Resumen de una alternativa mejor

The first screening experiments (Table 4-1) were standardized by means of Equation 4-3 and decomposed into two principal components (PC), which recovered 77.8 % of the total variation. The plot for RSD vs. Leverage (Figure A-3, Appendix IV) shows that all objects are within acceptable range. Graphical representations of the PCA are showed in Figure 5-9.

a) b)

Figure 5-9: a) Scores and b) loadings for the first screening with crude oil A.

The score plot (Figure 5-9a) shows groupings for an ionic strength of 0.09 (green objects) and ionic strength of 0.40 and 0.23 (red and blue objects). This may indicate that there exists an optimum ionic strength between 0.09 and 0.23, whereas increasing the ionic strength further past 0.23 does not promote large variations.

The loadings (Figure 5-9b) shows that the equilibrium IFT is strongly negatively correlated to the pH (r² = -0.98), but very weakly correlated to the ionic strength and Ca²⁺ content (r² = 0.07). These two relationships can be isolated, as displayed in Figure 5-10.

The most notable effect on the equilibrium IFT was obtained at caustic conditions, low ionic strength and no Ca²⁺, which produced an equilibrium IFT of 7.0 mN·m^-1. Additionally, this experiment reached equilibrium conditions in only 17 minutes, while all other experiments required an average measurement time of 84 ±46 minutes. Exp10 (the brown object in Figure 5-9a) was included to inspect the effect of adding a small amount of Ca²⁺ (136 ppm) to this experiment. This increased the equilibrium IFT to 21.1 mN·m^-1 and the measurement time required was 54 minutes. This observation indicates that metal-complexes (Equation 2-2 and Equation 2-3) may have formed, thus removing the interfacially acive dissociated acids

DataSet: A_screening, Subset: A_screening, Scores 1 vs 2

Exp2 Exp1

from the interface and presumably dissolving them in the water phase or stabilizing the CO/W emulsion film. This mechanism has been reported by several researchers [5, 10, 19, 20, 24, 31].

a)

b)

Figure 5-10: An isolation of the relationship between a) IFT and ionic strength and b) IFT and pH. Experiment 9 has been replaced with the object containing a low amount of Ca²⁺.

Figure 5-10 shows an almost linear relationship between the IFT and pH. This is not consistent with other reports on the pH-dependence of the IFT, which suggests that the IFT is at an optimum at neutral pH, but decrease at low and high pH [13, 18, 29, 32, 41, 43]. This has been attributed to the protonation of basic species at low pH and dissociation of acids at high pH. Thus, this increase in IFT at low pH may indicate that crude oil A does not contain significant amounts of basic species that become interfacially active at low pH, but rather promotes a decrease in the concentration of dissociated acids.

The second screening only considers two variables: ionic strength and molar fraction of Ca²⁺. It is therefore unnecessary to decompose the dataset into PCs. Figure 5-11 shows the variation in IFT as a function of variation in ionic strength (Figure a) and molar fraction of Ca²⁺ (Figure b), both at neutral pH conditions. The lines are polynomial trend-lines fitted by excel analysis tools. These indicate that IFT decrease with increasing ionic strength for both

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crude oils, while no trend can be spotted when modeling the IFT as a function of Ca²⁺ content.

However, Figure 5-11a does not show indications of the optimum hypothesized to exist between I=0.10 mol·kg^-1 and I=0.23 mol·kg^-1 for crude oil A. Instead, it appears as if the ionic strength causes a continuous decrease in the IFT, or that it has an optimum around I=0.35 mol·kg^-1.

a)

b)

Figure 5-11: Plots of a) IFT vs. ionic strength, and b) IFT vs. the molar fraction of Ca²⁺.

In summary, the first screening experiments indicate that there may be an optimum ionic strength for crude oil A around I=0.1-0.2 mol·kg^-1, while the second screening may be interpreted to contain an optimum closer to I=0.4 mol·kg^-1. Several reports [1, 2, 24, 38] have indicated that the ionic strength has an optimum effect on the IFT, as a result of reaching a saturation level in terms of ionic screening of charged species adsorbed at the CO/W interface. More levels for the ionic strength should be further inspected, to see if a trend can be elucidated once more measurement points are included.

pH is the most important variable for obtaining a reduction in the IFT for crude oil A. The plot of IFT as a function of pH for crude oil A implies that the relationship is seemingly linear. Further investigation of this relationship should include a pH value in between neutral pH and pH 11, to inspect the possible curvature of the response-surface.

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The concentration of Ca²⁺ does not appear to be significant at neutral pH conditions for either crude oils. However, the most drastic effect on the IFT for crude oil A is found to be at caustic conditions and low ionic strength with no Ca²⁺ present. This drastic reduction in the IFT is almost eliminated by adding trace-amounts of Ca²⁺. Therefore, the effect of Ca²⁺

should not be excluded from further experiments, but it may be more meaningful to inspect the effect of XCa2+ = 0.0 compared to XCa2+∈ (0.0, 0.1), instead of higher concentrations.

It is not yet known if higher concentrations of monovalent ions can eliminate the drastic reduction at caustic conditions and in the absence of Ca²⁺. However, Na⁺ has been proposed to not form strong complexes, but rather act as an efficient ionic specie in screening of the EDL formed in the vicinity of the CO/W interface [18, 30, 38]. Thus, it would be reasonable to assume that higher concentrations of Na⁺ will promote further reduction in the IFT, which has been reported by Jennings [20] and Sharma et al. [19]. This hypothesis may be tested by including measurements at high ionic strength, caustic conditions and in no Ca²⁺. Additionally, it is still unknown if the effect of caustic conditions and no Ca²⁺ is as drastic for crude oil B. At the time being, it can be hypothesized that the effect will not be as drastic, as a consequence of assuming that the acid concentration in crude oil B is lower than in crude oil A. It has not been reported that asphaltenes associate with Ca²⁺ in the same way as acids.