Phase separation kinetics of maya asphaltene emulsion and free-to-bound water transformation Francesca Ridi a , Nina Verdal b , Piero Baglioni a , Eric Y. Sheu c, * a Department of Chemistry and CSGI, University of Florence, via della Lastruccia 3-Sesto Fiorentino, I-50019 Florence, Italy b IPNS, Argonne National Laboratory, 2700 South Cass Avenue, Argonne, IL 60439, USA c Vanton Research Laboratory Inc., 1870 Arnold Industrial Place, Suite 1000, Concord, CA 94520, USA article info Article history: Received 12 April 2008 Received in revised form 30 July 2008 Accepted 25 August 2008 Available online 24 September 2008 Keywords: Asphaltene Kinetics Emulsion abstract Differential scanning calorimetry (DSC) and inelastic neutron scattering were applied to study the phase separation kinetics of Maya asphaltene water-in-oil emulsion by following the water separation from the upper phase. In addition, transformation of the separated water from the free state to the bound and/or restricted state was also investigated. Maya asphaltene in toluene with 0.1 M HCl forms emulsion follow- ing ultrasonic or high speed mechanical emulsification. Initially, water molecules in the emulsion are lar- gely free water. The emulsion gradually separates into two phases and at the 7th day over 90% of water molecules have situated at the bottom phase. In the mean time water molecules at the free state initially slowly transform into bound state in a much slower pace (over 100 days) than the phase separation kinetics. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Numerous reports have demonstrated the significance of asphaltenes in petroleum emulsion systems whether they are in the crude oil emulsions, the tar sand emulsions following separa- tion of sand, or the production streamline when steam or water is introduced [1–5]. Operationally, it is a common practice to sep- arate water from the crude oil emulsions, either by physical or by chemical means. Eliminating water totally from the emulsions ap- pears to be challenging [6]. Part of the reason is because some water molecules bound to the polar sites of the petroleum compo- nents, particularly the polar asphaltenes, and become either bound water or restricted water, which requires tremendous amount of energy to separate them from the adsorption sites. Khvostichenko et al. studied this phenomenon in a recent work [7]. In that study water binding to monomeric and aggregated asphaltenes was experimentally shown and qualitatively analyzed by the Flory– Huggin model. As water is bounded, the emulsion becomes stable, and water inseparable using the common de-emulsification pro- cesses. As such, water is carried through the production, transpor- tation, and operation streamlines, which is not economically viable. Aside from the fact that inefficient water separation directly im- pacts the processing efficiency and economic consequences, water separation kinetics from the emulsion is another key measure of the practicality of the emulsion breakage processes. The shorter it takes to break the emulsion, whether chemical or physical means are enforced for the separation, the better the process. In principle, an emulsion stabilized by emulsifiers of dual hydrophilic–lipophilic nature should phase separate after some time, depending on the emulsion droplet size, temperature, and pressure, etc. The main energy components in these systems are hydrophobic and bending rigidity (the interfacial tension). How- ever, there is another factor in an asphaltene stabilized emulsion, and that is the water–asphaltene interactions. An asphaltene emul- sion containing only free water should behave similarly to an emulsifier stabilized system. However, the stability would radi- cally change if the polar interactions between water and asphal- tene set in and water becomes restricted or bounded. To the best of our knowledge, there are no prior reports that demonstrate water molecules in a petroleum emulsion transforms from a free state to a restricted or bound state during storage. It is however logical to deduce such a physical consequence from the fundamental thermodynamic concept that a thermodynamic dri- ven emulsion system should energetically move toward the lowest free energy state and transformation of water molecules from a free state to a bound state should drastically lower the system free energy. This can happen and lead to phase separation or happen in the water rich phase after phase separation, if the kinetic of free state to bound state is much slower than the phase separation kinetics. Previous reports showed slow asphaltene thermodynamic equilibrium, such as their aggregation kinetics [8]. It is thus conceivable that the transformation kinetics of water from the free 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.08.019 * Corresponding author. Tel.: +1 925 878 5142; fax: +1 925 947 1978. E-mail address: ericsheu@vantonlab.com (E.Y. Sheu). Fuel 88 (2009) 319–325 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel