Breaking of Water-in-Crude Oil Emulsions. 6 ... - ACS Publications

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Breaking of Water-in-Crude Oil Emulsions. 6. Estimating the Demulsifier Performance at Optimum Formulation from Both the Required Dose and the Attained Instability José G. Delgado-Linares,*,† Juan C. Pereira,‡ Miguel Rondón,†,§ Johnny Bullón,† and Jean-Louis Salager*,† †

Laboratorio FIRP, Universidad de los Andes, Mérida 5101, Venezuela Laboratorio de Petróleo, Hidrocarburos y Derivados, Universidad de Carabobo, Valencia 2005, Venezuela

Energy Fuels 2016.30:5483-5491. Downloaded from pubs.acs.org by KAROLINSKA INST on 01/23/19. For personal use only.

‡

ABSTRACT: Hydrophilic surfactant molecules with the proper formulation are able to break W/O emulsions stabilized by asphaltenes and other lipophilic amphiphiles as found in the effluent of petroleum wells. The demulsifier performance is here tested according to two critera. The first one, as in previous research, is the minimum dose of demulsifier used to attain the minimum stability at the so-called optimum formulation in a simplified bottle test. The second criterion is the value of this minimum stability at optimum formulation that has a direct relation with the separation time. Our findings show that, in a family of ethoxylated surfactants, the best demulsifier is a hydrophilic one, though not too much. When the demulsifier is a mixture of two surfactants, it usually exhibits an intermediate behavior between the components. However, the mixture sometimes appears to be better than any of the components alone with some synergistic effect that improves the performance.

1. INTRODUCTION The formation of a water-in-oil (W/O) emulsion occurs in almost all the cases of crude production because of the stirring of the W/O mixture and the presence of natural lipophilic surfactants1−4 which will be referred to in this paper as asphaltenes. The stabilization mechanisms involve the presence of asphaltenes as lipophilic surfactants adsorbed at interface as well as their contribution to the formation of a viscous or rigid film around the water drops that slows down the interdrop film drainage and retards or inhibits the coalescence.4−9 The role of the demulsifier, which is basically a hydrophilic surfactant according to the physicochemical formulation concept,10 is to counteract these stabilization mechanisms by attaining the socalled optimum formulation at interface. The demulsifiers that have been used in the past 100 years11 consist in a large variety of molecules from simple soap to polymeric or dendritic species. This is not surprising since, as will be discussed later, a formulation able to break an emulsion can in principle be obtained with any mixture of lipophilic and hydrophilic surfactants. However, the demulsifier performance, i.e., the required dose to be applied and its actual effect as the necessary time to break the emulsion, depends not only on the demulsifier used but also on other factors, as will be discussed later. Scores of studies have been undertaken to improve demulsifiers with different ionic or nonionic surfactants and surfactant mixtures. Adequate solutions have been reached for specific examples, and trends have been found for some selected crude oil, but the findings of most of these reports are empirical and limited in practice. A general understanding is thus required. It may be reached by disaggregating the different phenomena involved and studying them independently. The present paper uses a testing technique (presented and justified in the first two articles of this series12,13) which focuses © 2016 American Chemical Society

on the formulation of the optimum mixture between the two adsorbed surfactants, i.e., the adsorbed asphaltenes and the adsorbed demulsifier, in a situation in which other phenomena able to retard the film drainage and delay the coalescence are eliminated so that the role of the actual asphaltenes− demulsifier pair can effectively be estimated. In these simplified conditions, the performance of the demulsifier is reported not only as its required dose to reach the minimum stability but also as the actual value of that minimum stability that is related to the water separation time. In practice, the demulsifier optimization will often involve a compromise between both performance indices. 1.1. Emulsion (In)stability: Formulation Main Influence and Secondary Effects. It has been known for over 40 years that emulsion stability is extremely low when the interactions of the interfacially adsorbed surfactant or surfactant mixture are exactly equal for oil and water.14−19 In the 1970s, this equal affinity balance at interface was called “optimum formulation” in enhanced oil recovery because its occurrence results in an interfacial tension minimum and often in a threephase behavior. This happens when a very specific correlation is obtained between the formulation variables.20,21 A more sophisticated study of the general formulation concept presents this correlation as the surfactant affinity difference (SAD) or as its dimensionless equivalent, i.e., the hydrophilic−lipophilic deviation (HLD),22 as a very general expression that depends on the nature of oil, water salinity, surfactant, and cosurfactant characteristics, as well as temperature. An up-to-date review on this issue23 explains why the HLD approach is much more complete and accurate than the Received: March 21, 2016 Revised: May 24, 2016 Published: June 20, 2016 5483

DOI: 10.1021/acs.energyfuels.6b00666 Energy Fuels 2016, 30, 5483−5491

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Energy & Fuels

described with the Hydrophilic−Lipophilic Deviation (HLD) which takes into account all the formulation variables as follows22

hydrophilic−lipophilic balance (HLB) approach because it takes into account not only the surfactant effect but also the influence of other variables. As previously mentioned, at optimum formulation where HLD = 0, the emulsion is highly unstable as if no surfactant were present in the system24 whatever the value of other factors, such as surfactant structure, oil nature, salinity, asphaltenes content, and/or temperature. Reaching an optimum formulation is thus the main condition to attain a very unstable emulsion. The chemical dehydration of crude oil makes use of this general property that essentially consists in adding some hydrophilic surfactant (so-called demulsifier) to the emulsion to be broken. The demulsifier effect in the formulation is the exact opposite to that produced by the lipophilic natural surfactant. According to the principle proposed 25 years ago,10 this combination produces an asphaltenes−demulsifier mixture at optimum formulation. This technique has been corroborated by other research either that used an explicit optimum formulation correlation25 or that was indirectly related, viz. where a numerical expression was not explicitly mentioned.26−28 It is worthwhile remembering that the nature of the surfactant, its molecular weight, structure, hydrophilicity, and/ or branching produce an effect on HLD23,29 through the surfactant species or mixture characteristic parameter. These features thus contribute to the demulsifier effect; as a consequence, they may be included in the characteristic parameter. Other physical factors were found to influence the stability of the emulsion, such as fluid densities, drop size, low tension value, drop flattening, Gibbs−Marangoni effects, oil viscosity or elasticity, etc. All of these factors alter the oil film drainage velocity in W/O emulsions. Moreover, they tend to make the emulsion more or less stable, specifically at optimum formulation, but they do not change the general phenomenology. Even though the general trend of emulsion stability vs HLD is well-known, the actual maximum and minimum values can vary according to the scanned variables (e.g., salinity, ethylene oxide number, temperature, etc.).17,19,24,30,31 In dehydration practice, the minimum stability value at optimum formulation is important because it is related to the separation time and/or to the separator size. This value depends on the opposite effect of the asphaltenes and demulsifiers in the conditions of the emulsion interface, i.e., the nature and characteristic of the substances, their compatibility to mix, and the kinetic effect, which is not known except for some trends indicating that, at the exact optimum formulation point, the kinetics is much faster than on the sides.32 Emulsion stability also depends on dynamic effects, such as interfacial rheology, fluid viscosity, alcohol, and other dilution additives which produce secondary effects, most of the time measured out of the optimum formulation where they could change drastically. Nonequilibrium effects, like tension dynamic variations or surfactant adsorption, may be important too, and some studies33,34 have shown that, close to optimum formulation, they can be much quicker. The present experiments were performed in the same simplified conditions as those used in the previous articles of this series12,13,35 in order to test the formulation main effect and reduce the secondary ones. 1.2. Interfacial Formulation Change When Adding a Demulsifier. The concept of generalized formulation is

HLD = ln S − k ACN + σ + f (A) − a T(T − Tref ) (1)

for ionic surfactants

HLD = bS − k ACN + β + ϕ(A) + c T(T − Tref ) 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 effect (∼ma Ca, i.e., close to a linear variation versus alcohol concentration), and aT and cT are positive temperature coefficients. It is worthwhile underlining that the temperature effect is opposite for anionic and nonionic species due to different entropic, structural polarity, and hydration influences, as discussed elsewhere.20−22 If the oil phase is not an n-alkane, ACN is replaced by its equivalent EACN.36−39 The surfactant parameter is sometimes called “critical curvature”, although it is a dimensionless number40 and although it has been compared to other surfactant characteristics.41 It has been shown that σ and β linearly increase with the number of carbon atoms in the surfactant tail,42 and, in the case of β, it linearly decreases with the ethylene oxide average number EON.43,44 Optimum formulation is attained when HLD = 0. If the system studied corresponds to a real petroleum reservoir, many variables are fixed, e.g., the aqueous salinity, the crude oil EACN, as well as temperature. Moreover, most of the time, there is no alcohol, i.e., f(A) = 0, or the alcohol effect is incorporated in the surfactant parameter so that f(A) disappears from HLD.23 Thus, the surfactant characteristic parameter called SCP here (which is σ or β depending of the surfactant type) has a given value at optimum when HLD = 0, which may be calculated from the previous equation (1 or 2) entering the oil EACN, the temperature, and the water salinity, for this particular system: SCPSYS = k EACN + a T(or − c T)(T − Tref ) − ln S (or bS)

(3)

Hence, attaining an optimum formulation for such a system consists in using a (pure or mixture) surfactant with a characteristic parameter SCP = SCPSYS. The linear mixing rule between two surfactants has been found to be exact or at least approximate,45 if the selective partitioning is not too severe.46 Selective partitioning comes from the fact that there are different equilibria between the different surfactants in the oil and water phases, and with respect to what is adsorbed at interface. It produces a deviation between the mixture composition in the whole system, which is known from the experimental conditions, from its composition at interface, which corresponds to the correlations (1 and 2) for optimum formulation. Such partitioning is not very significant with a mixture of relatively similar surfactants. In the present case, this is likely to happen if the demulsifier is similar to asphaltenes in molecular size and structure, and if they are compatible enough to be fairly mixed at interface. In this case, the interfacial mixture will satisfy the following linear equation45 which is basically the same, although more accurate, than the suggested mixing rule that used HLB surfactant characteristics.12,13 5484


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Energy & Fuels XDSCPD + XA SCPA = SCPSYS

characteristic parameter SCPD* for which the minimum stability is reached according to eq 8. Consequently, as shown in the first paper of this series,12 there are two equivalent ways to determine the optimum formulation according to relation 5 when the asphaltenes type (SCPA) and water and oil nature as well as temperature (SCPSYS) are determined: (1) scanning the concentration (CD) of a given demulsifier (SCPD), or (2) having a given dose (CD) and scanning the nature of the demulsifier (SCPD). The first scanning is generally used because it is easier to obtain a continuous change of concentration along a scan, but the second one could be appropriate to compare the performance of two demulsifiers. In what follows, the demulsifier dose is used to scan formulation as in two previous articles of this series,13,35 and the stability vs CD is plotted, CD* being the value at which the minimum stability is reached, i.e., the optimum formulation. Such stability vs CD plots will be exhibited in our results below not only to determine CD* but also to estimate the performance as far as a very low stability is concerned. By plotting the value of CD* for a given surfactant demulsifier as a function of the asphaltenes concentration CA, attained by diluting the crude oil with a solvent, in the second article of this series13 it was shown that there is a linear relationship in a log CD* − log CA plot at low concentrations. In other words, at low concentration of asphaltenes, probably low enough to keep the asphaltenes as a lipophilic surfactant adsorbing at interface (i.e., with not enough asphaltenes available to go out of the interface), the general relationship was found to be as follows with a slope n = 1

(4)

SCPSYS is fixed for the given system, characterized by oil EACN, brine salinity, and temperature. XA and XD represent the mole fractions of the asphaltenes (A) and demulsifier (D) species at interface, and it is assumed that the two surfactants occupy the available interface area, i.e., that XA + XD = 1. XDSCPD + (1 − XD)SCPA = SCPSYS

(5)

Since the simplified experimental conditions consist in, as in previous articles of this series,13,35 a low concentration of both asphaltenes and demulsifier in the bulk oil and water phases, it may be assumed that the adsorbed A and D amounts are proportional to their concentration in the oil and water bulk phases, which is known to be the case in the initial part of almost all adsorption isotherms. In other words XA = kACA and XD = kDC D as well as kACA + kDC D = 1 (6)

where CA is the concentration of asphaltenes in bulk oil, and CD the concentration of demulsifier in bulk aqueous phase at equilibrium with the interface. The k are constants depending on the nature of surfactant, oil, and water and system temperature, and correspond to the initial tangent slope in the isotherm plots. Equation 5 may be rewritten as follows to attain the optimum demulsifier fraction at interface XD* and its optimum concentration in water CD* for a given species with an SCPD characteristic parameter and a system having an oil (EACN) and water (S) phases at temperature T according to eq 3:

log C D* = n log CA + constant

XD* = [SCPSYS − SCPA ]/[SCPD − SCPA ]

Such a relationship will be shown to be valid for all the data reported here below. The constant, which depends on the nature of A and D, may be called log kC so that

and C D* = kD[SCPSYS − SCPA ]/[SCPD − SCPA ] (7)

Since XD < 1, SCPSYS is somewhere in the range from SCPD to SCPA. SCPSYS indicates an exact balance of affinity of the surfactant (mixture) for the oil and water phases; hence, if the asphaltenes A are lipophilic, it implies that the demulsifier D is hydrophilic for this system. It is worthwhile mentioning that this is a necessary condition which is sometimes not well understood because, in practical cases of dehydration, the demulsifier is mixed with the oil phase and has to be displaced through the oil phase to the water drop interface by convection (some homogenization), diffusion, and adsorption. This also means that the demulsifier has to be soluble in oil in spite of being hydrophilic. This is why, in most cases, an aromatic solvent is added to a commercial demulsifier product to improve its solubility in oil. Equation 5 can then be rewritten as follows for a given proportion XD of demulsifier at interface, and hence a given concentration CD in the system, which is CD = (1 − kACA)/kD according to eq 6

log C D* = log CA + log k C may be written as C D* = k CCA (10)

Consequently, from eq 6, eq 10 becomes XD* = k intXA at interface

(11)

This means that, at optimum formulation, the CD* and CA concentrations in the bulk are proportional, as well as the interfacial molar fractions XD* and XA. This is why this part of the plot is called “proportional regime”. When the asphaltenes concentration is above a certain value called CAT for “threshold”, typically 1000 ppm, the CD* is found to stay basically constant with the increase in CA.13 This second part of the CD* vs CA plot, i.e., a horizontal line, will be reported for all our data. As explained in the second article of this series,13 above the threshold, the excess asphaltenes are not adsorbed and they stay in the oil phase. As a consequence, the adsorbed natural surfactants which are mixed with the demulsifier at interface are in the same proportion with the demulsifier, as indicated in eq 11, to attain SCPSYS according to eqs 5 and 7. The corresponding CDT* at CAT and above is very close to the required dose of demulsifier with the real crude (provided that the EACN effect is taken into account if the solvent has not the same EACN as the rest of the crude oil, i.e., its maltenes). This explains an a priori puzzling fact in which 50 ppm of demulsifier has a significant effect on 10 000 times more asphaltenes. In

SCPD* = [SCPSYS − (1 − XD)SCPA ]/XD or SCPD* = SCPA + [SCPSYS − SCPA ]/kC D

(9)

(8)

As shown in Figure 2 in the first paper of this series,12 for a given demulsifier (here SCPD), there is a concentration CD* in the system at which an optimum formulation for minimum stability is attained, according to eq 7. Similarly, for a given concentration of demulsifier (CD), there is a surfactant 5485


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Energy & Fuels Table 1. Characteristics of the Ethoxylated Nonyl Phenols

a

surfactant

ethylene oxide number (EON)

molecular weight

HLB

β (SCP)a

label

C9H19C6H4(C2H4O)6OH C9H19C6H4(C2H4O)8OH C9H19C6H4(C2H4O)11OH C9H19C6H4(C2H4O)30OH

6 8 11 30

440 572 704 1540

10.9 12.3 13.8 17.1

0.5 −1.5 −4.5 −23.5

NP-440-6 NP-572-8 NP-704-11 NP-1540-30

Defined as α − EON, with α = 6.5.21,42

other words, only a very small amount of asphaltenes is adsorbed at interface and mix there with the demulsifier.

2. EXPERIMENTAL PROCEDURE 2.1. 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) considerably diluted in cyclohexane to produce the CD value to be tested. 2.2. Demulsifiers. Two families of surfactants were used as demulsifiers. In order to compare with other publications, we used the series of ethoxylated nonyl phenol (manufactured by Witco Corporation) with an average ethylene oxide number called EON, with a Poisson distribution of ethylene oxide length according to the analysis47 (Table 1). Commercial surfactants were also used, such as triblock EON-PON-EON Pluronics from BASF sold as PE 4300 and PE 9400, respectively, labeled here CP-1750-14 and CP-4600-19, where the first number indicates its molecular weight and the second its HLB number. An ethoxylated (20 EO) sorbitan monooleate, sold as Tween 80 and labeled here ESM-1310-15, was also used. The first number indicates its molecular weight and the second its HLB number. 2.3. Performance Test (Modified Bottle Test). As in the previous articles of this series,12,13,35 the conditions are selected so as to focus on the formulation influence and to reduce secondary effects: (1) the demulsifier is placed in the aqueous phase before emulsifying so that the demulsifier convection and diffusion through oil is eliminated; (2) the interfacial adsorption of the demulsifier from water is thus relatively quick; (3) the crude oil is considerably diluted so that the rheological aspects of the oil film drainage are not important; and (4) the crude diluent used (cyclohexane) does not produce the precipitation of asphaltenes in this crude.48,49 The demulsifiers selected are prepared in different water solutions down to 10 ppm concentration, and the crude is diluted in cyclohexane to obtain solutions containing down to 100 ppm of asphaltenes, which are determined by the precipitation with an excess of heptane (n-heptane/ crude oil ratio = 40 in volume). The 10 mL samples containing 5 mL of each phase are then placed in a closed test tube that is slightly shaken (care is taken not to produce an emulsion) and left to equilibrate for 24 h so as to avoid any nonequilibrium effect that could alter the emulsion morphology.33,34 The emulsification is then carried out in a beaker using an ultraturrax turbine at 11.000 rpm during 30 s. The emulsion is then poured in a 10 mL graduated tube (this moment is taken as time “zero”) and left to equilibrate at ambient temperature (22 ± 2 °C). The emulsion persistence, i.e., what is referred to as “stability” in the plots, is the time required for the separation of half of the water, i.e., 2.5 mL, as used in previous reports13,35,50 after optimization.17,51

Figure 1. Emulsion stability versus demulsifier concentration CD for different ethoxylated nonyl phenols at CA = 500 ppm.

the same family as that of the surfactants studied in a previous paper,13 though from a different manufacturer and at different ethoxylation grades. The Hamaca crude oil is an extra heavy one, i.e., it is more viscous and contains more asphaltenes than the ones previously used in this series of articles.12,13 Such crude is often believed to be more difficult to dewater, but it is not necessarily the case here, since the diluted conditions used correspond to a low concentration of asphaltenes and a low viscosity. As can be seen (Figure 1), in all the surfactant cases, a minimum stability determines CD*. The aspect of these plots is similar to that previously published but with a wider demulsifier SCPD zone. The NP-440-6 is slightly hydrophilic (its SCPD is very close to SCPSYS); as a consequence, from eq 5, XD is large as well as CD*, according to eq 6. It is, however, an excellent emulsion destabilizer with a fairly wide range for its effective dose below CD*. This is not of much interest in practice because of the associated higher cost of the CD* high dose unless a low separation time is required, as in an off-shore situation. Note that, in such a case, a 300−400 ppm dose could be satisfactory in spite of not being exactly at optimum. It is also observed that an excess dose above optimum would be highly inconvenient, including with the loss of many species present in the commercial mixture staying in the water rather than adsorbed at interface. Another consequence of the elevated dose of demulsifier would be an O/W or double emulsion with a shift of formulation because of the partitioning. NP-1540-30 is a highly hydrophilic surfactant that also exhibits a high CD* value, but its performance is worse than that of NP-440-6 because it is not efficient to lower the stability, even at optimum. Its stability remains essentially unchanged with the dose, which means that the actual interfacial formulation is likely to be constant for the reasons explained

3. RESULTS AND DISCUSSION. EFFECT OF THE DEMULSIFIER NATURE AND DOSE ON EMULSION BREAKING 3.1. Emulsion Breaking Performance for Simple Surfactants (Commercial Ethoxylated Nonyl Phenols). 3.1.1. Results Obtained from the Measurement of Emulsion Stability and Its Minimum at CD* Dose. Figure 1 indicates the variation of the emulsion stability in the case of an oil phase containing 500 ppm (CA) of asphaltenes. The surfactants are of 5486


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Energy & Fuels elsewhere.46,51 It is thus clear that a demulsifier that is too hydrophilic is not a proper choice because a considerable amount of surfactant will not go to interface but into the water bulk, as noted in a previous paper.12 Middle-of-the road hydrophilic species NP-572-8 and NP704-11 exhibit a good performance in both ways; i.e., they have a low C D * and a low minimum stability. Previous research34,52,53 has shown that smaller surfactant molecules are likely to diffuse and adsorb at interface more quickly than larger ones; this might also have a small, but significant, effect favoring a low stability when using these demulsifiers, even at a low dose. However, the problem in these cases is that a narrow range for attaining a low stability and a quick increase of stability in both sides of the optimum requires a very accurate dose. It is worthwhile noting that NP-440-6 is even better as far as the low stability is concerned, probably because of another effect reported elsewhere,32−34 which is a significant increase in (pseudo)equilibration velocity at or close to three-phase behavior. The performance of the demulsifier then implies that these two effects should be taken into consideration because it is not known whether they are independent or partially related. This is why it is important to study the influence of the different variables on both effects. Figure 2 displays the two performances (concentration and stability) as a function of the nature of the demulsifier, i.e., its

Since the asphaltenes are the lipophilic surfactant and the demulsifier is the hydrophilic one, an increase in CA that tends to increase XA should be compensated for by increasing CD which also tends to increase XD. However, the tendencies for XA and XD to change should be canceled since there are not independent, and the mixture at interface to produce optimum formulation cannot change since, according to eqs 5−7, there is a unique value of XD* for the system. Figure 3 shows the relation between CD* and CA for the NP surfactant species referred to in the previous section. Each one

Figure 3. Plot of CD* versus CA for ethoxylated nonyl phenols in Hamaca crude oil diluted with cyclohexane.

of the four plots exhibits two straightline segments. The first segment at low concentration of asphaltenes and demulsifier is a straight line of unit slope, which means that CA and CD* are proportional, as indicated in eqs 9 and 10 previously discussed. Above the breaking point, also called threshold,13 a horizontal segment with a slight variation can be observed if the solvent (cyclohexane, in this case) and the crude do not have the same EACN. This zone corresponds to a saturation of asphaltenes at interface with an excess in the oil phase, probably close to the interface, called “segregation” elsewhere.55 As can be seen in Figure 3, the threshold depends on the surfactant, but it seems to be around 1000 ppm of asphaltenes. At CA = 500 ppm, the CD* values for the different surfactants are the ones that can be observed in Figure 1 for a stability minimum. In Figure 3, the best demulsifier to destabilize the asphaltene effect, as regards its required dose to obtain an optimum formulation, is the one corresponding to the lower proportional regime straight line, here NP-572-8, which is slightly lower than NP-704-11. This seems to be confirmed at a high asphaltene concentration, i.e., for CD*T above the threshold, but it is not known whether this can be generalized because secondary effects are likely to occur. It is worthwhile mentioning that the actual values of CD* are similar but not equal to the data reported elsewhere for the same kind of surfactants but different crude oils.13 At any rate, the trends are exactly the same, regardless of possible differences in the commercial products. 3.2. Effect of Surfactant Mixture on Emulsion Breaking Performance. Commercial nonyl phenol ethoxylates are mixtures of oligomers, generally with the same alkyl chain, but with a distribution of their ethylene oxide group number EON, often of the Poisson type.56,57

Figure 2. Variations of demulsifier concentration at optimum CD* and emulsion stability as a function of EON for different ethoxylated nonyl phenols, at CA = 500 ppm.

ethoxylation degree for a CA concentration of 500 ppm which will be found to be in the proportional regime. Undoubtedly, this kind of plot is useful to select an overall satisfactory performance in the NP-572-8/NP-704-11 region that, in this case, is well-defined. Further selection should check the CD* value for the crude and the compatibility with usual additives. This range is consistent with what is reported elsewhere with other crudes.12,54 3.1.2. Proportional Regime in the CD* vs CA Plot. In the previous section, eq 4 was maintained for a given crude oil (asphaltenes SCPA) and at a given asphaltenes concentration (CA and thus XA). A change in demulsifier (change in SCPD) was compensated by a change in CD*, thus XD. Now, both surfactants, i.e., asphaltenes (SCPA) and demulsifier (SCPD), stay the same and what is changed is the asphaltenes concentration CA and thus a tendency to change XA, that will be compensated by changing the demulsifier contribution CD, thus the tendency to change XD in eq 4. 5487


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Figure 4. (Left) Stability versus demulsifier concentration for two ethoxylated nonyl phenols and their equimolar mixture, at CA = 500 ppm. (Right) Plot of CD* versus CA for two ethoxylated nonyl phenols and their mixture.

Figure 5. (Left) Stability versus demulsifier concentration for two triblock copolymers and their equimolar mixture, with CA = 500 ppm. (Right) Plot of CD* versus CA for two triblock copolymers and their mixture.

As previous research has shown,46,52 such a mixture is likely to present a strong partitioning of the low ethoxylation species in the oil phase; it is thus unlikely that it will present an average mixing behavior. This is not exactly the case, and the mixture is closer to the poorest component. 3.2.2. Mixture of Two Commercial Demulsifiers. The second mixture consists of two EON-PON-EON triblocks larger molecular weight surfactants that are commercially used in demulsifying formulations, viz. CP-1750-14 and CP-4600-19, which have been previously checked individually in one of our previous articles.35 The larger molecular weight CP-4600-19 is more hydrophilic than the lighter one CP-1750-14. Figure 5 shows that its CD* is about 5 times lower. This could be explained from eq 7 since SCPD significantly differs from SCPA. However, as regards the separation time, the larger MW species yields better results, which is consistent with a greater hydrophilicity but contradicts what is expected from a “slower” molecule. As in the nonyl phenol ethoxylate family, the range of very low stability around the minimum seems to be wider for the component with higher molecular weight. The 1:1 mixture is basically the same as the best of the two components for CD* and for the minimum stability. Hence, in

This means that they are mixtures, but, in general, they do not contain different products and they tend to behave as pure products with the same EON number. In many surfactant applications, mixtures between pure or commercial products are used to seek a synergistic effect, as is the case in enhanced oil recovery where a mixture of two or three different components is the usual solution.58−62 The following results show how a mixture of different surfactants behaves as far as an emulsion breaking is concerned, and how to compare the mixture behavior with that of a single species. In other words, two hydrophilic surfactants are tested as well as their 50% mixture, using the same previously discussed stability vs CD and CD* vs CA plots. 3.2.1. Mixture of Very Different Ethoxylated Simple Surfactants. Two of the previously tested surfactants are mixed: NP-572-8, which exhibits a high performance for both the dose and the minimum stability, is mixed with NP1540-30, a much poorer surfactant in terms of dose and instability effect. Figure 4 indicates that the 1:1 mixture exhibits an intermediate behavior as far as the two performance indices are concerned. The CD* is roughly in the middle (in a log scale) as well as the minimum stability, with an also intermediate minimum range. 5488


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Figure 6. (Left) Stability versus demulsifier concentration for two demulsifiers of different nature and their equimolar mixture, with CA = 500 ppm. (Right) Plot of CD* versus CA for two demulsifiers of different nature and their mixture.

triblock surfactant is not important because the CD that has actually been selected does not necessarily need to have the exact same value as CD* that is determined by the stability minimum. Moreover, the lowest CD value in the range in which the stability is very low is lower than what is obtained with any of the components. In practice, a very good performance, about 10 times less stable than with the best surfactant alone, is reached with the mixture at 50−60 ppm and more. Obviously, mixing a good demulsifier with a poor one could produce a powerful synergy, in which each surfactant would bring its advantageous contribution. In this case, the CP-4600-19 produces a quick breaking at a low CD*, and the ESM-1310-15 a wide favorable range of demulsifier concentration that allows the decrease of the selected dose much below CD*. This is certainly very interesting when considering cost issues. It is not easy to find a simple explanation for this synergy. Further studies should be conducted to understand why it happens or how to predict it. It should also be noted that the equal proportion of both surfactants was arbitrary in the tests reported here and that another proportion could yield better results and could prove cheaper if it contains less CP-4600-19, which is an expensive component.

this case, it can be said that the mixture behaves in the exact same way as the best component that seems to dictate its behavior. 3.2.3. Mixture of a Commercial Demulsifier and an Ordinary Surfactant. The third kind of mixture to be tested consists of two surfactants of very different structures. One of them is known as a good commercial demulsifier for crude oils, the triblock CP-4600-19. The other one, which is generally used in other applications, particularly with polar oils in stable cosmetics or foods O/W emulsions, is the ethoxylated (20EO) sorbitan monooleate, here called ESM-1310-15. Its characteristics in SCP and molecular size are relatively close to those of the component that was mixed with the CP-4600-19 in the previous result, but with poor performance35 for breaking crude oil emulsions. Figure 6 left indicates that CP-4600-19 is a good candidate as a demulsifier with a low CD* and a fairly low minimum stability, as well as a relatively narrow, but reasonable, dose range for a low stability. As can be seen in Figure 6, right, CP-4600-19 is also a good demulsifier as far as CD*T is concerned with respect to the other tested surfactants in our series. On the contrary, ESM-1310-15 has a higher required dose CD* and a poor effectivity to reduce stability in the same way as NP-1340-30 does, which also has a medium hydrophilicity and a low molecular weight. Moreover, no CD*T was found for ESM1310-15 below 1000 ppm, which implies that, in practice, it should be rejected. Contrary to the two previously mentioned mixtures, this one is not made up of similar surfactant species with some difference in their size and hydrophilicity, but of two completely different molecules: a good demulsifier and a bad one. When this mixture was about to be tested, we were expecting either some kind of behavior average or some determinant effect of the good demulsifier, as was the case in the previous tests. It was surprising to observe a completely different behavior for what was called a synergistic mixture. Figure 6 (left) shows that the 1:1 mixture exhibits a very good performance, much better than that observed with the good surfactant CP-4600-19 alone. This performance improvement includes a very low stability at CD* and a very wide range of low stability around CD*. The fact that CD* is higher for the mixture than for the

4. CONCLUSIONS The first conclusion that can be drawn from our experiment results is that the demulsifier should be highly hydrophilic, though not too much, so as to exhibit a good performance at optimum for both a low required dose and a swift breaking of the W/O emulsion. The second conclusion is that mixing quite different species could help produce favorable and quick interactions between the two demulsifier components and asphaltenes with a remarkable performance improvement both in the required dose and in the swift W/O emulsion breaking. It should finally be kept in mind that the mixture concept includes two different situations. The first one consists in the intermolecular mixture of two different surfactants as reported above, as well as the so-called lipophilic linker63−65 effect. The second one consists in an intramolecular mixture in a single molecule (the so-called extended surfactants66,67) which, in some cases, present a fair demulsifier performance.35 5489


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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.). Present Address §

Grupo de Recobro Mejorado, Universidad Industrial de Santander, Bucaramanga 680002, Colombia. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Our gratitude goes to the French-Venezuelan Cooperation Program PCP “Petroleum Emulsions” for sponsoring the exchange of graduate students and professors between the two countries.

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