Breaking of Water-in-Crude Oil Emulsions. 8. Demulsifier Performance

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Breaking of Water-in-Crude Oil Emulsions. 8. Demulsifier Performance at Optimum Formulation is significantly Improved by a small Aromatic Content of the Oil José G. Alvarado, José G. Delgado-Linares, Ana María Forgiarini, and Jean-Louis Salager Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03994 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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Breaking of Water-in-Crude Oil Emulsions. 8. Demulsifier Performance at Optimum Formulation is significantly Improved by a small Aromatic Content of the Oil José G. Alvarado1, José G. Delgado-Linares1,2*, Ana M. Forgiarini1, Jean-Louis Salager1 1Laboratorio FIRP, Universidad de Los Andes, Mérida 5101, Venezuela 2Center for Hydrate Research, Chemical and Biological Engineering Department, Colorado School of Mines, Golden 80401, Colorado, USA *[email protected] Abstract Asphaltenes tend to aggregate in different structures depending on the aromatic content of the oil phase. The different aggregates adsorb at interface as some kind of lipophilic surfactant which tends to stabilize water-in-oil emulsions. Hydrophilic demulsifier molecules are added to combine with asphaltenes until the optimum formulation is attained at HLD = 0, thus resulting in the emulsion instability. It is found that with the change of asphaltenic aggregate structure produced by the aromatic content of the oil, its surfactant-like effect at interface is also altered. The performance of dehydration is significantly improved with only 5% of aromatic additive in the oil phase. 1.Introduction Water-in-crude oil emulsions represent a serious problem in the operations of crude oil production and transportation. In order to break up those emulsions once the crude oil reaches the surface facilities, several physical and chemical methods have been developed over the past decades. Heating, electrical treatment and the addition of a chemical are some of the most important strategies used to separate the water emulsified (dehydration or dewatering) in the crude oil matrix.1,2 As explained some decades ago, the chemical treatment consists in adding a hydrophilic surfactant (demulsifier) to the crude oil emulsion so as to modify the interfacial physicochemical formulation.3 When the demulsifier perfectly compensates the stabilizing effect of the natural surfactants of crude oil (referred to as asphaltenes), the system reaches the so-called optimum formulation in which the emulsion stability exhibits a minimum.3-6

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The optimum formulation can numerically be expressed with the following equality:7,8 𝐻𝐿𝐷 = 𝑙𝑛𝑆 ― 𝑘𝐴𝐶𝑁 + 𝜎 + 𝑓(𝐴) ― 𝑎𝑇(𝑇 ― 𝑇𝑟𝑒𝑓) = 0

(1)

for ionic surfactants. 𝐻𝐿𝐷 = 𝑏𝑆 ― 𝑘𝐴𝐶𝑁 + 𝛽 + 𝜙(𝐴) + 𝑐𝑇(𝑇 ― 𝑇𝑟𝑒𝑓) = 0

(2)

for polyethoxylated nonionics where HLD is the hydrophilic-lipophilic deviation, S is the salinity in wt % NaCl, ACN is the alkane carbon number, σ and β are the surfactant characteristic parameters (in general referred to as SCP), T is the temperature, f(A) and ϕ(A) are the alcohol effects (∼maCa, where ma is a constant that depends on the alcohol type and Ca is the 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.9 Applying the equations 1 and 2 to a petroleum system containing the reservoir brine (S) and the crude oil (EACN) at temperature T, the optimum formulation is reached for a specific parameter SCP*SYS, namely: ∗ +𝑙𝑛𝑆(𝑜𝑟 𝑏𝑆) ―𝑘𝐸𝐴𝐶𝑁 ― 𝑎𝑇(𝑜𝑟 + 𝑐𝑇)(𝑇 ― 𝑇𝑟𝑒𝑓) = 0 𝑆𝐶𝑃𝑆𝑌𝑆

(3)

with ∗ = 𝑋𝐷𝑆𝐶𝑃𝐷 + (1 ― 𝑋𝐷)𝑆𝐶𝑃𝐴 𝑆𝐶𝑃𝑆𝑌𝑆

(4)

as stated in the last two papers of this series.10,11 SCPD and SCPA are the surfactant characteristic parameters of the demulsifier and asphaltenes at interface, and XD is the molar fraction of demulsifier in the interfacial mixture.

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1.1.

Asphaltenes as naturally-occurring surfactants stabilizing crude oil emulsions.

Crude oil emulsions are typically formed from systems at HLD > 0, in which asphaltenes act as lipophilic surfactants stabilizing W/O emulsions.12 Hence, chemical dehydration is based on the addition of a hydrophilic surfactant in an appropriate concentration to counteract the effect of asphaltenes at the water-oil interface, and to attain the SCPSYS value at which the emulsion stability is minimum, i.e. SCP*SYS (eq. 4). The term SCPA in equation 4 depends on the asphaltenes nature at interface, in other words, this is an interfacial contribution directly related to the asphaltenes aggregation state and polarity. Asphaltenes are chemical species with a variable molecular weight (from 750 to 1000 Da on average for a single molecule) and made up of an aromatic core containing about half a dozen of polycondensed benzene rings to which alkyl chains and polar functional groups, such as hydroxyl, carboxyl, etc. are bonded.13-16 They are insoluble in linear paraffins, such as n-heptane and npentane, but are soluble in aromatic solvents, like toluene and xylene.17-22 As shown in many publications aphaltenes are organized in crude oil through aggregation structures known as nanoaggregates13,14,22,23 also called “micelles” by some authors.13,24 The average diameter of nanoaggregates is about 2-3 nm and they only contain approximately 6 asphaltenes molecules.13,14 The size of nanoaggregates is not discussed in the present paper whose goal is to show that when the aromaticity of the oil phase changes, there is a variation of the asphaltenes effect as lipophilic surfactants which mix with hydrophilic demulsifiers to attain the optimum formulation and minimum stability as discussed in our 7 previous papers.5,6,10,11,25-27 Nevertheless nanoaggregates are known to grow in size as the concentration of asphaltenes in the solution increases, thus, the association of nanoaggregates originate larger structures known as clusters (5 nm or more).13,14,28-30 Although there is no single criterion for defining the concentration thresholds at which aggregation occurs, it is known that at very low concentrations (< 10 mg/l), asphaltenes molecules begin to form very small nanoaggregates.24,31,32 In the following of this paper, this first asphaltenic association (which possible involves two or three molecules of asphaltenes) is referred to as the “initial” asphaltenes nanoaggregates.

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On the basis of those works, it is possible to visualize a conceptual model including four stages of asphaltenes aggregation (Figure 1), namely: Molecules (a very low concentrations), “initial” nanoaggregates, nanoaggregates and clusters. This model will allow us to better explain the results obtained in this work when the oil phase aromaticity is changed.

Figure 1. Scheme of asphaltenes aggregation structures The asphaltenic aggregates, solvated by resin molecules, are adsorbed in the water-crude interface during the production stage, generating a rigid film of high mechanical resistance around water droplets that inhibits their coalescence and thus stabilizing water-in-crude oil emulsions.30,33-35 1.2.

Performance Evaluation of Demulsifiers

In the industrial practice, it is economically important to minimize as much as possible the dosage of dehydrating agents need to reach the HLD = 0 condition. For this reason, great efforts have been done to evaluate the formulation conditions at which surfactants from different nature exhibit the best performance as crude oil demulsifiers. An important advance in this direction was done by Rondon et al.;25 in this work a new methodology was proposed in which the asphaltenes concentration (CA) is changed by diluting the crude oil with a proper solvent (ciclohexane, toluene), and for each CA, it is possible to know the 4


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demulsifier concentration (CD*) at which the emulsion stability is minimum (stability*). In general, for CA1 < CA2 then stability*2 > stability*1 [see Figure 2 (left)]. The selection of optimum conditions (CD* and stability*) is carried out through the a “modified bottle test”.6,10,11,25 Grouping in a Cartesian system the CA values and the corresponding CD* values in a log-log scale, a CD* -CA map is obtained. Published work has showed that this map is composed by two regions divided by a certain asphaltenes concentration so-called CAT that represents a breaking point (T). The first region is called proportional regime and corresponds to CA’s < CAT; there is a linear relationship between CD* and CA with unit slope. In this regime, the relationship between CD and CA can mathematically be expressed as: log CD* = Log CA + constant, therefore it means that CD*/CA is constant, i.e. CD* and CA are proportional. The second region is a horizontal line known as constant regime that appears for CA’s > CAT, in this CA range the demulsifier concentration (CDT*) remains essentially constant; this seems to indicate that the demulsifier only compensates for the effect of a certain amount of asphaltenes (CAT), which is probably saturating the interface. The asphaltenes added in "excess", i.e. at CA > CAT, should then be located not at interface, but near it as a segregated polar oil.6,10,25 The CD*-CA map constitutes an important tool for comparing different demulsifiers in terms of concentration required to attain the optimum formulation. In Figure 2 (right), at certain CA (for instance CAT) the surfactant 2 exhibits a better performance as demulsifier that surfactant 1 because CDT*surf2 < CDT*surf1. These figures are summing up previously published results.6,10,11

Recent work has applied an additional criterion to evaluate performance of a demulsifier, namely the minimum stability at optimum formulation that has a direct relation with the emulsion separation time.10,11

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Figure 2. (left) Variation of the stability with respect to the concentration of demulsifier for two values of CA (CA1 and CA2), where CA2 > CA1. (Right) Performance evaluation of two demulsifier agent (Surf.1 and Surf.2) through a CD*-CA map. The demulsifier agent optimum dosage (CD*) is the criterion of comparison at a given CA.6,10,11 In this paper, the effect of the asphaltenes aggregation states on the performance of a polyethoxylated noionic demulsifier is studied by using the emulsion stability curves (stability CD) and the CD* - CA maps. The change in the aggregation of asphaltenes and as a consequence in their activity as lipophilic surfactant, is induced by diluting the crude oil in a cyclohexane-xylene mixture. Thus, Figure 2 could also indicate the comparison with the same surfactant but with different dilution solvents. 2. Experimental Procedure 2.1. Liquid phases The studied systems were composed of an equal volume (5 ml) of distilled water containing the demulsifier and a dilution of a heavy crude oil from Orinoco oil belt (Hamaca: 8 ºAPI, SARA analysis (wt.): 34.9%, 29.8%, 18.3%, 17% and density at ambient temperature: 1.02) in different 6


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solvents to attain the asphaltenes concentration CA to be tested. The solvent aromaticity is varied by mixing cyclohexane with xylene in different proportions. 2.2. Demulsifierhydrophilic surfactant A commercial polyethoxylated nonyl phenol with an average ethylene oxide number EON of 8, provided by Witco Corporation and labeled as NP-8EO, was used as demulsifier. This surfactant showed a good performance for breaking up crude oil emulsions in previous reports.5,10,25 2.3. Modified bottle test As in previous papers of this series,5,6,10,11,25-27 a modified bottle test was used to evaluate the interaction between asphaltenes and demulsifiers at optimum formulation, in order to minimize the secondary kinetic effects. The demulsifier is solubilized in water in prepared solutions at different concentrations. Crude oil is diluted in cyclohexane or in a mixture of cyclohexane and xylene, to cover asphaltenes concentrations from 30 ppm (1 ml of crude oil / 284,443 ml of mixture crude oil + solvent) to 10,000 ppm (1 ml of crude oil / 16 ml of mixture crude oil + solvent). The samples containing 5 ml of each phase (aqueous solution and diluted crude oil) are placed in closed test tubes that are shaken very lightly left to equilibrate during 24 hours in order to avoid eventual non-equilibrium effects.36,37 The samples are then emulsified in a beaker using an Ultraturrax blender at a speed of 30,000 rpm during 30 sec. The emulsions are poured in graduated tubes (at time “zero”), which are then left to rest at ambient temperature (22 ± 2 °C). Previous work aimed at studying the stability of emulsions, have shown that the optimum formulation results are not dependent on the percentage of separation of a phase, whatever it is (water or oil) to evaluate the emulsion separation time.38-40 Consequently, the time (in minutes) required for the separation of half the water (i.e. 2.5 ml) is recorded and referred to as the emulsion stability or persistence as in previous works of this series.6,10,11,25

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3. Results and Discussion 3.1. Performance of the NP-8EO surfactant as emulsion breaker Figure 3 shows the emulsion stability variation as a function of demulsifier concentration (CD), for systems containing different asphaltene concentrations in the oil phase (CA) (only cyclohexane diluent in this case). The first element to be highlighted in this figure is that each stability curve passes through a welldefined minimum (marked with an asterisk), which is thus an accurate CD* value. This behavior is expected according to the results published in the previous papers of this series.6,10,11,25 It is worth noting that a CD concentration slightly lower than CD*, produces a notable increase in emulsion stability, with extremely high values in absence of demulsifier, i.e. out of the considered range.

Figure 3. Variation of the emulsion stability with CD for the systems NP-8EO/Crude oil Hamaca (diluted in cyclohexane)/Water.

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It may be said in general, that when CA increases, both the emulsion minimum stability* and demulsifier CD* at the optimum formulation conditions tend to increase. The increase in the minimum stability* obeys to the rise in asphaltene adsorption at the water-crude oil interface, and as a consequence, the steric repulsion between water droplets is reinforced.25,30 One could then think that these asphaltenes adsorption increases the thickness of the film that covers the water droplets. Figure 4 shows the CD*-CA map for the systems described above, with a more extensive experimental data range than the one used before by Rondón, Pereira and Delgado-Linares.6,10,11,25 This is particularly significant beyond the previously noted breaking point threshold T, and in particular when there is some superposition of the stability curves, as it is the case here in Figure 3 at CA ~ 700-1,000 ppm and CA ~ 7,000-10,000 ppm. It is worth noting that working at high CA values, it is possible to evaluate the physicochemical behavior of the system at asphaltene concentrations close to that one found in the crude oil in a natural state.

Figure 4. CD*-CA map for the system NP-8EO / diluted Hamaca crude oil (cyclohexane) / water 9


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The data showed in Figure 4 seem to indicate a behavior similar to that one reported in the previous articles of this series10,11,25, namely a proportional regime along the concentration range 30 ppm < CA < 7,000ppm and a constant regime for CA > 7,000ppm (blue line). However, at 700ppm < CA < 1,000ppm CD* is constant and the system suggests the occurrence of other narrow constant regime that, as will be shown below, is due to a change in the aggregation state of asphaltenes; taking into account both proportional and constant regimes, a double-step behavior could be graphically defined on Figure 4 (dotted line). Figure 4 (dotted line) indicates that the first proportional regime is located between CA = 30 ppm and CAT = 700 ppm. This first step is perfectly matching the results showed by Rondón in a CD* vs CA plot25. The unit slope straight line in a log-log plot confirms the proportionality between CD* and CA in this regime. It is thus though that in this first proportional regime range the asphaltenes available in the oil phase essentially act as surfactants, and therefore are adsorbed in the interface in the form of nanoaggregates according to Figure 1. When CA increases, the number of asphaltenic aggregates adsorbed increases, according to the adsorption model proposed by Álvarez and Márquez which exhibits a very low elasticity at optimum formulation.30,40-42 It is thought that in this system the formation of the “initial” asphaltenic nanoaggregates shown in Figure 1 is not occurring due to the relatively low affinity between asphaltenes and the solvent used (cyclohexane). Previous works have shown the formation of small asphaltenic aggregates when the crude is diluted in an aromatic solvent, for example, toluene.24,31,32 It is thought that in these systems the partition of the asphaltenes occurs, thus the more polar molecules associate with each other forming a sub-fraction that is poorly soluble in the oil phase, and the less polar molecules intensely interact with the aromatic oil phase.

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The system of Figure 4 was prepared by diluting the Hamaca heavy crude oil in cyclohexane, which is a naphthenic solvent and therefore does not dissolve the asphaltenes as well as toluene or any other aromatic solvent. Consequently, the partition phenomenon is minimized and almost all of the asphaltenes (the more and the less polar ones) are likely to be associated somehow. In this sense, when CA = 30 ppm (very diluted system) the state of aggregation of the asphaltenes could be the same as at CA = 100 ppm, 500 ppm or 700 ppm. In the range 30 ppm < CA < 700 ppm (first proportional regime range), the formulation change that occurs at interface through the adsorption of lipophilic asphaltenes (xASCPA), is compensated by an increase of the demulsifier dose (CD*) with hydrophilic effect, so that HLD = 0 in this range as previously discussed.6,10,11,25 The first breaking point of the curve (transition point threshold T) located at CAT = 700 ppm, marks the beginning of the first constant regime. This plateau extents up to CAT´ at about 1500 ppm and it is characterized by the invariance in the optimum concentration of dehydrating agent (CDT* = 130ppm). This invariance zone at CDT* suggests no change in the interfacial physicochemical formulation. On the other hand, the emulsion stability, in the concentration range corresponding to the constant regime (see Figure 3), practically does not change. It thus seems that the asphaltenes which are added after the T point, i.e. from 700 ppm up to 1,000 ppm, do not adsorb at interface anymore but accumulate close to it in the oil phase. Therefore, experimental evidence seems to indicate that in the first constant regime the asphaltene nanoaggregates adsorbed at the interface have essentially the same size and polarity in all the concentration range. Thus, in this regime only a fraction of the added asphaltenes are adsorbed at the interface, probably the most polar one, while the rest (the less polar asphaltenes) is located in the bulk of oil, or segregated in the region near interface as a polar oil.22,30,43-45 The increase of interactions between the more lipophilic asphaltenes and the oil phase (maltenes in the crude oil–cyclohexane in this case) when the constant regime is reached, can be explained based on the influence of crude maltenes or diluent on the solubility parameter of the oil phase.32

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It is worth remembering that the solubility parameter takes into account the molecular interactions that govern the solute-solvent systems.46 Mathematically, it is defined as:

𝛿=

𝐸𝑉

(𝐸𝑞.4)

𝑉

Where δ is the solubility parameter (cal/ml)1/2, EV is the vaporization energy for an ideal gas (cal/gmol), and V is the molar volume (ml/gmol).46 In general, a substance will be soluble in a solvent if their solubility parameters are similar. Since in the first constant regime, the Hamaca crude oil-cyclohexane system has little crude (Figure 4), the solubility parameter of the solution (oil phase) must be very close to that of the cyclohexane. Therefore, the solubility parameter of the oil phase is expressed through equation 5:32,46 𝛿𝑜𝑖𝑙 𝑝ℎ𝑎𝑠𝑒 = 𝜙𝑐𝑦𝑐𝑙𝑜ℎ𝑒𝑥𝑎𝑛𝑒𝛿𝑐𝑦𝑐𝑙𝑜ℎ𝑒𝑥𝑎𝑛𝑒 + 𝜙𝑐𝑟𝑢𝑑𝑒 𝑜𝑖𝑙𝛿𝑐𝑟𝑢𝑑𝑒 𝑜𝑖𝑙

(𝐸𝑞.5)

Where δoil phase is the solubility parameter of the oil phase constituted by cyclohexane and Hamaca crude oil, ϕ and δ is the volumetric fraction and the solubility parameter respectively of cyclohexane and Hamaca crude oil, being ϕcyclohexane >> ϕcrude oil. Based on the equation 5, δoil phase comes close to the solubility parameter of asphaltenes as the dosage of crude oil increases, due to the presence of aromatic compounds such as alkylbenzenes and resins in the maltenes.46 For this reason, the less polar asphaltenic fraction increases its interactions with the oil phase, while the more polar fraction is adsorbed at the water-crude interface. Above CAT´ = 1,500 ppm, CD* again increases proportionally with CA. The rise in the demulsifier dose to reach the optimum formulation indicates that the interfacial physicochemical formulation changes with respect to that corresponding to the first constant regime. Thus, for CA's > 1,500 ppm

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the interface nature is modified possibly due to the fact that the asphaltenic nanoaggregates have been associated to form clusters. The clusters formation can also be noted on Figure 3, in which it is clear an increase in the stability* for CA > 1,500 ppm. Moreover, it seems that the interfacial saturation with clusters occurs at CAT” = 7,000 ppm instead of CAT’ = 1,000 ppm for the nanoaggregates. It is thus conjectured that at this saturation condition, the oil-side interface is covered by asphaltene clusters. Based on the changes occurring in the interfacial physicochemical formulation, expressed through the double-step type behavior seen in Figure 4, an asphaltenic adsorption model in the water-crude oil interface is proposed in Figure 5.

Figure 5. Schematic representation of the adsorption of asphaltenic aggregates at the water-crude interface with respect to CA. System: NP-8EO / diluted Hamaca crude oil (cyclohexane) / water. 13


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This model displays the two association structures of asphaltenes in the interfacial region. On the one hand, in the first step of the curve (30 ppm < CA < 1,500 ppm) the asphaltenes adsorbed at the water-crude oil interface are associated in nanoaggregates; on the other hand, from the second step (1,500 ppm < CA < 10,000 ppm) the asphaltenes adsorb at the interface as clusters. It is important to highlight that it is possible that only a fraction of the clusters (constituted by the most polar asphaltenes) will be in contact with the water-crude interface as a surfactant, while the rest (less polar asphaltenes) will act as polar oil over the interface. Another possibility is that the clusters are not homogeneous aggregates and have areas, which are more hydrophilic and thus more likely to be closer to the interface. This model is closely related to the adsorption model proposed by other authors,30,31,47 regarding the dependence of the aggregation state of asphaltenes with respect to CA. Additionally, this model allows the interfacial physicochemical formulation, expressed through CD*, to change with the state of aggregation of asphaltenes, which is not only depending on the asphaltenes concentration, but also of asphaltenes affinity for the diluent, in particular its aromaticity. 3.2. Influence of oil phase aromaticity on the emulsion breaking performance for the NP8EO surfactant As mentioned previously, the increase in the aromaticity of the oil phase will increase the asphaltenes solubility parameter and thus inhibit the interfacial adsorption of any kind of asphaltenic aggregates. As a consequence, at optimum formulation both the emulsion minimum stability* and the emulsifier CD* dosage are likely to diminish. In this regard, and consistent to the fact that commercial demulsifier products contain a large proportion of aromatic solvent, it is appealing to determine the influence of this kind of addition, on the phenomenology of chemical demulsification. In the present case, xylene is added in different proportions to the diluting cyclohexane to alter the diluent aromaticity.

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Moreover, as it is known from the industrial point of view, the economic aspect of the crude dehydration is very important. For such purpose, the minimum addition of aromatic solvent capable of generating relevant changes in the performance of the tested NP-8EO demulsifier will be determined in what follows. In this second study the solvent for the Hamaca crude oil will be a cyclohexane/xylene mixture, starting with a very low aromatic content. Figure 6 shows the results obtained when the crude oil is diluted in cyclohexane/xylene solvents in volume proportions of 95/5 and 90/10.

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Figure 6. Variation of the emulsion stability with CD for the systems NP-8EO / diluted Hamaca crude oil / Water. A) Dilution of Hamaca crude oil in the mixture cyclohexane/xylene 95/5 v/v. B) Dilution of Hamaca crude oil in the mixture cyclohexane/xylene 90/10 v/v. As in the previous Figure 3 case, the stability curves are well defined with a neat minimum for each asphaltenes concentration CA. It worth noting that when compared to the case of pure cyclohexane solvent in Figure 3, the addition of 5-10 vol% of xylene to the diluent considerably decreases the emulsion minimum stability* at the optimum formulation, even for high asphaltenes content close to real practical cases, as 10,000 ppm. As previously discussed, this reduction is probably due to the interfacial segregation of xylene,25,4345

which boosts the aromatic character of the region close to the interface, and as a consequence

the asphaltenes aggregates are more solubilized or more dispersed. In other words, it may be said that when the diluent becomes more aromatic, the asphaltenes aggregation and their interfacial adsorption are both inhibited. Therefore, at interface the number of adsorbed asphaltenic aggregates decreases, as well as the demulsifier adsorption compensating the asphaltenes at optimum, and thus the concentration CD*. Consequently, the steric repulsions between

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approaching water droplets are reduced, thus favoring their coalescence and reducing the stability*. Some authors have demonstrated that in systems with a high content of aromatic compounds, asphaltenes are mainly molecularly solubilized and are not able to stabilize emulsions.18,19,48 The reason they proposed was the increase of the asphaltenes dispersion/solubilization near the interface, which reduces the rigidity of interfacial film that usually opposes the coalescence of water droplets. Additionally, Figure 6 shows that in general, the stability at optimum formulation does not change much as a function of CA, as it was the case observed in the proportional regimes of system diluted in cyclohexane (Figure 3). Therefore, the size of asphaltenes nanoaggregates could only slightly increase with respect to CA, since the kinetics of aggregation between the asphaltenes molecules decreases.49 In relation to the CD*-CA map of these systems, Figure 7 indicates that the behavior in the wide range of CA studied is very similar to the one exhibited by the system diluted in pure cyclohexane (Figure 4).

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Figure 7. CD*-CA map for the system NP-8EO / Hamaca crude oil diluted / water. A) Hamaca crude oil diluted in the solvent mixture ciclohexane/xylene 95/5 v/v. B) Hamaca crude oil diluted in the solvent mixture cyclohexane/ xylene 90/10 v/v. Three breakpoints T are occurring at CAT95/5(90/10) = 700 (700) ppm and CAT´95/5(90/10) = 1,500 (2,000) ppm and CAT´´95/5(90/10 = 6000 (7000) ppm, thus generating two proportional regimes and two constant regimes. As in the pure cyclohexane diluent case, this suggests that the aggregation pattern of asphaltenes, in both cases of proportional regime, remains practically constant. Regarding the demulsifier optimum dose CD*, the corresponding values are quite smaller than those reported for cyclohexane dilutions, up to one order of magnitude lower for CA’s ≥ 7000 ppm (CD* = 120 ppm), resulting in an important cost saving in practice. The fall in the demulsifier CD* dose is clearly associated with the reduction of interfacial asphaltenes adsorption when the oil becomes more aromatic. It is clear that a change in the asphaltene aggregation state induced by the aromaticity of solvent, originates a change in the asphaltenes characteristic parameter at the interface (SCPA). As mentioned before, when the aromaticity of oil phase increases, the most polar asphaltenes are absorbed at the interface and as a consequence the SCPA decreases. When the aromatic nature of the diluent is increased by using mixtures with cyclohexane/xylene volume ratios of 50/50 (Figure 8A) and 75/25 (Figure 8B), the emulsion minimum stability* at optimum becomes even smaller as well as the required CD*.

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Figure 8. Variation of the stability with CD for the systems NP-8EO/diluted Hamaca crude/water. A) Dilution of Hamaca crude oil in the mixture cyclohexane/xylene 50/50 v/v. B) Dilution of Hamaca crude oil in the mixture cyclohexane/xylene 25/75 v/v.

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However, it is worth noting that the minimum stability reduction that occurs in systems with 50% or more xylene is only a very few minutes less than with those diluted with cyclohexane/xylene mixtures of 95/5 and 90/10. This indicates that there is a certain amount of xylene “in excess” that has little influence on the physicochemical formulation at the interface or close to it, and that it only plays the role of diluent in the bulk of oil phase.25,43-45 On the other hand, the systems formulated with the mixture cyclohexane/xylene 50/50 and 25/75 show a phenomenon not observed in the previous systems, namely the robustness of the stability curves. Robustness is a very attractive concept in practice, since it can be achieved in many ways and improve performance with some kind of formulation trick as recently reviewed.50 In the present case, it is seen that the low stability occurs over a wide range of demulsifier concentration thus helping in selecting a proper instability even if it is not exactly at optimum formulation, i.e. using a less expensive CD dose. Another advantage of such robustness is that a dose error, that can move the formulation away from a deep instability, is not likely to happen and thus the formulation adequacy is less critical in industrial processes in which the oil nature changes from time to time (e.g. refinery desalting process). It is believed that such a robustness might be associated with the great dispersion of the asphaltenic aggregates both near the interfacial region and in the bulk of the oil phase. By adding xylene in a high-volume proportion, the molecular solubility of asphaltenes in the oil phase is increased, therefore, for a certain CA value (for example, Figure 8; CA = 10,000 ppm) the number of asphaltenic aggregates adsorbed at the interface (the more polar ones) remains practically constant in a wide range of CD. This then promotes irrelevant changes in the interfacial physicochemical formulation as CA increases. The robustness of the physicochemical formulation is very important from the point of view of industrial practice, since it reduces the risk of overdosing the demulsifier which could eventually result in the formation of very stable O/W (inverse) emulsions. In relation to the CD*-CA maps for the 50/50 and 25/75 systems (Figure 9) there are several other elements that deserve to be analyzed. Firstly, the 50/50 curve system (Figure 9, left) shows a

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double-step type behavior as in the cases mentioned above. The first break point occurs at approximately CAT= 190 ppm; this value is 3.7 times lower than those corresponding to systems diluted with cyclohexane and with mixtures cyclohexane/xylene (90/10 and 95/5).

Figure 9. CD*-CA maps for the systems of NP-8EO/diluted Hamaca crude oil/water. A) Hamaca crude oil diluted in the solvent mixture ciclohexane / xylene 50/50 v/v. B) Hamaca crude oil diluted in the solvent mixture cyclohexane/ xylene 25/75 v/v. This result is obvious when considering that the kinetics of aggregation of asphaltenes could decrease due to the high content of aromatic solvent of the oil phase, as explained above. In this sense, the first constant regime is much wider for this system, and as a consequence the change in the interfacial physicochemical formulation is irrelevant as CA increases. On the other hand, when the crude oil is diluted with the 25/75 mixture (Figure 9, right), the CD*CA map seems to actually exhibit three constant regimes. The first one (30 ppm < CA < 100 ppm) could correspond to the formation of the smallest asphaltenes nanoaggregates (what was called the “initial” asphaltenes nanoaggregates in Figure 1), which practically do not affect the interfacial physicochemical formulation. These results are in accordance with the experimental evidence showed by some authors in relation to the single molecules adsorption or the formation of very tiny aggregates in very diluted systems (< 10 ppm of asphaltenes).24,30-32,47

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This “initial” asphaltenes nanoaggregates could be located at the interface as a thin film that covers the water droplets. Therefore, this asphaltenes arrangement generates steric repulsions weaker than those produced by the larger asphaltenes nanoaggregates and clusters, which are formed from the stacking of asphaltenes molecules at higher concentrations. In the Figure 9 (right) with 75% of xylene, the first transitional point Ti (CA ≈ 100 ppm) corresponds to the first asphaltenes aggregation change, from “initial” nanoaggregates to regular nanoaggregates. On the other hand, the transitional point T’ (CA ≈ 1,300 ppm) marks the beginning of the formation of clusters. One could state that after the first constant regime (up to CA ≈ 100 ppm), the rest of the CD* versus CA map corresponds to similar situations to those observed in Figures 4 and 7, in terms of the formation of asphaltenes aggregates. It is thus reasonable to modify the model of interfacial adsorption of asphaltenes proposed in Figure 5 to include the formation of the three asphaltene aggregation structures as in Figure 10, namely, “initial” asphaltenes nanoaggregates (CA < 100 ppm), regular nanoaggregates (100 ppm < CA < 1,500 ppm) and clusters (CA > 1,500 ppm).

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Figure 10. Schematic representation of the adsorption of asphaltenic aggregates at the water-crude interface with respect to CA. System: NP-8EO / diluted Hamaca crude oil (cyclohexane / xylene 25/75 in vol.) / Water By analyzing the global behavior of emulsion stability and the dehydrating agent dosage, very important aspects can be drawn from the point of view of industrial practice. First, Figure 11 shows the trend of emulsion minimum stability* as a function of CA for systems formulated with cyclohexane and cyclohexane/xylene mixtures. As mentioned previously the addition of a small amount of xylene (5 %vol. in the solvent mixture) drastically decreases the minimum stability* with respect to the system diluted with cyclohexane. On top of it, it is worth noting that such a decrease becomes irrelevant in practice for greater proportions of xylene, particularly at high asphaltenes concentration as in the crude oil real cases.

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It may be said that it is obvious that a commercial demulsifying product should contain a small amount of aromatic solvent to considerably improve the dewatering performance. It is thus no wonder that more than 80% of a demulsifier product barrel is an aromatic solvent.

Figure 11. Variation of the optimum stability (stability*) with CA in the NP-8EO /Hamaca crude oil (diluted) / systems. Additionally, it is observed in Figure 11 that for xylene proportions greater than 5% in the solvent mixture, the emulsion stability practically does not change. From the industrial point of view this aspect has a great relevance, since it does not make sense to invest resources in the addition of aromatic solvent above 5%, unless formulations with an important robustness is required, with thus more solvent but less demulsifier. In any case, the decision must be preceded by an economic evaluation. To help in such a dilemma, Figure 12 indicates the variation of the optimum dose of dehydrating agent (CD*), with respect to the aromaticity of cyclohexane/xylene mixture, at CA = 10,000 ppm, i.e. close to a real crude oil situation. 24


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Figure12. Variation of CD* as a function of the volumetric percentage of cyclohexane in the cyclohexane / xylene mixture used as a diluent of the Hamaca heavy crude oil. It is amazing to observed that only 5 vol.% of xylene in the solvent mixture causes CD* to decrease more than 10 times with respect to the system formulated with cyclohexane only (e.g. from CD* = 1,300 ppm to CD* = 120 ppm). The practically constant CD* above 5% xylene means that in such situation the oil phase segregated at interface essentially contains xylene. 4. CONCLUSION CD*-CA maps for systems diluted with cyclohexane show a double-step type behavior when the asphaltenes concentration is increased. The appearance of two proportional regimes and two constant regimes is a clear indication of changes in asphaltenes aggregation and adsorption at the interface. In the proportional regimes, the composition of oil-side interface varies with CA and as a consequence the global interfacial formulation changes as well. On the contrary, in the constant regimes the asphaltenes aggregates adsorbed are the same in number and nature and the interfacial formulation does not change. From these results, a new model is proposed in which three levels of 25


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asphaltenes aggregation is presented, namely: tiny (“initial”) nanoaggregates eventually single molecules, regular nanoaggregates and clusters. Each breaking point on the CD*-CA map could represent a change in the asphaltenes aggregation state. When the oil phase aromaticity is slightly risen by using a xylene/cyclohexane with a small proportion of xylene (e.g. 5 vol.%), the emulsion stability* significantly decreases as well as the demulsifier optimum dose CD*. This is interpreted associating the addition of an aromatic compound with the inhibition of the asphaltenes aggregation and interfacial adsorption. As a consequence, the demulsifier concentration for compensating the effect of asphaltenes at the optimum formulation decreases as well. The variation in demulsifier concentration CD* and emulsion stability* at the optimum formulation produced by a further increase in xylene content in the oil phase (from 5 to 75 vol.%) is not significant. Nevertheless, the robustness of the emulsion stability curves increases with the xylene content. These results are of considerable importance for the industrial practice, because they suggest some strategies for improving the performance of a crude oil demulsifier like the addition of a small amount of an aromatic solvent.

Acknowledgments Total Raffinage Chimie is thanked for supporting JGA’s PhD dissertation.

References (1) Van Dyke, K. 1987, Petroleum Extension Services, University of Texas at Austin, TX USA. (2) Treating Oilfield Emulsions. 1990. Petroleum Extension Service. The University of Texas at Austin. (3) Salager, J.L. International Chemical Engineering. 1990, 30, 103 – 116.

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Breaking of Water-in-Crude Oil Emulsions. 8. Demulsifier Performance

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