Breaking of Water-in-Crude Oil Emulsions. 6. Estimating the Demulsifier Performance at Optimum Formulation from Both the Required Dose and the Attained Instability

The demulsifier of crude oil, also called its dehydrator product, is a hydrophilic surfactant according to the physicochemical formulation concept, which destabilizes the effect of asphaltenes (natural lipophilic surfactants) adsorbed at the water-oil interface. The water content in the produced crude oil has to be removed, and the asphaltenes cause a slowdown of interdrop film drainage and the inhibition of the water drops coalescence. The actual demulsifier role is to counteract IJ eliminate the asphaltenes stabilizing effects, and its final performance depends on many factors with their very complex interactions, which are difficult to deal with and optimize.

Most studies on improvement of demulsifier with different ionic or non-ionic surfactants and surfactant mixtures are empirical and limited in practice. However, a general understanding can be achieved by disaggregating different phenomena involved and thereby studying them independently as in a new approach.

The recent article of Delgado-Linares et al. (2016) [1] which appeared as the 6th part of a series of publications in the Journal Energy & Fuels uses a testing technique focusing on the formulation of the optimal mixture between two adsorbed surfactants (asphaltene and demulsifier). In this method the other influences which can retard the film drainage and the delay of coalescence are eliminated, so that the role of actual asphaltene-demulsifier pair theoretically described a long time ago [2] can effectively be numerically estimated for a clear comparison of the effect of different formulations.

The systems studied were composed of an equal volume (5mL) of distilled water containing demulsifier and a dilution of a heavy crude oil in cyclohexane to produce an asphaltene concentration (C_A) which is at equilibrium at interface with the demulsifier low concentration (C_D) contained in the bulk aqueous phase, with no excess of asphaltenes in the oil bulk, which could (partially) hide the actual interfacial formulation effect.

Various families of surfactants are used as demulsifiers. There are the ethoxylated nonyl phenols with an average ethylene oxide number EON whose hydrophilicity can be easily changed,  typical commercial demulsifier surfactants such as triblock EON-PON-EON (polyethyleneoxide-polypropyleneoxide-polyethyleneoxide), and an ethoxylated sorbitan monooleate, a cheap regular surfactant used in many applications.

Results are obtained from the measurement of emulsion stability taken as the time for the separation of half of the emulsified water, particularly its minimum value at optimum concentration of demulsifier in water, so-called C_D*. The oil phase dilution, in general to result in 500 ppm or less of asphaltenes, insures the occurrence of a so-called “proportional regime” [3] in the asphaltenes-demulsifier mixture at interface. In this case a linear mixing rule is attained in the behavior of the asphaltene-demuslifier pair and it allows to make numerical calculations in the interfacial formulation, as proposed a long time ago and recently reviewed in details [4].

The part of the study using ethoxylated nonyl phenols showed that there is an optimum selection corresponding to a significantly hydrophilic surfactant with about 10 ethyleneoxide groups for which both the dose (C*_D) and the minimum stability are low, as a compromise between the two extremes with lower and higher ethoxylation.  If the demulsifier hydrophilicity is too low, close to three-phase behavior (EON=5), the stability is very low but the dose to apply is too high. On the contrary if the hydrophilicity is too high (EON=30), most of the surfactant fractionate into the bulk water and thus does not sufficiently adsorb at interface where it should go to mix with asphaltenes.

Another part of the study dealing with a mixture of two different demulsifier species from the same type, i.e. nonylphenols with 8 and 30 EO groups, exhibited an intermediate behavior as far as the two performance indices as concerned.

The study of the mixture of commercial demulsifiers of the triblock type indicated that both the hydrophilicity of the components and their molecular weight have their importance. The larger molecular weight species (> 4000 Daltons) is, as expected, better when it is more hydrophilic, in spite of being slower to diffuse to interface than the smaller ones. This is probably an evidence that the test method focusses on the interfacial formulation influence, eliminating transport processes effects through the bulk which could hide the asphaltene-demulsifier interaction actual result concerning the water-in-oil emulsion coalescence.

Interesting results are attained by mixing two demulsifiers of very different stuctures. When a large triblock demulsifier (called D1 in what follows) and a three times smaller and less hydrophilic ethoxylated sorbitan ester (called D2) are mixed, the combination of the two structures can result in an impressive synergy. As seen in the figure, mixing the good demulsifier D1 (very hydrophilic, thus with a low C*_D1 and a low minimum stability) with a bad one D2 (not very hydrophilic thus with a high C*_D2 , and a too high minimum stability) could produce a powerful demulsification efficiency zone.

The D1/D2 50/50 mixture exhibits an outstanding dual performance (low stability and low dose) better than the D1 good component alone, and with a wide synergistic range of concentration. It is obvious from the figure that a dose located at the star point (✭), i.e. with a small concentration of demulsifier mixture and a quite low stability, is probably an exceptional selection when considering cost issues.

This study proves that demulsifier should be a significantly hydrophilic surfactant, though not too much so as to exhibit a good performance at optimum for both a low required dose and a swift breaking of water-in-oil emulsion. It also shows that mixing quite different species resulting in synergy could 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 water-in-oil emulsion breaking.  

REFERENCE

 [1] Delgado-Linares J.G., Perreira J.C., Rondón M., Bullón J., Salager J.L, Breaking of Water-in-Crude Oil Emulsions. 6. Estimating the Demulsifier Performance at Optimum Formulation from Both the Required Dose and the Attained Instability. Energy & Fuels, 2016, Volume 30, pp 5483-5491.

[2] Salager J.L. The fundamental basis for the action of a chemical dehydrant. Influence of the physical and chemical formulation on the stability od an emulsion. International Chemical Engineering, 1990, volume 30, pp 103-116.

[3] Rondón M., Pereira J.C., Bouriat P., Graciaa A., Lachaise J., Salager J.L. Breaking of Water-in-Crude oil Emulsions. 2. Influence of asphaltene concentration and diluent nature on demulsifier action. Energy & Fuels, 2008, Volume 22, pp 702-707

[4] Antón R.E., Andérez J.M., Bracho C., Vejar F., Salager J.L. Practical surfactant mixing rules based on the attainment of microemulsion–oil–water three-phase behavior systems. In Interfacial Processes and Molecular Aggregation of Surfactants, Narayanan R. Ed., published in Advances in Polymer Science, 2008, volume 218 pp 83-113