Measurement Of Asphaltene Flocculation In Bitumen Solutions
Authors: Fuhr B.J. , Klein L.L. , Reichert C. 
Published: March 2010

The ability to transport bitumen, in solutions of low viscosity solvents, can be seriously affected by asphaltene precipitation. Solvents differ dramatically in their ability to dissolve asphaltenes and often those which are inexpensive and readily available have poor solvating properties. Titrating a solution of bitumen in the solvent of interest with pentane and observing the onset of asphaltene flocculation yields the flocculation threshold for that solution. This information is often more useful than merely the dissolution properties of the solvent for asphaltenes or the standard asphaltene precipitation determination. A series of experiments were performed with various solvents utilizing apparatus assembled in our laboratory which measured the increase in light scattering at the onset of flocculation. Plots of the information obtained from the titration curves were constructed allowing solvent effects to be readily observed and measured.

Data is presented which compares the ability of n-pentane to initiate the flocculation of asphaltenes from various bitumen and crude oils indicating the stability to the asphaltene solution in the oil. The solubilizing characteristics of several solvents for an Athabasca bitumen are presented showing that a high aromatics diesel fuel is a better solubilizer than toluene. Some chemical additives such as surfactants, acids, and bases were found to delay the onset of asphaltene flocculation in the bitumen solutions.

Introduction

Solvent dilution is an important technique for viscosity reduction of heavy oil, bitumens and upgrading residues to aid in removal of water and solid contaminants or to improve the transport properties of the oil.The choice of a solvent depends upon its dissolution capability to ensure that asphaltenes are not precipitated producing additional problems. Often the most appropriate solvents are in short supply or not available(1) and information about the solvating power of readily available solvents is desired. A measurement of the asphaltene content by standard methods(2) generally does not provide the information about flocculation or precipitation kinetics which are of practical importance.

Work by Bichard(3) and Hotier and Robin(4) discussed methods for the study and measurement of solvating power, flocculation properties, and precipitation potentials by titrating samples with a solvent and observing the onset of asphaltene flocculation and precipitation. The initial studies of flocculation were accomplished by means of spot tests or optical microscopy(3–5) but these are tedious and time-consuming and often difficult to reproduce. The spot formed by placing a drop of a solution on filter paper can be compared with a series of reference spots to estimate the degree of asphaltene precipitation. The spot test is based on the fact that flocculated asphaltene particles diffuse more slowly than the surrounding liquid when the mixture is placed on the filter paper. When flocculated particles form in the solution a dark region is observed in the middle of the spot. This test is the basis of ASTM method D2781 for measuring the compatibility of oils. In optical microscopy as described by Bichard(3) the particles in suspension are observed directly within an aliquot of the mixture when placed on a microscope slide.