Breaking of Water-in-Crude Oil Emulsions. 7. Demulsifier Performance

Article pubs.acs.org/EF

Breaking of Water-in-Crude Oil Emulsions. 7. Demulsifier Performance at Optimum Formulation for Various Extended Surfactant Structures José G. Delgado-Linares,* José G. Alvarado, Francia Véjar, Johnny Bullón, Ana M. Forgiarini, and Jean-Louis Salager* Laboratorio FIRP, Universidad de los Andes, Mérida 5101, Venezuela ABSTRACT: The performance of several extended surfactants as water-in-crude oil emulsion breakers was evaluated using two criteria: (1) the demulsifier dose required (CD*) to attain the minimum stability at the so-called optimum formulation, and (2) the corresponding low minimum stability value. These surfactants were found to behave in the same way as typical commercial demulsifiers do; i.e., they require a lower dose CD* when their hydrophilicity is slightly greater. The reported data for a dozen different extended surfactants indicate how the two performance indices are altered by changing the structure characteristics, such as the propylene oxide number, the ethylene oxide number, and the ionic polar group (carboxylate, sulfate, phosphate). The best performance as a demulsifier seems to depend on the proper combination of these structures to attain a well-fitting compromise. and β are referred to as the surfactant characteristic parameters, hereafter abbreviated as SCP. 1.1. Dehydration Corresponds to W/O Emulsion Breaking. The principle of breaking a brine-in-crude (W/O) emulsion produced at the well output was first described in 199015 and consists in adding a hydrophilic surfactant called a demulsifier or dewatering agent to the emulsion. The demulsifier mixes at interface with the naturally occurring lipophilic surfactants referred to here as asphaltenes (but also containing resins, naphthenic acids, and other slightly polar species). When the interfacial mixture reaches the optimum formulation of the system containing the reservoir brine (S) and oil (EACN) at temperature T, then the σ* or β* parameter, which is characteristic of the surfactant mixture (called Hydrophilic−Lipophilic-Balance of the optimal mixture, HLB*m, in previous articles of this series,16−19 and, more accurately, SCP*SYS in Part 620) may be calculated as

1. INTRODUCTION As reported in the late 1970s and early 1980s1−3 and discussed in details elsewhere,4 emulsions are very unstable at the socalled optimum formulation in which the surfactant species at interface have exactly the same affinity for the oil and water phases. Such optimum formulation is the physicochemical state at which the interfacial tension passes through an ultralow minimum, as is required for chemical enhanced oil recovery.5 In this situation, a maximum solubilization occurs in a three-phase system consisting of a bicontinuous microemulsion in equilibrium with excess oil and water.5−7 This is a morphology also sought for other applications,8,9 e.g., detergency, solubilization of polar oils, toxic drug trapping in the blood, etc. Several decades of research and development that started with empirical data processing10 have shown that the condition to reach such optimum formulation may be written as a correlation between the formulation variables, such as the following zero expression of the surfactant affinity difference SAD11−14 or its dimensionless value the hydrophilic−lipophilic deviation HLD13

SCP*SYS = σ * (or β*) = k EACN + a T (or − c T)(T − Tref ) − ln S (or bS)

SAD/RT = HLD = ln S − k ACN + σ + f (A)

(3) 20

Hence, according to our previous article, the interfacial condition to reach an optimum formulation may be written as follows

− a T(T − Tref ) = 0 for ionic surfactants (anionic and cationic)

(1)

XDSCPD + (1 − XD)SCPA = SCP *SYS

SAD/RT = HLD = bS − k ACN + β + ϕ(A)

where SCPD and SCPA are the surfactant characteristic parameters of the demulsifier and asphaltene at interface, and XD is the molar fraction of demulsifier in the interfacial mixture. 1.2. Performance Tests. The classic bottle test consists in adding a demulsifier in a test tube containing a W/O emulsion

+ c T(T − Tref ) = 0 for polyethoxylated nonionics

(2)

where S is the salinity in wt % NaCl, ACN is the alkane carbon number, σ and β are the surfactant characteristic parameters, f(A) and ϕ(A) are the alcohol effects (∼maCa, i.e., close to a linear variation versus alcohol concentration), and aT, cT, b, and k are positive coefficients. If the oil phase is not an n-alkane, ACN is replaced by its equivalent EACN.6 The parameters σ © 2016 American Chemical Society

(4)

Received: May 27, 2016 Revised: August 15, 2016 Published: August 16, 2016 7065

DOI: 10.1021/acs.energyfuels.6b01286 Energy Fuels 2016, 30, 7065−7073

Article

Energy & Fuels Table 1. Characteristics of the Extended Surfactants surfactanta

molecular weight

C12(PO)14(EO)12 C12(PO)14(EO)12-SO4Na C12(PO)14(EO)12-PO4Na2 C12(PO)14(EO)7-SO4Na C12(PO)14(EO)2-PO4Na2 C12(PO)14(EO)2-SO4Na C12(PO)14(EO)2-CO2Na C12(PO)14(EO)20-SO4Na C12(PO)4(EO)10 C12(PO)10(EO)10 GA16(PO)10(EO)20

1526 1628 1650 1408 1210 1188 1152 1980 858 1206 1718

σ

β/HLB

label used

−0.7/13.5

ES12-14-12 ES12-14-12-S ES12-14-12-P ES12-14-7-S ES12-14-2-P ES12-14-2-S ES12-14-2-C ES12-14-20-S ES12-4-10 ES12-10-10 ESGA16-10-20

−2.8 −5.2 −2.6 −4.5 −2.2 −2.1 −3.1 −3.6/18.2 −2.1/17 n.d./12.4b

EO = ethylene oxide group (−CH2CH2-O−); PO = propylene oxide group (−CHCH3CH2O−), C12 = C12H25−; C16 = C16H33− and GA = Guerbet alcohol branching.49 bn.d.= not determined.

a

in the 1950s,6,7 surfactant mixtures are known to produce favorable effects in attaining low tension and high solubilization, it is also known that they present a serious disadvantage, called fractionation between the oil/water phases and the interface. This phenomenon, which has been studied in detail elsewhere,22 in particular for polyethoxylated nonionics,23−26 results in the selective partitioning of the different species, so that the mixture is not the same in oil, in water, and at interface. As a consequence, the actual interfacial formulation depends on the partitioning of the different species, which is influenced by the temperature, oil EACN, and salinity, as well as the water-tooil ratio, and the surfactant total concentration.27 Even if tricks using opposite fractionation for anionic and nonionic surfactants are available,28 it is preferable to avoid mixing highly dissimilar species. An alternative is to make an intramolecular mixture associating a hydrophilic regular surfactant and a lipophilic linker29−31 in a single molecule called an extended surfactant.32 Its structure is designed to include an intermediate part between the head and the tail. This part can be a poly(propylene oxide) chain next to the alkyl group; it can also include a poly(ethylene oxide) chain close to the headgroup.32−34 Different kinds of extended surfactants have been used for different applications,35−48 not only with a linear tail but also with a ramified or branched tail,49−51 and with different heads like the common sulfate, carboxylate, or phosphate, as well as polyglucosides, xylitol, or other natural hydrophiles.52−56 Extended surfactants have been shown to solubilize polar oils much better than common surfactants do, particularly with large multibranch molecules like triglycerides.32 They are thus likely to exhibit a good interaction with asphaltenes.

and to measure the emulsion persistence, generally referred to as stability. The demulsifier is usually added as a solvent solution to help its solubility in the oil phase and to avoid its precipitation, an unwelcome problem to overcome since the demulsifier is a hydrophilic surfactant. As the demulsifier concentration is increased, its contribution at interface (XD) is increased as well, and at some “optimum” concentration, called CD*, the emulsion becomes unstable. The main problem with this kind of experiment is that the demulsifier works only after it is adsorbed at interface, which is a rather slow process. In industrial practice, the demulsifier is introduced in an oil continuous phase; it can thus take quite some time for it to be transported through the oil by convection, then by diffusion, before finally being adsorbed at the water drop interface. To avoid this kind of delay that could lead to a wrong conclusion regarding the demulsifier action in a mixture with asphaltenes at interface, a simplified bottle test method was used in the present series of articles (see section 2.2 below).16,17 The demulsifier is placed in the water phase so that there is no delay for convection or diffusion transport before adsorption at interface. The oil phase is highly diluted with a solvent (here cyclohexane) so that all the asphaltenes migrate to the interface as a surfactant species. The so-called proportional regime shown in Part 2 of this series17which we will refer to in section 3, belowclearly shows that, in this method, there is no excess of asphaltenes as a polar oil segregated close to the interface,21 which could alter the oil EACN or the film interdrop drainage due to its viscosity and/or its precipitation effects. As was shown in the previous papers,16,17 under such conditions, the composition of the demulsifier−asphaltene mixture at interface is constant over a range called the proportional regime,17,18,20 which allows the calculation of the proper contribution of the demulsifier at interface XD or the equivalent optimum demulsifier dose CD*. In three articles of this series,16−18 the performance of the demulsifier was first estimated according to the concentration CD* to be added to reach an optimum formulation. In the last article,20 the actual minimum stability value was also considered to be a performance index since a quicker dewatering is very important in practice, in particular for offshore wells. In the present article, both criteria are used to evaluate and compare demulsifiers. 1.3. Extended Surfactants. As seen in the previous article,20 mixtures of demulsifiers can exhibit remarkable synergetic effects with a better performance than the components alone. Although, since Winsor’s suggestion back

2. EXPERIMENTAL PROCEDURE 2.1. Products Used. Liquid Phases. The systems studied were composed of an equal volume (5 mL) of distilled water containing the demulsifier and a dilution of Hamaca crude oil (8° API, 11% asphaltenes, acid number 2 mg KOH/g, kinematic viscosity 225 000 cSt at 100 °F) highly diluted in cyclohexane to produce the asphaltene concentration CA to be tested. Demulsifiers. All the surfactants used as demulsifiers are extended surfactants which were alkoxylated by our industrial partner LIPESA before the final synthesis steps that were carried out in our laboratory following procedures described elsewhere.44 Most of these extended surfactants have a C12 n-alkyl tail, i.e., a very common hydrophobe coming from dodecyl alcohol. The species tested have different headgroups, e.g., monoanionic as carboxylate or sulfate, or dianionic as phosphate, generally connected to the central extension with two 7066


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Figure 1. (Left) Variation of emulsion stability vs ES12-14-12-S demulsifier concentration CD at different asphaltene concentrations CA. (Right) Variation of minimum stability vs CD* at corresponding CA (right) with the proportional regime up to the threshold point T indicated as CD*T.

corresponds to a constant proportion between CA and CD*, as explained in detail elsewhere.20 As discussed previously,17 for CA values above the threshold, the asphaltenes in excess which are not absorbed stay in the oil phase; hence, the proportions of both natural surfactant and demulsifier in the interfacial mixture are constant. As a consequence, the required demulsifier concentration (CD*T) remains constant as well. As Figure 1 shows, this ES12-14-12-S surfactant, which is a highly hydrophilic species, behaves like a typical demulsifier, as reported in previous papers.17−19 Figure 1 also shows that its low stability at CD*T is rather good for a highly hydrophilic surfactant. It seems that its structure of extended surfactant and its large size, comparable to asphaltene molecules, tend to produce a synergy better than that yielded by smaller molecules, such as ethoxylated nonyl phenols.20 In the case illustrated by Figure 1, the CD*T (380 ppm) for a highly hydrophilic structure (with 12 EO and a sulfate group) is lower than the CD*T of the majority of the nonyl phenols studied in a previous paper20 (CD*T values ≥ 500 ppm for hydrophilic species with 11 or more ethylene oxide units). It would thus be highly justified to carry out further systematic studies to find out whether and how the performance is related with the structure. The roles of the tail and the headgroup in altering the surfactant characteristic parameter (SCP) in the HLD equation are well-known58,59

ethylene oxide units or more. The intermediate central part is a poly(propylene oxide) chain that is mostly hydrophobic. Some species have no anionic group but a long polyethoxylate chain as the only hydrophilic headgroup. All the extended surfactants used are hydrophilic and readily soluble in the aqueous phase. Their main characteristics as well as the nomenclature used are shown in Table 1. 2.2. Performance Test (Modified Bottle Test). As in previous articles,16−18 a modified bottle test is used to focus on the interaction between the asphaltenes and the demulsifiers at optimum formulation with minimum secondary effects, such as convection and diffusion delays. The demulsifier is prepared as various aqueous solutions down to 10 ppm concentration. The Hamaca crude oil is diluted in cyclohexane to prepare solutions containing over 100 ppm of asphaltenes. The 10 mL samples containing 5 mL of each phase are left to equilibrate for at least 24 h so as to prevent any nonequilibrium effect that could alter the emulsion morphology. The emulsification is carried out in a beaker using an Ultraturrax blender at a speed of 11.000 rpm during 30 s. The emulsion is then poured in a graduated tube (time “zero”) which is left to rest vertically at ambient temperature (22 ± 2 °C). The emulsion persistence, called stability in this article and plots, is the amount of time in minutes required for the separation of half the water, i.e., 2.5 mL, as justified elsewhere57 and as used previously.17−20

3. RESULTS AND DISCUSSION 3.1. Performance Test on a Typical Extended Surfactant. The basic studies conducted in previous articles, in particular the last one,20 are carried out here with a typical extended surfactant named ES12-14-12-S that contains both intermediate parts [C 12 (PO) 14 (EO) 12 SO 4 Na] so as to determine how this kind of large surfactant behaves in comparison with others that have head and tail only. The left plot in Figure 1 displays the variation of emulsion stability versus demulsifier concentration CD for systems containing different asphaltene concentrations CA. As noted in previous articles,17,18 there is a clear minimum stability in each curve that defines the optimum demulsifier concentration CD* for the corresponding asphaltene concentration CA. The blue diamond symbols on the right-hand side plot (Figure 1) show these minimum stability points vs CD*. The square dots indicate the same points as a CD* vs CA plot in which the breaking point, called the threshold T,17 corresponds to CD*T. The straight line CD* vs CA below the threshold is found to exhibit a unit slope in the log−log scale plot. This zone has been called the proportional regime17 because it

σ = σ0 + 2.25kNC for anionics

(5)

β = β0 − EON + 2.25kNC for polyethoxylated nonionics (6)

in which NC is the length of the linear tail (number of carbon atoms), EON is the average number of ethylene oxide groups, and the constants σ0 and β0 are the surfactant characteristic parameters at the extrapolation NC = 0, which depend on the headgroup. However, the role of the central part of the extended surfactants is still not well-understood. Some studies33,34,40,49,51 have shown that a PON (number of propylene oxide units) increase produces a rise in lipophilicity, although less significant than an increase in the alkyl group length. From the point of view of lipophilicity, each propylene oxide unit is equivalent to only half a methylene group33,34 in the alkyl tail. Moreover, the increase of an EO group in alkylphenol or alcohol ethoxylate is 7067


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The species ES12-14-12 with no ionic head is hydrophilic because of its 12 EO groups, but it is of course much less hydrophilic than the other two surfactants. Consequently, according to eq 4, a larger CD* concentration is expected for ES12-14-12, as discussed elsewhere.17,20 Besides, the phosphate head surfactant has two charges. Hence, it is more hydrophilic than the sulfate head one, as corroborated by its Critical Micelle Concentration44 (value not shown) and σ values (see Table 1). Lower σ values correspond to more hydrophilic surfactants. It is thus expected that phosphate extended surfactant has a lower CD* dose. However, as seen in Figure 3, it is the sulfate head species that has the lower CD*. This may be due to the fact that it occupies a smaller area at interface44 or that the phosphate species is too hydrophilic and migrates too much in water, as was found for highly ethoxylated nonyl phenol.20 Both ionic head species exhibit good performances, e.g., about 10 min as far as the minimum stability at CD* is concerned, with the sulfate species having a lower CD*. However, in this case, it is worth taking into consideration the wide range of low stability exhibited by the purely nonionic head species. This undoubtedly represents a strong advantage of this surfactant over the others, particularly since it occurs in the 100−500 ppm range, i.e., below the CD* (600 ppm). As a matter of fact, the performance of the nonionic extended surfactant at CD = 100 ppm is better than the one exhibited by the other species, even at their higher optimum CD*. This result clearly indicates that the demulsifier performance not only depends on the minimum dose CD* to attain the least stable emulsion. As suggested in the Part 6 article of this series,20 the evaluation should also include the attained minimum stability value. It should be noted that the good performance shown in Figure 3 for the nonionic species is not related to its lowest stability at optimum, but to its wide low stability range, which is unusual. In the majority of the cases we tested, the low stability takes place over a narrow range of formulation, as it happens for a minimum tension or a maximum solubilization with no known exception.4,64 This wide range is thus an outstanding feature that may be referred to as an exceptional performance robustness. This outstanding effect may be explained as follows. The EON is the average chain length in a typical Poisson distribution; i.e., in such commercial products, many different oligomers could partition22,24,27,59 in different ways between the phases and the interface. It is well-known that, when the total concentration of a polyethoxylated commercial surfactant decreases, the species tending to migrate to interface becomes more hydrophilic.24,65,66 This effect is particularly strong when the total concentration is very low,65,66 as in the present case of ES12-14-12 nonionic demulsifier. It is worthwhile mentioning that the effect is opposite and less significant for anionic mixtures and even less so with anionic−nonionic mixtures.28 Consequently, when the nonionic species ES12-14-12 concentration decreases below its CD*, its interfacial contribution is likely to become more hydrophilic, and thus more efficient to compensate for the asphaltene hydrophobicity. In other words, in the term XDSCPD of eq 4, the XD decrease is compensated by the SCPD absolute value increase so that the demulsifier interfacial mixture effect XDSCPD stays equal in practice, thus maintaining a low stability instead of the usual rapid increase This favorable fractionation effect is probably due to the wide distribution of ethoxylation around EON = 12 in surfactant ES12-14-12.

equivalent to the reduction of the alkyl tail by 3 methylene groups.60 From this trade-off, adding a single EO unit may be equivalent to shortening the extension by 6 PO units. Previous research has adapted classical correlations for optimal formulation in enhanced oil recovery applications to include the effect of EO and PO units.14,61 This kind of surfactants has been shown to exhibit an abnormal (although interesting) behavior compared to ordinary surfactants, as, for instance, the fact that they produced the surfactant and lipophilic linker combination effect in a single molecule.9,29−31,38,62 This intramolecular mixture resulted in a considerable improvement in tension/solubilization performance with polar oils, an outstanding advantage to produce microemulsions with such oils.9,33,34,38 Another abnormal, but interesting, behavior was the unexpected increase in performance of extended surfactants containing more than 10 PO when ACN increases, as reported for solubilization39 and corroborated for tension.42 It is important to keep in mind that recent studies41,44 have shown that the first 2−3 propylene oxide units close to the hydrophilic head are partially hydrated; they are thus more or less lying flat on the interface rather than being perpendicular to it,63 as the lipophilic tail is expected to be. This orientation is likely to produce some disorder close to the interface, thus allowing the formation of a microemulsion without alcohol, and to avoid the asphaltenes rigid association when these are mixed with the surfactant.18 As shown in a previous report,44 the order of PO and EO intermediate units (sequential or homogeneous arrangement) must be controlled as a progressive polarity change in the molecules. The interaction between asphaltenes and surfactant thus depends on the relative location of the surfactant polar groups. In Figure 2, the likely locations of the different portions of an extended

Figure 2. Location of PO groups as the tail large extension with an excess in lipophilicity and a short twisted part close to the interface.

surfactant at the interface are shown. The picture corresponds to an extended surfactant with an ionic head, 2 EO, and 7 PO before the n-dodecyl tail. 3.2. Effect of the Anionic Hydrophilic Group. The left plot in Figure 3 displays the variation of CD* vs CA for three demulsifiers of the series ES12-14-12-X containing a dodecyl tail with a long intermediate extension with 14 PO, then with 12 EO, and finally with a mono- or dianionic head, such as a sulfate or phosphate, or without any ionic head. 7068


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Figure 3. (Left) Plot of CD* versus CA for three extended surfactants of the series ES12-14-12-X. (Right) Stability versus demulsifier concentration for three extended surfactants of the series ES12-14-12-X, with CA = 500 ppm.

Figure 4. (Left) Plot of CD* versus CA for three extended surfactants of the series ES12-14-2-X. (Right) Stability versus demulsifier concentration for three extended surfactants of the series ES12-14-2-X, with CA = 500 ppm.

Figure 5. (Left) Plot of CD* versus CA for extended surfactants with a sulfate group. (Right) Stability versus demulsifier concentration for extended surfactants with a sulfate group, with CA = 500 ppm.

have no partitioning problems because the effects of the two head parts cancel each other out.28 The carboxylate (ES12-142-C) species presents the largest CD* without a very low stability; it is thus the worst of the three. The sulfate (ES12-142-S) and phosphate (ES12-14-2-P) species more or less have the same minimum stability (30 min) with a lower CD* dose for the phosphate, as expected because, with its two charges, it

This is why another study on hydrophilic group effect was carried out with an insignificant ethoxylation degree, i.e., 2 EO groups added so as to easily connect the anionic part to the poly(propylene oxide) chain. Figure 4 shows the CD* vs CA and stability vs CD plots for an extended surfactant series with 2 EO groups only. They are less hydrophilic than the previous ones with 12 EO and probably 7069


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Energy & Fuels is more hydrophilic. It is important to remark that the CD*T values of the saturation regime of the two surfactants are similar. Figure 4 shows that, as the surfactants hydrophilicity is increased, both performance indices improve. As a consequence, we can state that the phosphate head is favorable in the 2 EO series but disadvantageous in the 12 EO one, probably because the long 12 EO chain makes the surfactant too hydrophilic. This result indicates that the presence of the EO group is an important issue not only for its hydrophilicity but also because of its influence on the size of the molecule and its exact location with respect to interface. Thus, the greater the EO number, the heavier the molecule, which could affect its diffusion velocity toward the water−oil interface. 3.3. Effect of the EON Hydrophilic Chain Contribution. ES12-14-12-P (Figure 3) and ES12-14-2-P (Figure 4) surfactants have a highly hydrophilic anionic headgroup with two charges, but the second one, which has 2 EO only, is hence less hydrophilic. In other words, we could say that the first surfactant is too hydrophilic, and that this is the reason why it is not as efficient as the second one when considering CD*, even at or above CD*T. To ascertain whether such a trend can be generalized, the effect of the number of EO units is studied in a series of four available extended surfactant molecules with a sulfate headgroup (i.e., less hydrophilic than phosphate) and with a wider EO range than that used in the two previous phosphate species. Figure 5 displays the same plots for species having a C12 tail with 14 PO units and 2, 7, 12, and 20 EO units before the sulfate anionic head. The CD* vs CA left plot (Figure 5) shows that there is a significant difference for the larger and most hydrophilic molecule ES12-14-20-S, with a lower CD*, in both the proportional and the saturated regions. This lower CD* with the more hydrophilic surfactant is consistent with the general trend previously seen in eq 4, as well as with the fact that the minimum stability is not as low as with less hydrophilic ones, as seen here with only 7 or 12 EO. This trend, already observed in the ethoxylated nonyl phenol series in a previous paper,20 appears in both regimes, including above the threshold. The best performance as a compromise between a low CD* and a low minimum stability occurs for the EO7 species (see right plot in Figure 5). The lower hydrophilicity (when compared to that observed in the EO 12 and EO 20 species) could result in less partitioning in water,67 and the smaller size of the molecule is likely to favor a quicker motion to interface and adsorption, thus a faster breaking of the emulsion. Figure 6 shows the minimum stability performance vs EON at different CA concentrations, including above the threshold, which is here at about CA = 500 ppm. Then, the EO7 species, which is hydrophilic, but not excessively, is by far the best option whatever the asphaltene concentration. As the previous graphs show, the CD* vs CA plot for various extended surfactants indicates that the general phenomenology and the trends observed are basically the same as those observed with nonionic commercial demulsifiers.17,18 It is also worthwhile mentioning that the magnitude of the minimum stability is consistent with, and even better than, the results obtained with commercial demulsifiers.68−71 3.4. Effect of the Polypropylene Extension Length PON. Another way of altering the extended surfactant

Figure 6. Stability at optimal formulation versus ethylene oxide number for extended surfactants with a sulfate group for different CA.

characteristic parameter is to change its extension length, i.e., its propylene oxide number PON. Since the propylene oxide chain is a hydrophobic part of the tail, an increase in PON is expected to significantly increase the lipophilicity because of the three carbon atoms making up each unit. However, this is not so: adding one PO unit has been found to be roughly equivalent to half the change produced by adding a methyl group in an n-alkyl chain,33,40,61 i.e., a small change in the surfactant characteristic parameter according to eqs 5 and 6). Obviously, this equivalence is an approximation because the poly(propylene oxide) chain is branched, and such a nonlinearity is known to significantly alter the tail hydrophobicity.51,72,73 Nevertheless, it may be said that the most important effect of a poly(propylene oxide) intermediate in an extended surfactant lies in the fact that the tail becomes longer and bulkier without being much more hydrophobic. As a consequence, the demulsifier PO chain tends to migrate where the asphaltenes are located, i.e., on the oil side of the interface. It should be remembered that, as Figure 2 shows, the first 2−3 PO units are in a twisted intermediate zone, thus producing a desirable disorder in the oil phase close to the interface. This creates favorable rheological effects and reduces the necessity of a lipophilic linker or alcohol to avoid an asphaltene ordering and to reduce a possible rigidity in the oil phase close to the interface. In order to pinpoint the actual influence of the poly(propylene oxide) chain length, two extended surfactants were selected (ES12-4-10 and ES12-10-10), with 4 and 10 PO units, respectively, both with a 10 EO head without ionic part, as ES12-14-12 has been shown to be particularly effective as a demulsifier. Figure 7 clearly shows the influence of the PO intermediate chain length by comparing the ES12-4-10 and ES12-10-10 plots in the proportional and the saturation regimes. It can be seen that both extended surfactants break the emulsion in a short period of time (2 min for ES12-4-10 and 4 min for ES12-1010). Moreover, ES12-4-10 has fewer PO units. It is thus slightly less lipophilic, and as a consequence, it exhibits a lower CD*, as is usual for a more hydrophilic surfactant.17,18 ES12-4-10 has also a lower molecular weight and is likely to diffuse, adsorb, and mix faster with asphaltenes. It should be remembered that the twisted part of the PO chain corresponds to the first 2−3 PO units. Hence, this disorder effect on the asphaltene gel zone is identical for both surfactants. 7070


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Figure 7. (Left) Plot of CD* versus CA for nonionic extended surfactants. (Right) Stability versus demulsifier concentration for nonionic extended surfactants, with CA = 500 ppm.

Since the headgroup can be made with an ethoxylated chain and an ionic group, a compromise must be reached to obtain the proper hydrophilicity with the two polar parts. With 12 EO, a phosphate group is too hydrophilic, and a sulfate is more adequate. With only 2 EO, the phosphate group is appropriate, better than the sulfate and even better than the carboxylate. With a sulfate group, a 12 EO chain produces a species that is too hydrophilic, and 2 EO produces a species that is not hydrophilic enough. The best performance is reached with 7 EO. The surfactant characteristic parameter may be changed by altering the number of propylene oxide units PON. Since the propylene oxide chain forms part of the lipophilic part, reducing PON is equivalent to increasing hydrophilicity. With 10 EO as the only headgroup, 4 PO results in a surfactant that is more hydrophilic, thus more appropriate, than with 10 or 14 PO. The lower molecular weight also tends to speed up the kinetics effects during the mixing with asphaltenes. Hence, ES12-4-10 results in the fastest breaking rate of the present study (less than 2 min). A high molecular weight nonionic extended surfactant [C12(PO)14(EO)12], which happens to be a mixture of many different oligomers, was found to produce a low W/O emulsion stability in a wide concentration zone below optimum concentration CD*.

Additionally, Figure 7 shows that ES12-4-10 has a similar low stability range around a lower CD* dose, which accounts for its better performance. It is worthwhile mentioning here that the HLB of ES12-4-10 is about 18 (β = −3.6), a value close to that of the demulsifiers with the best performance reported by Al-Sabagh et al.74 and Pereira et al.,18 with HLB values of 18.6 and 19, respectively. In the present case, the overall results are similar. Nevertheless, the reason is different from the case of some ethoxylated demulsifiers and some ionic head extended demulsifiers, because the increase in hydrophilicity is currently produced by a decrease in PON, which also implies a reduction in the molecular weight. Figure 7 shows that ES12-14-12 (HLB = 13.5, β = −0.7), which is less hydrophilic than ES12-4-10 (HLB = 18.2, β = −3.6) and ES12-10-10 (HLB = 17, β = −2.1), requires a higher CD* as expected. ES12-14-12 is larger and thus probably slower to move to the interface and mix with asphaltenes. However, even if it is not as efficient as ES-12-10-10 and ES12-4-10, it is still attractive in practice because of its wide low stability zone, which may be qualified as an excellent robustness. An even larger molecule with a Guerbet49 alcohol double tail and a high EON to compensate for the extra large tail, though not sufficiently because of its lower HLB (12.5), is not efficient. Its poor performance is probably due to its unfavorable characteristics such as its excessive hydrophilicity and its high molecular weight, as well as an apparently very high threshold. It is clear that not only the hydrophilicity of the surfactant but also its structure can influence its placement at interface and its interaction with asphaltenes. Thus, both performance indices must be taken into consideration since they cannot readily be predicted because of the many possible effects,70,75 and sometimes also because of the need to find a compromise between opposite effects. The situation is probably even more complex in practice, since secondary effects, such as bulk and interfacial rheology at optimum formulation, are known to alter the value of the minimum stability, in particular with heavy crudes.76

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +58-274-2402957 (J.G.D.-L.). *E-mail: [email protected]. Fax: +58-274-2402957 (J.-L.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Ali Bahsas from Universidad de Los Andes, Dr. César Scorzza from BIOFARCO, and our industrial partner LIPESA, for their valuable contribution in the synthesis and analysis of the extended surfactants tested.

4. CONCLUSIONS Extended surfactants follow the trends we previously observed with regular surfactant demulsifiers; i.e., they require a lower CD* dose to attain the best instability when they are highly, but not excessively, hydrophilic, i.e., for −3.5 < SCP < −2.

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REFERENCES

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