Article pubs.acs.org/EF
Emulsion Stability Studies Based on the Critical Electric Field (CEF) Technique Jan H. Beetge* Champion Technologies, Incorporated, 3130 FM 521 Road, Fresno, Texas 77545, United States ABSTRACT: The production and processing of crude oil most often involve the formation of undesirable emulsions, because of the presence of water and high shear conditions. Crude oil emulsions need to be resolved to produce dry oil as well as oil-free water in a timely manner. The type of equipment, process, and chemical treatment required is determined by the emulsion stability as well as the mechanism of stabilization. Traditional “bottle tests” are used to mimic the system to develop suitable demulsifiers for chemical treatment. More recently, critical electric field (CEF)-based techniques were developed in an effort to measure the stability of an emulsion in quantitative terms. The internal phase ratio (IPR)−CEF technique was subsequently proposed as an expansion of earlier work, to gain additional information related to the mechanism of stabilization as well as the mechanism of demulsifier function. This paper reports new insights and advances made with applications of CEF- and IPR− CEF-based techniques. The principles and approach followed here are not limited to oil field applications only but could be relevant to water-in-oil emulsions in various other industries or applications.
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certain critical point, the magnitude of the electric field is sufficient to break the thin film barriers between the individual drops in a string, forming a conducting column, which is indicated by a very sharp increase in conductivity between the two electrodes. The electric field strength at this critical point was originally referred to as Ecritical but later more commonly called the CEF. The CEF value is therefore a quantitative reference of the relative stability of an emulsion; a high CEF value would therefore be indicative of high stability and vice versa. Eow and Ghadiri9 provided a good overview of the technology related to electrostatic coalescence of water drops. Kilpatrick and co-workers10−12 designed and manufactured a different CEF cell, where the gap is set with Mylar spacers of known thickness. They favored slower voltage ramp rates of 0.25 V every 5 s, using a 100 V computer-controlled DC power supply with overcurrent protection. The critical voltage was recorded as the intersection between the tangent lines of the two slopes in the observed current versus voltage plot. Reasonable good correlations were found between CEF data and more traditional emulsion stability evaluation techniques.13 Grutters et al.14 reported good results for CEF measurements made in a study of the role of asphaltenes in the stability of crude oil emulsions. Significant insights were gained by expanding the CEF technique to include rheological measurement as a function of the electric field strength in a number of electrorheology studies.15−19 In our earlier efforts,20,21 we have identified a commercial unit used for “electrical stability testing” (EST) of drilling mud. The unit produces an alternating current (AC) signal at a frequency of 340 Hz and a voltage ramp rate of 150 V s−1 up to a maximum of 2 kV, able to generate an electric field as high as 12.3 kV cm−1 in an electrode with a gap size of 1/16 in. We have
INTRODUCTION Undesired crude oil emulsions are formed during the production and processing of crude oil, when the two immiscible phases of oil and brine are contacted under conditions of shear, in the presence of natural surfactants, solids, waxes, clays, asphaltenes, napthenic acids, or any other stabilization component. Some combinations of these conditions can cause very stable emulsions, and various methods are employed to accelerate the resolution to ensure economical processing of the crude oil. An elevated water content can also lead to a significant increase in the corrosion rate of pipelines, transport vessels, or processing equipment. It is a general requirement that emulsions should be broken and the water separated out as early as possible in the production process. Commercial demulsification involves suitable processing equipment, most often in combination with a chemical treatment program. Commercial demulsifiers or emulsion breakers are very specifically developed for each application to ensure optimal economical treatment. Various techniques are used to measure the stability of an emulsion as well as to evaluate the effect of a demulsifier on the stability of an emulsion. The critical electric field (CEF) technique is of particular interest to deliver a reliable method to determine emulsion stability in quantitative terms. This technique was first reported by Sjöblom and co-workers1 in their effort to measure the stability of crude oil emulsions, following some earlier work on dielectric spectroscopy of crude oil emulsions.2−5 According to their method, a small sample of a crude oil emulsion is positioned between two parallel plate electrodes, separated with a gap size of 0.5 mm. The sample is then subjected to a stepwise direct current (DC) voltage ramp of 1 V/s up to 100 V. They also used steps of 0.05 V/s in some of their later work.6 Variations in electrode gap distances, as low as 0.1 mm7 and as high as 1.0 mm,8 have been applied in various later studies using DC power sources. They provide photographs that show how water droplets line up in strings between the electrodes as they become polarized in the increasing electric field. At a © 2012 American Chemical Society
Received: July 3, 2012 Revised: September 7, 2012 Published: September 10, 2012 6282
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voltage ramp of 150 V/s. Emulsion coalescence is detected at a threshold electrical current value of 0.06 mA through the sample, as an indication of the rapid increase in conductivity in the sample at coalescence. The voltage required to reach the threshold current is recorded to calculate the CEF value at this critical point. The syringe is manually pumped with several strokes between each CEF reading to ensure thermal equilibrium as well as complete displacement of any residual water that may have collected on the electrodes after a voltage ramp. Figure 1 shows a diagram of the experimental setup.
found that this unit produces fast results at 1−5 s per CEF measurement in most of our work, and our results seem to be in good agreement with results obtained by the DC approach by Sjöblom and Kilpatrick. Other researchers followed to adopt this AC unit with good results in subsequent work.18,22 It is very important to note that the value determined by the CEF technique is cumulative, containing both flocculation and coalescence components. Water droplets need to be brought into contact (flocculated) with each other to form a connecting string of droplets between the electrodes, before they can rupture (coalescence) to form a conducting bridge. The CEF value is thus a cumulative value, reflecting both a barrier to flocculation (energy required to bring droplets in contact with each other) as well as a barrier to coalescence (energy required to rapture the thin films that are separating the individual droplets). We have described an experimental method in our earlier work,20 where the contribution of each of the two processes can be extracted from internal phase ratio (IPR)− CEF data, by determining CEF values for a number of identically prepared emulsions, with only small variations in the water content (IPR). A plot of observed CEF data as a function of the corresponding IPR (where the water content is expressed as a volume fraction of the total emulsion) yields the IPR−CEF plot. The slope of the IPR−CEF plot is directly proportional to the barrier to flocculation, and the extrapolated intercept is directly proportional to the barrier to coalescence. Where CEF data give a quantitative indication of the relative stability of an emulsion, IPR−CEF data also provide quantitative insight into the mechanism of stabilization, in terms of flocculation and coalescence behavior in the emulsion. Important contributions have also been made recently by other researchers in this field.8 The effect of a chemical demulsifier can then also be determined in a similar fashion to reveal the mechanism of function, which could ultimately be correlated with the chemical structure. Much experience has been gained since our earlier work, and it is the intent of this paper to report some of the advances made. We also want to highlight potential applications for CEF- and IPR−CEF-based experimental approaches, as well as the interpretation and implications of the information obtained by it.
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Figure 1. Diagram of the experimental setup. Some of the work was also performed on fresh crude oil samples at location. In our typical procedure, the fresh crude oil sample was allowed to stand for a few minutes to separate the free water. The free water was separated and retained to be returned to portions of the top oil at various concentration levels to determine CEF readings at different levels of water content. The water content of the top oil was determined separately by centrifuge, sometimes with the aid of toluene dilution and demulsifier addition, to be able to determine the exact water content of the emulsion in every experiment. In our work, we recognize that a homogeneous emulsion is essential for good results with the CEF technique. Our experience has taught the importance of having good control over the emulsification process to ensure homogeneous emulsions for good repeatable CEF measurements. This is critical to determine reliable values for the flocculation and coalescence coefficients from IPR−CEF relationships, especially if the range of water content variation is small. More recent work23 seems to be in strong agreement, with specific reference to droplet size. The use of a speed-controlled blender made a significant improvement in the variation of CEF data, and our observed standard deviation is typically about 2% of the CEF value. We adopted the protocol to make a minimum of 10 individual CEF readings, with allowance to remove one data point if an outlier can be identified as a substantial deviation from the rest of the data set.
EXPERIMENTAL SECTION
A special electrically heated cup was constructed from a 130 cm length of copper tubing with a diameter of 8 cm. The copper baseplate of the cup was equipped with a Waring blender blade and base assembly, which connects with a speed-controlled motor drive from Chandler Engineering. Agitation can be controlled as low as 500 rpm for most viscose crude oils or any other higher settings up to 12 000 rpm. The cup is sealed off at the top with a special copper lid with an imbedded electric heating element to secure a constant composition by preventing condensation on the inside of the lid. A tapered polytetrafluoroethylene (PTFE) insert seals off an opening that allows for easy access to inject fluids from the top at any interval during an experiment. The cup is not pressurized, which limits the temperature range to ambient pressure. A special CEF flow cell is attached on the outside of the cup, with a 1/8 in. fluorinated ethylene propylene (FEP) tube protruding 5 mm into the cup and 15 mm from the bottom of the cup. A 1 mL plastic syringe is connected to the CEF cell by a Luer lock fitting, directly opposite the insert, and used to extract a sample of the fluid into the flow cell. Two parallel electrodes are inserted in the CEF flow cell, with a fixed gap of 1/16 in. between the electrodes. The sample volume between the two electrodes is estimated to be about 3.1 μL. An electrical stability meter (part 131-50) from Ofite is connected to the electrodes in the CEF cell to generate a 340 Hz AC signal with a
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RESULTS AND DISCUSSION IPR−CEF Relationship. The majority of crude oil emulsions can be considered to be kinetically stabilized, and the immiscible phases will separate given sufficient time. Phase separation is dictated by Stoke’s law, where the settling rate of a droplet is directly proportional to the density difference between the two phases as well as the gravitational constant applied but inversely proportional to the viscosity of the continuous phase. The droplet size has the biggest effect, because the settling rate increases with the square of the droplet radius. Phase separation is thus significantly accelerated if droplets are allowed to flocculate and coalesce with a reduced barrier; droplets grow fast in size, and the settling rate is exponentially increased. On the other extreme, if flocculation is
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related to the close packing of spheres. According to the Kepler conjecture,24 the best packing arrangement of spheres would give a packing density = π/(18−1/2) or ∼0.740 48, which would approximate a value of Φ−1 ≈ 1/0.740 ≈ 1.35 associated with the most dense packing arrangement in a theoretical (monodisperse) emulsion. The value of Φ−1 = 1.35 was then adopted as a practical reference and very well supported by our experimental results, even though the droplets are not monodisperse in the actual emulsions. The data from an IPR−CEF study (CEF versus Φ−1 plot) is extrapolated to Φ−1 = 1.35 to determine the barrier to coalescence in relative terms, and the CEF value at Φ−1 = 1.35 is then defined as the coalescence coefficient. A high coalescence coefficient is thus associated with a high barrier to coalescence. It is argued that the coalescence coefficient is not influenced by the water content and that the increase in the CEF value at Φ−1 higher than 1.35 (lower water content) is determined by the barrier to flocculation, the additional electric field strength required to flocculate the droplets. Thus, the flocculation coefficient is defined as the slope in the IPR−CEF relationship. A high slope is associated with a high barrier to flocculation and vice versa. The IPR−CEF relationship is accordingly expressed as
inhibited, the settling rate will be significantly retarded and coalescence can only happen after droplets have been compressed by droplet settling under gravity. The stability of an emulsion is thus a function of the degree of flocculation and coalescence allowed by the system, and this is the behavior that we aim to assess with IPR−CEF studies. The IPR−CEF relationship has been derived from basic principles for a simple system. In this approach, we propose that the stability of our model emulsion is expected to be inversely proportional to the probability of a collision between drops. emulsion stability∝1/probability of collision
(1)
In analogy to the fundamentals of statistical thermodynamics, we assume that the probability of a collision between droplets is directly proportional to the number of droplets, n, present in the system and inversely proportional to the number of possible positions, g, where a droplet could be in the system at any given point in time. probability of collision∝n/g
(2)
Furthermore, the number of possible positions, g, where a water droplet can be in the system at any given point in time can be determined as g = Vt /Vd
CEF = Cf Φ−1 + I
(3)
and
where Vt is the total volume of the system and Vd is the volume of a droplet in a monodisperse system. However, the volume of a droplet can be calculated as
Vd = Vi /n
Cc = I + 1.35Cf
(4)
(5)
Considering eqs 1, 2, and 5, we can conclude that the stability of an emulsion can then be expected to be proportional to the inverse of the IPR, Φ. emulsion stability∝Vt /Vi = 1/Φ
(8)
where the flocculation coefficient, Cf, is represented as the slope in the plot of the observed CEF value against the corresponding Φ−1 value and the coalescence coefficient, Cc, is determined at an adjusted intercept of Φ−1 = 1.35 according to eq 8. The observed intercept, I, is determined according to eq 7 and only used for calculation. Figure 2 shows the interpretation of an IPR−CEF plot in graphic representation, indicating the determination of the flocculation and coalescence coefficients from the IPR−CEF plot.
where Vi represent the volume of the internal phase (sum of the volume of all of the droplets in the system). It follows from eqs 3 and 4 that g = Vt /Vd = nVt /Vi
(7)
(6)
If we assume that the CEF values are directly proportional to the stability of the emulsion, we can conclude that the CEF values will be directly proportional to the inverse of the IPR and call this the IPR−CEF relationship. It needs to be emphasized that droplet collision does not imply droplet coalescence. The “efficiency” of collision inducing interfacial film rapture strongly depends upon the characteristics of such an interfacial film. This “efficiency” is indicated by the coalescence coefficient observed at a given temperature or by the pre-exponential factor in its temperature dependency to be discussed later. In our earlier work, we made assumptions that the CEF value is cumulative. The electric field required to rupture the droplets will be additive to what is required to flocculate them, and in the absence of a barrier to coalescence, a theoretical emulsion with infinitely high water content will have an extrapolated CEF value of zero at Φ−1 = 1. However, in practice, we have noted several cases where the barrier to coalescence was completely removed by demulsifier addition, and these cases consistently show zero CEF values at Φ−1 values between 1.3 and 1.4 as result of a negative intercept. After careful consideration and substantial experimentation, we concluded that this value is
Figure 2. Schematic explanation of the IPR−CEF concept.
This concept can also be described (or thought of) alternatively by recognizing that the CEF measurement is a cumulative value made up by the sum of the barriers to flocculation and coalescence. The barrier to coalescence is independent of the water content (IPR), but the barrier to flocculation is directly proportional to the inverse of the IPR. The flocculation component, at any given water content, can 6284
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be in good agreement after a sufficient number of CEF measurements were made under similar conditions. This may be due to high polydispersity as well as possible inhomogeneity in samples from the bottle test, considering the very small samples size of about 3 μL between the electrodes inside the CEF flow cell. As a result, we take great care to make and maintain emulsions in a very consistent fashion, using a speedcontrolled agitator at well-defined agitation levels associated with specific time intervals. IPR−CEF Titrations. The original intent of the IPR−CEF approach was to evaluate the flocculation and coalescence behavior of an emulsion by making small changes in the water content. The purpose was to maintain the characteristics or properties of the emulsion by making a series of emulsions under identical conditions, with only small changes in the water content, small enough to claim identical emulsions and identical formation conditions. However, with experience, we have noted that the range of the water content can be very large in some cases. As good example is given in Figure 4 which
then be determined as the difference between the measured CEF value and the coalescence component. It is clear that, in a general sense, emulsions with very high water content will most likely be stabilized by a barrier to coalescence, while the barrier to flocculation might be the dominant stabilization factor in emulsions with very low water content. The coalescence coefficient will reflect aspects such as rigidity of the interface and thin liquid film rapture dynamics, while the flocculation coefficient will be an indication of factors that slow the contact or “collision rate” between droplets, such as the viscosity of the medium.8 Figure 3 shows earlier results20 refitted in support of the practical choice of Φ−1 = 1.35 as a reference point to calculate
Figure 3. Earlier results refitted to accommodate close packing of spheres.
the coalescence coefficient. A demulsifier was used to decrease the barrier to coalescence with increasing concentration. The barrier to coalescence is completely eliminated at a demulsifier concentration of 100 ppm and above. It is clearly shown that the IPR−CEF plot tends to pivot around a Φ−1 value of 1.35 as the flocculation coefficient (slope) decreases at higher demulsifier concentrations, without any change to the coalescence coefficient. The Φ−1 = 1.35 reference point was also confirmed in several other experiments, and we believe that the use of this reference point will provide accurate values for the coalescence coefficient. It needs to be mentioned that some explorative efforts were made to study the effect of the droplet size and droplet size distribution on CEF data, but more work needs to be performed. This is relevant, especially considering the effect that increased polydispersity can have on the packing density and, hence, the validity of the argument to use the intercept value at Φ−1 = 1.35 as a reference point (to separate flocculation and coalescence behavior from the cumulative result). It is also relevant, considering the effect that droplet size and polydispersity can have on emulsion stability in general. Our results have shown that, although a high polydisperisity did not induce any significant shift in the reference point of Φ−1 = 1.35 (in the presence of a very effective demulsifier), there was a substantial increase in the standard deviation of the individual CEF measurements. For example, the standard deviation in CEF measurements made on emulsion samples forming a traditional bottle test, where the bottles were shaken on a mechanical shaker, were more than 5 times higher than the same result obtained by the experimental setup described in Figure 1. However, the average CEF values always seemed to
Figure 4. IPR−CEF plot to demonstrate the wide range of water fractions in which the relationship was maintained.
shows a plot of results obtained on location at the North Slope of Alaska, using fresh crude and brine from the same well. The water content in Figure 4 ranges from 9.86 to 48.7 vol %, and the IPR−CEF relationship was well-maintained over this wide range of water contents. Flocculation and coalescence coefficients were determined with reasonable accuracy as Cf = 0.69 kV cm−1 and Cc = 3.12 kV cm−1. As an illustration of how the IPR−CEF plot can be used to characterize, categorize, or compare different crude oils, it was noted that this crude oil had very similar coalescence behavior compared to the Cc = 3.22 kV cm−1 determined for the crude oil from Bohai Bay in Figure 3 but the flocculation coefficient is about 3 times higher compared to the Cf = 0.23 kV cm−1 from the data in Figure 3. However, this is not true for all crude oil samples. In the production system studied, the crude oil in Figure 4 was mixed with the crude oil from a large number of producing wells, spread over a significant area to simulate the actual production scenario. Figure 5 shows an IPR−CEF plot of a crude oil sample taken from the combined production stream. A significant inflection point is noted at Φ−1 = 3.53, which equate to a water content of about 28%. The IPR−CEF relationship seems to be maintained at both sides of the inflection point content, and the coalescence coefficient of Cc = 3.14 kV cm−1 observed above the inflection 6285
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The crude oil was titrated with synthetic brine, made up according to results from a brine analysis. This example covers a water content range from as low as 1.6% to as high as 38.6% to support the IPR−CEF relationship at both sides of the inflection point. The two pairs of flocculation and coalescence coefficients as well as the location of the inflection point all serve as quantitative descriptors of emulsion behavior. It is important to note that the required treatment of an emulsion can differ significantly for small changes in the water content in the vicinity of the inflection point, because the mechanism of emulsion stabilization differs dramatically at opposite sides of the inflection point. The presence and location of the inflection point may thus have significant practical implications. In our work, we have seen several examples of crude oils that exhibit an inflection point, but we have also seen many cases where the IPR−CEF relationship is maintained over a very wide range of water contents. We concluded that crude oil can be characterized by what we call a “CEF titration”, where water (field or synthetic brine) can be titrated into a crude oil to follow the IPR−CEF relationship. The observed flocculation and coalescence coefficients as well as the location of the inflection point (if present) are all quantitative descriptors that characterize emulsion behavior. In one of the most extreme cases, we saw the IPR−CEF relationship maintained over an extremely wide range of water contents, from 0.34 to 42.8 vol %, as shown in Figure 7. This work was performed on an off-
Figure 5. IPR−CEF plot of a crude oil mixture from different producing wells.
point (at a lower water content) is in very good agreement with the sample from the single well in Figure 4. However, the flocculation coefficient of Cf = 0.32 kV cm−1 is almost half of the value observed for the single well sample. It was noted that the barrier to coalescence is completely destroyed below the inflection point (at a higher water content). It is speculated that the natural surfactant or other stabilizing species that causes the high coalescence coefficient is diluted beyond a critical concentration with the increased interfacial area and no longer dominates the interfacial behavior above this critical water content. The flocculation coefficient in Figure 5 is dramatically increased below the inflection point to dominate as the stabilizing factor. The results in Figure 5 are also another example in support of the choice of Φ−1 = 1.35 as a reference point to determine the coalescence coefficient. One interesting aspect to note here is that an IPR−CEF study comparing crude oil samples taken from the coalescer inlet and outlet gave the same IPR−CEF relationship, including the inflection point. This observation is explained by reasoning that the bulk of the surface active species remained in the crude oil fraction during phase separation to show the same emulsion behavior before and after the separation vessel. We also noted a case with a prominent inflection point in the IPR−CEF relationship, but without complete destruction of the barrier to coalescence. Figure 6 shows the results of an IPR− CEF titration of an off-shore crude oil sample from west Africa.
Figure 7. IPR−CEF relationship for Kashagan crude oil with synthetic brine.
shore crude oil sample from the Caspian Sea. Although the emulsions were very unstable with a flocculation coefficient at 0.03 kV cm−1 and a coalescence coefficient of 0.16 kV cm−1, we were still able to determine the IPR−CEF relationship with reasonable accuracy over an extremely wide range of water contents from 0.3 to 42.8%, following the IPR−CEF titration methodology. Observations on the Flocculation Coefficient. In another study, Athabasca bitumen from vacuum residue was diluted at a fixed concentration of 10 mass % in solvent mixtures with different heptane−toluene compositions. Diluted bitumen and deionized water emulsions were made up under identical conditions, with small variations in the water content. CEF measurements were made at identical time intervals. Figure 8 shows the results of an IPR−CEF study performed at room temperature, in solvent compositions ranging from pure toluene to H75T25 (heptol mixture containing 75 vol % heptane and 25 vol % toluene).
Figure 6. IPR−CEF titration of a west African crude oil with synthetic brine. 6286
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an asphaltene layer deposited on a silica substrate by atomic force microscopy (AFM) measurements in heptane/toluene mixtures of various compositions. They report a transition of force profiles from a repulsive force to a weak attraction force at a toluene fraction below 0.2 in the heptol, which seems to be in reasonable agreement with our observation. Yarranton et al.26 worked on bitumen from the same source and reported the onset of asphaltene at hexane/toluene ratios between 1.2 and 1.6 for asphaltene concentrations ranging from 0.1 to 10 kg m−3, which correspond to toluene fractions between 0.38 and 0.45 in heptol and in good agreement with our observations represented in Figure 9. Kilpatrick et al.27 studied a number of crude oil samples from different sources and showed the onset of asphaltene precipitation around the 40% toluene level. Other work13,28,29 correlates asphaltene precipitation with the stability of crude oil emulsions and reports optimum stability at comparable toluene fractions in heptol. In microscope studies, we have also seen strong evidence of attraction between droplets (flocculation) beyond this critical heptol composition. Figure 10 shows a micrograph of deionized water emulsified
Figure 8. IPR−CEF study of diluted bitumen in various toluene/ heptane mixtures.
It was noted that a low heptane content in an excess of toluene had a relatively small effect on the stability and mechanism of stabilization but that the effect of heptane addition became more pronounced with the addition of heptane above the H50T50 (50 vol % heptane and 50 vol % toluene) level. There was a dramatic change in the mechanism of stabilization with the addition of heptane above the H65T35 solvent composition, where the IPR−CEF relationship swung from positive to negative slopes and the intercept significantly increased. Figure 9 shows flocculation and coalescence
Figure 10. Micrograph of an emulsion with 10% bitumen in heptol with 25% toluene content.
with diluted bitumen in heptol with 25 vol % toluene content. The corresponding flocculation and coalescence coefficients for this system were determined as Cf = −0.075 kV cm−1 and Cc = 3.29 kV cm−1, respectively. Additional qualitative video material further confirmed strong attractive forces between these droplets as the emulsion moved through the microscope flow cell. The negative slope in the IPR−CEF plot illustrated in Figure 8 and the resulting negative value for the flocculation coefficient are explained in terms of competition between droplet flocculation and droplet line-up in the electric field. The probability for a droplet−droplet collision increases with an increase in the water content of the system, similar to the ability to form a connecting line between the electrodes. Thus, a higher water content will therefore lead to bigger flocs of water drops. With increasing attractive forces between droplets, there will be an increasing tendency to line-up as clusters rather than individual drops in the electric field. It is proposed that the negative slope in the IPR−CEF relationship is the result of competition between cluster formation and single-drop line formation. For strong attractive forces, cluster formation is
Figure 9. Observed flocculation and coalescence coefficients as a function of the solvent composition.
coefficients calculated from the data in Figure 8. There seems to be a dramatic inflection point just above 60 vol % heptane in the heptol mixture, where the flocculation coefficient is dramatically reduced and the coalescence coefficient accordingly increased. One aspect of significance is the change in the flocculation coefficient from a positive value (positive slope) to a negative value (negative slope) with the addition of the heptane content in the heptol mixture. In our earlier work, we accepted that, on the basis of the theoretical principles given above, the flocculation coefficient would be a direct reflection of the barrier to flocculation. By implication, a zero slope will then indicate the absence of a barrier to flocculation. However, by extrapolation, we can try to explain this observation by saying that a negative slope must then represent not the absence of a barrier but rather some kind of attractive force. Xu and coworkers25 studied interaction forces between a silica probe and 6287
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favored by an increase in the water content to such a degree that the observed CEF values increase with an increase of the water content. We are led to believe that the flocculation coefficient obtained in an IPR−CEF study could indicate not only the magnitude of the factors limiting flocculation but also those that might be driving flocculation, such as demulsifiers or other chemical additives. The flocculation coefficient can therefore assume negative or positive values, although positive values seem to be a lot more common. Temperature Dependence of the IPR−CEF Relationship. To determine an additional level of information, the IPR−CEF relationship can also be obtained at different temperatures to determine activation parameters. Figure 11 Figure 12. Arrhenius plot of the flocculation and coalescence coefficients of a crude oil emulsion system.
coalesced droplet pair. Following this approach, the activation energy for the flocculation step was calculated from the slope in Figure 12 to be 16.9 and 5.0 kJ mol−1 for the coalescence step. The quantitative results from a typical plot shown in Figure 12 can be used to characterize an emulsion, to compare different emulsions, or to understand differences in the mechanism of stabilization. This type of evaluation can also be used to study the effect of a chemical additive, such as a demulsifier, or to understand the mechanism of the demulsifier function, expressed in quantitative terms. Demulsifier Development. Earlier, we have shown20 how the IPR−CEF relationship can be used to gain insight into the mechanism of demulsifier function, especially in how the flocculation and coalescence behavior is affected as a function of the demulsifier concentration. In another recent study,31 the CEF technique was also used to study the effect of demulsifiers on the stability of real crude oil systems and compared to results from low-field nuclear magnetic resonance (NMR) and interfacial behavior studies. In this work, we also show the value of a more detailed demulsifier concentration CEF study and insights that could be gained from it. Figure 13 shows a typical plot of CEF data collected as a function of the demulsifier concentration. In this case, a crude oil sample from west Africa was emulsified with synthetic brine that was made up according to the actual brine analysis. The temperature was maintained at 170 °F at a fixed brine content of 10 mass %, and emulsification was performed with agitation at 1800 rpm for 2 min using the
Figure 11. Arrhenius plot of CEF data collected from different crude oil emulsions.
shows a typical Arrhenius plot for CEF data obtained at different temperatures for a few different samples. Both the slope and the intercept contain additional and relevant information about the overall stability of the emulsion. The Arrhenius relationship for CEF data was also reported by other researchers.30 Paso et al.7 report that the increase in CEF values associated with the decrease in the temperature was noted to be consistent with the increase in the viscosity of the continuous phase. However, Wang et al.17 report an observation made with Gibbs field crude oil, where the CEF data decrease with the temperature but then stabilize at a fixed value above a temperature of about 50 °C, even though the viscosity of the oil decreases with a further increase in the temperature. Overall, it seems that the flocculation coefficient as well as the associated activation parameters derived from the IPR−CEF relationship could possibly be a reflection of the rheology of the emulsion and most likely dominated by the viscosity of the continuous phase. For a more detailed and informative level of analysis, the flocculation and coalescence coefficients can be determined at different temperatures to enable a more detailed Arrhenius plot, as shown in Figure 12. If the analogy to the theory of the absolute rate of reaction from chemical reaction kinetics can be applied to the observed behavior in Figure 12, as proposed for the cumulative CEF value, then the activation energy for both the flocculation and coalescence behavior can be calculated from the observed slopes. The pre-exponential factor, in analogy to the theory, can in both cases be interpreted as an indication of the probability that an interaction or collision between two droplets will result in either a flocculated or
Figure 13. CEF-based demulsifier concentration study. 6288
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We noted a strong “overtreat” tendency beyond 150 ppm, and the maximum performance at this point was recorded at 0.647 kV cm−1. Thus, the demulsifier was capable of reducing the barrier to separation (cumulative CEF value) to about 40% of its original value. However, demulsifiers work in combination with plant equipment, and in this case, a reduction to about 0.9 kV cm−1, at a demulsifier concentration of about 40 ppm, was sufficient and most likely optimal for the economic production of the crude oil according to specification. As a final note, it needs to be mentioned that the CEF data have been compared to results from traditional bottle test in a number of instances where series of demulsifiers were tested in parallel, with the same fluids under the same conditions. We have seen reasonable good correlations in most of our comparisons, considering the experimental errors that may exist in bottle test data as well. It is also important to note that the bottle test is used to determine several performance parameters, such as the amount of water resolved, the rate of water resolution, the water content of the oil phase at a fixed point above the interface, and the amount of unresolved emulsion, as well as qualitative information, such as interface appearance or water quality. To add complexity, some of these parameters can be positively or negatively correlated; for example, an increase in the rate of “water drop” is often associated with a reduced “dryness” of the oil. These contrasts in demulsifier performance criteria also need to be considered when we make comparisons to CEF data. The best comparisons are made by generalization, ranking the demulsifiers in a sequence of performance, from best to worst, which in most cases seems to be well-correlated with a similar ranking of the CEF data.
setup described in the Experimental Section. A suitable commercial demulsifier was subsequently injected into the emulsion at increasing concentrations at regular intervals of 10 min, under constant agitation at 500 rpm. Figure 13 shows the results of CEF data taken after each demulsifier addition to provide a plot of CEF as a function of the demulsifier concentration. Surfactant systems are traditionally studied in terms of adsorption isotherms, according to various theoretical approaches32−34 to determine the relationship between surfactant concentrations in the solution and at the interface. In its most common form, the Gibbs adsorption equation is written as the relationship between the surface tension and the logarithm of the concentration of the surfactant in solution. The relationship between the surface or interfacial tension and the surfactant concentration in the bulk is used to calculate the values of specific parameters, such as surface access concentrations, critical micelle concentrations, and the interfacial area covered by each molecule. The theoretical approach in these adsorption models is most often expressed in terms of logarithmic function of the concentration. Because the emulsion behavior is dictated by the interfacial behavior of the surface active species at the interface, it makes sense that CEF results from a demulsifier concentration study may also be plotted in term of the logarithm of the demulsifier concentration, similar to an adsorption isotherm. The observations made in our study generally confirm this analogy, and it seems fit to evaluate results from a CEF study in similar terms to those accepted for interfacial adsorption behavior. There are several important parameters that could be determined from such an experiment. First, we can determine the onset of the demulsifier effect, the minimum concentration where the demulsifier start to show an effect. We can also determine the “effectiveness” of the demulsifier; once it started to work, how well does it work. This can be expressed in quantitative terms as the slope of the plot, subsequent to the onset. We also want to know when does the demulsifier stop to work, at what concentration does the effect of the demulsifier disappear, and the performance level at this point (at what performance level does the demulsifier stop to work). Finally, it is very important to know what happens with further demulsifier addition beyond this point. It is wellknown that demulsifiers can “overtreat”, where too much demulsifier can actually start to increase the stability of the emulsion once this critical concentration is exceeded. When we design a demulsifier, we want to know which structural aspects of the individual demulsifier molecules or which combinations of demulsifier molecules affect the onset of function, the effectiveness, the performance limit, and its associated concentration, as well as the behavior beyond this concentration. This information can serve as a guide to develop improved chemical structures and product formulations. Ideally, we want to decrease the onset concentration, increase the effectiveness (slope), and increase the concentration of the performance offset or subsequent “overtreat” situations. The commercial demulsifier used in Figure 13 is a complex mixture or formulation of various chemical structures. The data showed the onset of the demulsifier effect, as observed by the CEF technique, at a concentration as low as 0.91 ppm. The effectiveness (slope) is low at concentrations above the onset but improves significantly beyond an inflection point at about 10 ppm. A subsequent inflection point is noted just above 40 ppm, where the effectiveness started to decrease with a lower slope until a critical point at a concentration of about 150 ppm.
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CONCLUSION Measurements made with the CEF technique provide a relative indication of the stability of oil external emulsions, but the CEF value is a cumulative number, which includes the flocculation and coalescence behavior in a single measurement. We have elaborated on the development of the IPR−CEF technique to separate the flocculation and coalescence components in quantitative terms. This information will allow for insight in the mechanism of emulsion stabilization but may also reveal the function mechanism of demulsifiers, which could ultimately be correlated with the chemical structure or chemical formulation. The flocculation and coalescence coefficients are useful to compare different emulsions or to reveal the effect of chemical additives on the mechanism of emulsion stabilization or destabilization. These techniques can be used to study and develop new chemical demulsifiers in the field or laboratory, monitor changes in crude-oil-processing plants or refineries, or direct adjustments in chemical treatment of these processes. New insights include the application of the intercept at Φ−1 = 1.35 as a more practical reference to determine the coalescence coefficient, as derived from the theory of close packing of spheres. The relevance of this reference was often confirmed in the many experiments that followed since our earlier work, where we have proposed the intercept at Φ−1 = 1 as a theoretical reference. Although the initial intent was to make small changes in the water content of identically produced emulsions, we have later learned that there might be additional information revealed by what we call an “IPR−CEF titration”, where CEF measurements are made over a very wide range of water contents. We have noticed that the IPR−CEF relationship is maintained over 6289
dx.doi.org/10.1021/ef301115r | Energy Fuels 2012, 26, 6282−6291
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a very wide range of water contents, but some crude oil systems show a well-defined inflection point at various IPRs. It is speculated that the inflection point is the result of a critical dilution of the domination surface active species with the increase in the interfacial area, beyond which point another mechanism of stabilization dominates the behavior of the emulsion. This may be a valuable tool to determine concentrations of surface active species or solids at the interface or in the bulk phases. It was also shown that the temperature dependence follows the Arrhenius relationship, for not only the cumulative CEF data but also the flocculation and coalescence coefficients. Activation parameters can thus be calculated for the flocculation and coalescence behavior of a specific emulsion system or similarly the effect introduced by an additive, such as a demulsifier. It was also shown that the flocculation coefficients can assume a negative value, which we speculate to be associated with strong attractive forces between droplets. Finally, we have demonstrated how the effect of an additive, such as a demulsifier, can be evaluated by a concentration study to reveal useful parameters, such as the onset of performance, efficiency, performance limits, and onset of overtreat situations. We believe that this work could be relevant and of value in a variety of emulsion applications.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The author thanks the owners and management of Champion Technologies for the opportunity to conduct this research as well as the permission to publish this work. A special word of appreciation goes to Lee Nuebling for the design and construction of the equipment used in this work.
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