It also shows that the larger the difference in solubility parameter between the solute and the solvent is, the more difficult the solute is dissolved, i.e., the more easily the sludge is generated.
ln(Xa) = -[MWa x (φs)2 x (δa - δs)2]/(Da x R x T) (1)
where,
Xa = Quantity of asphaltene (mole fraction)
MWa = Molecular weight of asphaltene
Da = Density of asphaltene
φs = Volume fraction of solvent
R = Gas constant
T = Temperature
δa, δs = Solubility parameter of asphaltene and solvent
δ = [(ΔHv - R x T)/V]1/2 (2)
ΔHV = Heat of vaporization (25℃, cal/mol)
V = Mole volume (25℃, cm3)
4.1.2 Solubility parameters
The solubility parameter (δ) in formula (2) shows that the solubility depends on the boiling point and the molecular volume (the value of the molecular weight divided by the density). Table 2 shows an example of the solubility parameters, and Fig. 11 shows the solubility parameter of asphaltene. It is proven from the table and figure that the δ of the aromatic solvent such as benzene and toluene is around 9 while the δ of asphaltene is 8 to 10, and since the value of the aromatic solvent is located in the range of the asphaltene value, their solubility is high. On the other hand, δ of the aliphatic solvent such as, hexane and cetane is below 8, and large in dissociation from asphaltene, and the compatibility is bad.
Fig. 11 Solubility parameter of asphaltene
4.1.3 Applications to prediction of compatibility
The condition of sludge generation when C oil is diluted with A oil was calculated by the formulae (1) and (2), and the quantity of the dry sludge was actually measured, and the results were compared with each other. (Fig. 12). It can be theoretically and experimentally understood that most sludge is generated in the vicinity of 70%/30% in the A/C ratio. The thermodynamic method is thus applicable to predict the compatibility in the A/C blend.
4.2 Test method for stability prediction
4.2.1 Test method for degree of digestion of asphaltene4)
This test method (hereinafter, referred to as P value measurement method) is to experimentally obtain the solubility parameters of the solute and the solvent and to predict the stability of heavy oil from these values. The principle of measurement is to obtain the point of coagulation of asphaltene by titrating heavy oil with the blended solvent system of toluene/heptane, and to judge the stability by the toluene/heptane ratio in the titration. (Fig. 13)
(1) Prediction of blending stability
The stability in blending dissimilar oil can be predicted by the P value measurement. Fig. 14 shows that the stability is most degraded in the vicinity of the center between C oil excellent in stability and C oil low in stability when they are blended. This relationship is changed by the difference in solubility (Pa) of asphaltene and difference in solubility in malten (Po), and when these values are known, the calculation for prediction is possible.
Fig. 12 Estimation of slash generation in A/C blend
