Colloids and Surfaces A: Physicochemical and Engineering Aspects
On water-in-oil emulsions stabilized by fine solids
Introduction
Solids-stabilized emulsions are often encountered during the extraction of bitumen from oil sands, crude oil de-watering, separation of fines from shale oil, and separation of oil from wastewater. The knowledge that fine solids can stabilize emulsions dates back to the beginning of the century when Pickering originally noted that colloid particles that were wetted more by water than by oil could act as an emulsifying agent for oil-in-water emulsions [1]. Since then, several workers have studied the stability of solids-stabilized emulsions [2], [3], [4].
It has been found that the fine particles adsorbed at the droplet surface act as a barrier preventing droplets form coalescing [5]. The breaking up of such emulsions has been of major concern for economic and environmental reasons. Considering the extraction of bitumen from oil sands as an example, the diluted bitumen recovered from bitumen froth contains ∼0.5% of fine solids and 3% of emulsified water droplets <3 μm in diameter. The emulsified water in the diluted bitumen causes serious corrosion problems in the downstream upgrading units due to the chlorides dissolved in the water. By using an aliphatic diluent in recovering the bitumen, the amount of water in the settler product could be reduced from 3 to 0.3% by weight with a corresponding reduction in chlorides [6]. Hence, the amount of water carrying through to the upgrading process could be reduced by a factor of 10. Unfortunately, the hydrocarbon recovery in the settler also dropped from 98 to 84%. It has been found from laboratory tests that the non-polar solvent causes the dispersed water droplets to flocculate, partially coalesce and settle out of the oil phase as a concentrated secondary emulsion between the oil and water phases. Such secondary emulsions, which are generally produced following a demulsification process, are common in the crude oil de-watering and in hydrometallurgical industries. Hence, it is critical to determine the nature of the water-in-oil emulsions and to find an economical method to break them.
Water-in-oil emulsions from heavy oils are thought to be stabilized by asphaltenes, and a great deal of research has been devoted to this area [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. However, it has been found that fine clays less than one micron in diameter play a more significant role here. The concentrated secondary emulsions that settle out after diluting the bitumen with an aliphatic solvent consist of ∼50% water, 3% fine solids and the remainder hydrocarbon. Once broken, these secondary emulsions could be reconstituted only if the fines were present. To examine the fines further, they were separated from the oil and water by centrifuging and washed with toluene until clean [6]. The cleaned fines were found to be bi-wettable. When added to a water/heptane system, the fines distributed in the water phase. However, when added to water/toluene system, they dispersed in the toluene phase. Similarly, the contact angle of water on a cake of the fines was high in heptane but low in toluene measured through the aqueous phase.
The above findings suggest that a non-polar diluent changes the contact angle of the bi-wettable fines, which in turn influences the stability of the emulsions. This concept has been confirmed for oil-in-water emulsions [7], [8], [9], [10], [11]. Previous experimental results with a model oil, Bayol-35, show that asphaltene-treated clays partition at the oil–water interface and stabilize oil-in-water emulsions [7]. The adsorption of the clays at the oil–water interface is of multilayer type. Moreover, desorption of the clays from the oil–water interface is hysteretic, leading to difficulties in the demulsification process. Recent experimental results from batch tests have shown that the contact angle of the treated clays is a function of the pH of the aqueous phase, and that significant effect of pH on the adsorption of clays at the oil–water interface has been observed [11].
There is a lack of work on solids-stabilized water-in-oil emulsions despite of their importance. In this communication, we shall present experimental results related to water-in-oil emulsions stabilized by fine solids. Various parameters, especially the wettability of particles, will be examined to determine how they influence the type and the stability of the formed emulsions.
Section snippets
Clays and their contact angles
Kaolinite clays (Hydrite UF) from Georgia Kaolin Company were used. The equivalent spherical diameter of the primary clay particles was 0.2 μm. In order to be effective in stabilizing water-in-oil emulsions, clays have to be treated to render them hydrophobic. The procedure for modifying the clay particles and the method for measuring clay contact angles were the same as previously reported [7].
Hydrophilic colloidal silica
Hydrophilic colloidal silica particles were either prepared by the process of Stober et al. [12]
Emulsions stabilized by clays treated with asphaltenes
When the treated clays were used to prepare the emulsions, the following were observed: The colorless Bayol oil became yellowish or brownish after the clays were dispersed in the oil, depending on the concentration and the wettability of the clays, indicating that the adsorbed asphaltenes were desorbed from the clays’ surface. Although the treated clays could stabilize emulsions, the desorption of asphaltenes from the clays’ surface resulted in changes in the wettability of the clays during
Conclusions
The stability and type of emulsions stabilized by solids depend on the hydrophobicity of the particles and the phase they reside in prior to emulsification. Hydrophilic colloidal silica particles could only stabilize oil-in-water emulsions for a short period of time regardless of the phase they reside in prior to emulsification. If hydrophobic particles (colloidal silica or polystyrene latex microspheres) were suspended in aqueous phase prior to emulsification, they could only produce
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