps due to the reduced deformability of the more viscous oil drops.

A lack of pressure gradient maximum during phase inversion, in contrast to what has been observed before from experiments carried out in horizontal flows, could be attributed to the large drop sizes encountered in the current work. Because of the orientation of the flow gravity does not promote phase separation in a pipe cross-section and dispersed flow can easily be achieved at low mixture velocities, such as 1.5 m/s, for which the flow would not be fully dispersed in the respective horizontal system. As a result larger drops form in our vertical pipe, compared to those formed at fully dispersed flows in horizontal systems. The large drops will also deform more easily. Perhaps a further increase in the mixture velocity, accompanied by a decrease in drop size, would also result in a peak in pressure gradient in the current system during phase inversion. In fact there are some small peaks close to the phase inversion point in upward flows (see Figure 6). It should also be pointed out that in the previous investigations, where changes in phase continuity were related to pressure gradient changes (Pal, 1993; Nadler and Mewes, 1995, 1997; Ioannou et al., 2005), phase inversion was identified at only one location in the pipe and the increase in pressure gradient was not matched with the change in phase continuity at different locations in the pipe cross-section. In other studies the peak in pressure drop was used as an indication of phase inversion but was not related to phase continuity changes. It is possible, therefore, that a maximum in pressure gradient appears not at the exact point of complete phase inversion but close to it, as the location of the small peaks in upward flow seems to suggest.

In Situ Hold-Up and Velocity Ratio

The average in situ hold-up of the dispersion is obtained from the quick-closing valves with [+ or -]3% manual operating error. In agreement with the previous results on phase continuity and pressure gradient it was found that the route of approaching phase inversion (from oil or from water continuous dispersion) also had little effect on the hold-up data, provided that a long enough experimental running time is allowed (to assure the previously stagnant liquids remaining in the test section after shutting off the quick-closing valves are removed before a new measurement is taken). The in situ hold-up results in upward and downward flows averaged over the two inversion routes are shown in Figures 7a and 7b respectively. It can be seen that in both flow directions the dispersed phase travels faster than the continuous phase for oil-in-water dispersions when [[epsilon].sub.o] <70% and for water-in-oil dispersions when [[epsilon].sub.o] >85%. Also, the difference between continuous and dispersed phase velocities gradually decreases as the flow approaches phase inversion. Figure 8 presents the average in situ oil to water velocity ratio (S=[U.sub.o]/[U.sub.w]) calculated by Equation (2) at immediately before and after the phase inversion point:

S = [[epsilon].sub.o][[phi].sub.w]/[[epsilon].sub.w][[phi].sub.o] (2)

where [[epsilon].sub.w] and [[epsilon].sub.o] are the input fractions of water and oil respectively and [[phi].sub.w] and [[phi].sub.o] are the measured average in situ hold-up of water and oil respectively. As can be seen, complete phase inversion is not accompanied by equal in situ average velocities (S=1) between the two phases. To further investigate this behaviour, the in situ oil hold-up and velocity ratio at the complete phase inversion points (according to Figure 5) are plotted in Figures 9 and 10 respectively at various mixture velocities.

[FIGURES 7-10 OMITTED]

It can be seen that the critical velocity ratio where inversion occurs is similar for the two flow directions and tends to a constant value, S [approximately equal to] 0.77. Discrepancies appear at the lower velocity 1.5 m/s for downward flow, where the visual observation showed that at high oil fractions very large oil drops (similar to plugs) appeared in the middle of the pipe.

Detailed measurements of the in situ hold-up profile in a pipe cross-section were also obtained with one of the sensors of the impedance probe. The results for 2 m/s mixture velocity are shown in Figure 11 for different input oil fractions ([[epsilon].sub.0]), for upward and downward flows, where R is the pipe radius. Most are for oil-in-water dispersions apart from [[epsilon].sub.o]=86% in upward flow and [[epsilon].sub.o]=76% and 80% in downward flow that are for water-in-oil dispersions (indicated with dotted lines and full marks). As can be seen, the profiles of o/w dispersions in both flow directions exhibit a centre peak as the oil fraction increases up to 50%, which is consistent with the findings by Farrar and Bruun (1996) in upward kerosene-water flows. With further increase of the dispersed phase input fraction the profiles become gradually flatter while approaching phase inversion and the two phases are more evenly distributed over the pipe cross-section. This phase distribution profile seems to build up the preparation for the occurrence of complete phase inversion in the whole pipe cross-section. It is interesting that although there are differences in the local hold-up between upward and downward flows for a given [[epsilon].sub.o], as illustrated in Figure 11, which would explain the differences in the complete phase inversion points between the two directions, phase inversion does happen at both directions at the same velocity ratio, especially at the higher velocities (see Figure 10).

[FIGURE 11 OMITTED]

Chord Length and Drop Size Distribution

By cross-correlating the signals of the two sensors of the dual impedance probe the velocity of the dispersed phase drops can be found. This velocity can then be combined with the time duration of the interactions between the dispersed phase and either of the two probe sensors to provide a distribution of the drop chord lengths that have been intersected by the sensor. If a Sauter mean chord length is defined as [L.sub.32]=[[sigma][P(L)[L.sup.3]]/[sigma][P(L)[L.sup.2]] where P(L) is the number density of chord length L, Figures 12a and 12b show the radial profile of L32 measured at 2 m/s mixture velocity in upward and downward flows respectively for different input oil fractions before and after phase inversion, following the w/o [right arrow] o/w inversion route.

[FIGURE 12 OMITTED]

As can be seen from Figure 12b for downward flows, in the o/w dispersions (denoted by empty marks) with increasing oil fraction the dispersed phase size at the wall is not significantly affected, but it increases a lot at the pipe centre particularly for fractions between [[epsilon].sub.o]=0.3 and [[epsilon].sub.o]=0.4. At oil fractions 40% and above the L32 profile has a peak at the pipe centre and shows a significant decrease at one radial position; this characteristic position moves towards the wall with increasing oil fraction (i.e. from 0.3R at [[epsilon].sub.o]=0.4 to 0.78R at [[epsilon].sub.o]=0.64). The downward o/w flow at high oil concentrations, therefore, seems to consist of two different regions, a core region with large oil drops and a wall-annulus region with small oil drops. Once this pattern is established (at about [[epsilon].sub.o]=0.4) any further increase in the oil fraction extends the size of the region towards the wall but affects less the size of the drops. This may be because there exists a maximum drop size in the core region under a given mixture velocity (or turbulence level) and any further increase in the dispersed phase fraction cannot increase the drop size in the centre any more but can affect (increase) the size of the smaller drops closer to the wall, extending the core region. After the o/w dispersion completely inverts to w/o (full marks) smaller water drops are formed due to dilution as shown in Figure 12b.

Chord lengths [L.sub.32] in upward flows of o/w dispersions (see Figure 12a) have large values in the pipe centre similarly to downward flows, which also reduce to low values after inversion to w/o mixtures (water drops). However, the [L.sub.32] profiles in o/w dispersions at the highest oil fractions ([[epsilon].sub.o]=0.74 and 0.8) seem to be almost uniform across the pipe or slightly increase closer to the wall. In addition, the differences in drop size between the pipe centre and wall regions are not as great as in downward flow even for the lower oil fractions.

This disagreement between upward and downward flow dispersed phase size and distribution (also reflected in the hold-up profiles, see Figure 11) suggests that the structure of the two-phase mixture in the two flow directions while approaching phase inversion is different. It is interesting, however, that despite these differences phase inversion appears at the same velocity ratio in both directions. Perhaps a mechanism based on the momentum of the two phases during inversion is more relevant to phase inversion in pipeline flow. This would agree with the theory proposed by Yeh et al. (1964) and Nadler and Mewes (1995) of zero interfacial shear stresses during inversion.

From the local measurements of chord length the area-weighted integrated distributions of chord length (L) over the pipe cross-section can be obtained and are shown in Figures 13a-f for different input oil fractions ([[epsilon].sub.o]) at 2.0 m/s downward flow. In o/w flows (see Figures 13a-d) as the dispersions become denser the likelihood of the probe intersecting large chords rises. This is understandable since the coalescence rate of oil drops will increase with oil concentration, which will result in many large drops in the flow. Particularly for [[epsilon].sub.o]=64% before the phase inversion point (74%) intersected chords as long as 19 mm (=D/2) can frequently be seen. Visual observations (see for example Figure 17) showed that drops were still spherical at this fraction but it is expected that as the oil fraction further increases such large chord lengths may also originate from smaller deformed drops. The distributions after phase inversion to w/o (Figures 13e and 13f) confirm the presence of many small water drops.

[FIGURE 13 OMITTED]

One of the criteria suggested for phase inversion is that at the inversion point the system surface energies of the two possible dispersions, oil-in-water and water-in-oil are equal (Tidhar et al., 1986; Brauner and Ullmann, 2002). The system free surface energy consists of the liquid-liquid interfacial energy and the liquid-solid wall surface energy. The latter is found to be much smaller in the current experimental system than the former, which can be calculated from the dispersed phase size. To estimate the interfacial energy the chord length distribution is converted to a drop size distribution based on the algorithm developed by Hu et al. (2006). From these distributions the Sauter mean diameter ([D.sub.32]) is found, [D.sub.32]=[sigma][P(D)[D.sup.3]]/[sigma][P(D)[D.sup.2]] where P(D) is the number density of drops with diameter D. The interfacial energy ([E.sub.S]) is then equal to [E.sub.S]=6[[phi].sub.d][phi]/[D.sub.32] where [phi].sub.d] is the in situ dispersed phase hold-up and [sigma] is the interfacial tension. The variation of interfacial energy ([E.sub.s]/[sigma]) with input oil fraction for mixture velocities of 1.5 and 2.0 m/s is presented in Figure 14. Points very close to phase inversion in the water continuous mixtures cannot be obtained as at these high oil fractions drops are no longer spherical and drop diameters cannot be calculated from the measured chord lengths. However, the trends in the curves show that in both upward and downward flows there is a difference in the interfacial energies of the two dispersions before and after phase inversion, which is greater for downward flows, and suggest that the criterion of equal surface energy at phase inversion point may not always hold. This interfacial area difference at phase inversion was also observed by Luhning and Sawistowski (1971) in dispersions formed in stirred vessels. Hence, other factors may also be important to phase inversion such as drop coalescence and break up rates.

[FIGURE 14 OMITTED]

Type II Experiments

In the experiments of type II, the appearance of phase inversion is observed by keeping constant the flow rate of the initial continuous phase and increasing that of the initial dispersed phase until inversion is observed and beyond. Figure 15 illustrates the critical input oil fractions in both upward and downward flows that lead to the occurrence of complete phase inversion for the two different approaching routes, o/w [right arrow] w/o and w/o [right arrow] o/w. It can be seen that in vertical downward flow the amount of input oil required for inversion increases as the mixture velocity increases until it reaches an almost constant value. It is understandable that at low mixture velocities buoyancy would have a greater effect on the in situ hold-up and the amount of in situ oil is expected to be higher than the input one. As a result phase inversion will appear at lower input oil fraction. While, for upward flow the critical oil fraction is found to decrease with the mixture velocity. The opposite will happen in the upward flow where buoyancy will now favour lower in situ oil hold-up than the input one. As shown in Figure 15, when the mixture velocity increases above 3.5 m/s the buoyancy effect seems to be negligible and the critical input oil fraction for inversion becomes independent of mixture velocity. This is more obvious for downward than for upward flow, perhaps because the dispersed drops close to inversion have smaller size in downward than in upward flow and therefore are less affected by buoyancy.

[FIGURE 15 OMITTED]

No obvious ambivalent region is found in type II experiments, which is consistent with the previously described results of type I experiments. Interestingly, during type II experiments, it was also observed that a particular dispersion, for example water-in-oil formed by increasing the oil flow rate up to the inversion point could be readily inverted back to the oil-in-water dispersion by slightly reducing the oil flow rate. This characteristic may further prove the absence of clear ambivalent region in pipeline systems, although it has been reported extensively for mechanically agitated vessels. Some small discrepancies of the complete phase inversion points between repeated experiments, as seen in Figure 15, are attributed to the differences that can experimentally exist in the system set-up and operating conditions as well as contamination and temperature variation.

Previous works have indicated that phase inversion is independent of mixture velocity (Arirachakaran et al., 1989; Soleimani, 1999; Ioannou et al., 2004), which would agree with the current findings for the higher mixture velocities used.

PHASE INVERSION PROCESS

On the basis of the visual observations in this work, supported by the experimental work by Liu (2005), a sequence of flow configurations before and after phase inversion is proposed, which is schematically shown in Figure 16 for an o/w to w/o transition. In the o/w dispersion at low oil fractions, oil is entrapped into the water continuum in the form of spherical drops (graph A). As the input oil fraction increases a relatively dense dispersion with larger drops is formed due to drop coalescence (graph B). If more oil is fed into the pipe the drops will be more closely packed and the dispersion will become more concentrated. Although the close packing would occasionally force the drops, especially the larger ones to deform into various shapes (e.g. ellipsoid and strangely elongated shapes in graph C), the majority of the dispersed drops would still be spherical. At this stage drops can stay together exhibiting negligible coalescence even for systems with no added surfactants, as shown by Figure 17. The formation of such concentrated o/w dispersions is attributed to the electrical double-layer effect around the oil drops because of preferential adsorption of ions from the continuous water phase, which was found to significantly suppress coalescence in both less dense (<10%, Collins and Knudsen, 1970) and dense dispersions (< 45%, Pal, 1993).

[FIGURES 16-17 OMITTED]

With further increase of the dispersed oil fraction, much closer to complete phase inversion point and within the transitional region, the flow pattern can change from dispersed-dominant flow to the complex-structure-dominant flow where complex multiple dispersions and large elongated drops are present. The dispersion could have either water at the wall (and considered as water continuous, as depicted by graph D in Figure 16) or oil at the wall (oil continuous, graph E), as clearly visualized by Liu (2005), with the complete phase inversion point located in between these two cases. The oil at the wall case seems to exist for only a narrow range of oil fractions. Because of the structures formed at the transitional region the conductivity of the mixture at a point inside the pipe would fluctuate between oil and water continuous values as the probes are alternatively wetted by the oil and water continuous complex structures. Once the oil fraction is beyond the critical value for complete phase inversion, a water-in-oil dispersion is formed (graph F in Figure 16). The transitional region is found in this study to be over 4-6% input oil fraction and is expected to narrow down with an increase in the mixture velocity.

The above process would also describe the w/o to o/w inversion. In this case, however, the continuous oil phase is non-polar and there is a higher possibility of water drops to coalesce due to the absence of the double-layer effect. This would justify the lower dispersed water fraction required to invert an oil continuous dispersion, compared to the dispersed oil fraction required to invert a water continuous dispersion as well as the shorter range of the oil continuous transitional region.

CONCLUSIONS

Phase inversion in co-current oil-water vertical flows at both upward and downward directions is experimentally investigated in this study. Two inversion routes (w/o to o/w and o/w to w/o, respectively) are followed to study the behaviour of phase inversion and associated phenomena. Experiments are carried either by keeping the mixture velocity constant and increasing the dispersed phase fraction (type I experiments) or by keeping the continuous phase superficial velocity constant and increasing the dispersed phase superficial velocity (type II experiments). A conductivity probe at the pipe centre and a glue-on HFA probe at the pipe wall indicate that phase inversion does not happen simultaneously at all locations in the pipe cross-section. The input oil fraction at which inversion is detected at the wall signifies that the new continuous phase has spread into the whole pipe cross-section and is defined as the complete phase inversion point.

Based on the above experimental investigations around the complete phase inversion point, the following conclusions can be drawn:

* The results from type I experiments in both upward and downward flows indicate that frictional pressure gradient reaches a minimum at the complete phase inversion point. Drag reduction as well as the effect of drop size on mixture viscosity are suggested as possible reasons for this behaviour;

* No obvious ambivalent region is found in type I and type II experiments at both flow directions. There is however, a narrow range of input phase fractions ([DELTA][[epsilon].sub.o]<4-6%) where complex structures may form;

* The phase inversion point is found by the type II experiments to depend on mixture velocity for low and medium mixture velocities;

* In type I experiments the phase inversion points were found to be different for the two flow directions. However, the velocity ratios where complete inversion appeared, acquired the same constant value in both flow directions apart from the lowest velocity investigated;

* In contrast to the previously postulated phase inversion mechanisms, it was found, based on drop size measurements, that the interfacial energies of the dispersions before and after phase inversion are not equal. Other phenomena, such as increased coalescence rate before inversion, supported by the large drops observed, could also be responsible for the appearance of the phenomenon.

A number of issues would still need to be resolved in order to establish a mechanism of phase inversion, such as turbulence modification as the system approaches phase inversion and during the phenomenon and the existence (or absence) of ambivalent range. More work is also necessary to relate the velocity and momentum of each phase to the complex structures and multiple dispersions formed close to inversion.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the financial support from the Engineering and Physical Sciences Research Council (EPSRC, grant number GR/R56044/01). B. Hu is also grateful to EPSRC and Overseas Research Students Awards Scheme (Universities U.K.) for providing financial support for the studentship.

Manuscript received June 21, 2005; revised manuscript received November 1, 2005; accepted for publication November 7, 2005.

REFERENCES

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Chesters, A. K., "The Modelling of Coalescence Processes in Fluid-Liquid Dispersions: A Review of Current Understanding," Chem. Eng. Res. Des., Trans. I Chem E (Part A) 69, 259-270 (1991).

Collins, S. B. and J. G. Knudsen, "Drop-Size Distributions Produced by Turbulent Pipe Flow, of Immiscible Liquids," AIChE J. 16(6), 1072-1080 (1970).

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Hussain, S. A., "Experimental and Computational Studies of Liquid-Liquid Dispersed Flows," PhD Thesis, University of London, London, U.K. (2004).

Ioannou, K., B. Hu, O. K. Matar, G. F. Hewitt and P. Angeli, "Phase Inversion in Dispersed Liquid-Liquid Pipe Flows," 5th Int. Conf. on Multiphase Flow, Yokohama, Japan (2004).

Ioannou, K., O. J. Nydal and P. Angeli, "Phase Inversion in Dispersed Liquid-Liquid Flows," Exp. Therm. Fluid Sci. 29(3), 331-339 (2005).

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Liu, L., O. K. Matar, E. S. Perez de Ortiz and G. F. Hewitt, "Experimental Methodology for Investigating Concentrated Liquid-Liquid Dispersions," 5th Int. Conf. on Multiphase Flow, Yokohama, Japan (2004).

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Bin Hu (1,2) and Panagiota Angeli (1 *)

(1.) Department of Chemical Engineering, University College London, Torrington Place, London, WC1E 7JE, U.K.

(2.) Department of Chemical Engineering, Imperial College London, Prince Consort Road, London, SW7 2BY, U.K.

* Author to whom correspondence may be addressed.

E-mail address: [email protected]
Table 1. Experimental studies of phase inversion in liquid-liquid
pipeline flows

Author (year) Main aspects of the work

Tidhar et al. (1986) 1. Surface energy

Arirachakaran et al. (1989) 1. Pressure drop;
 2. Phase inversion point;
 3. Pipe diameter;
 4. Temperature;
 5. Viscosity

Pal (1993) 1. Surfactants;
 2. Drag Reduction.

Luo et al. (1997) 1. Mixture velocity;
 2. Temperature;
 3. Pressure;
 4. Dispersion viscosity;
 5. Pressure drop

Nadler and Mewes (1997) 1. Oil viscosity;
 2. Temperature;
 3. Pressure drop

Angeli and Hewitt (1998, 2000) 1. Wettability;
 2. Mixture velocity;
 3. Pressure gradient

Soleimani (1999) 1. Pressure drop;
 2. Flow pattern

Gillies et al. (2000) 1. Surfactants;
 2. Intensity and nature of shear
 process;
 3. Solids content of oil

Ioannou et al. (2004, 2005) 1. Wettability;
 2. Pipe diameter and material;
 3. Pressure drop;
 4. Phase distribution

Liu et al. (2004); Liu (2005) 1. Flow structure;
 2. Drop size;
 3. Phase inversion point

Table 2. Properties of the test liquids at 25[degrees]C

Liquid Exxsol oil D140 Water

Density (Kg/[m.sup.3]) 828 998
Viscosity (mPa s) 5.5 0.993
Surface tension (mN/[m.sup.2]) 20 72
Interfacial tension (mN/m) 36.6 [+ or -] 0.3
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