Abstract

One step in asphaltene extraction is to wash the precipitated asphaltenes with the precipitant to remove trapped resinous material. To assess the effect of washing on asphaltene properties, asphaltenes were extracted with three different degrees of washing. Asphaltenes from three source oils (Athabasca, Cold Lake and Lloydminster) were examined. In all cases, increased washing decreased asphaltene yield and increased asphaltene density slightly. Increased washing increased molar mass and decreased the solubility of the extracted asphaltenes significantly.

A new washing method using a soxhlet apparatus removed the greatest amount of resinous material and yielded asphaltenes with significantly different properties from conventionally washed asphaltenes. The asphaltenes from the different source oils exhibited similar properties after conventional washing. However, there were significant differences in their properties after applying the soxhlet method. Hence, the soxhlet method allows for a more sensitive comparison of asphaltenes.

Introduction

With the recent utilization of heavy oil reservoirs and offshore fields, petroleum producers have been faced with increasing production problems as a consequence of asphaltene deposition. These asphaltene deposits have the potential to disable production operations anywhere from the oil reservoir to the production lines and storage tanks. In order to develop effective methods for mitigating asphaltene deposition it is necessary to characterize the asphaltenes, their phase behavior, their aggregation state, and their surface properties.

It has proven difficult to achieve consistent property measurements of asphaltene because they are a solubility class and not a pure component. Asphaltenes are defined as the fraction of a crude oil that dissolves in toluene and is insoluble in n-alkanes (e.g. n-heptane or n-pentane). It is well established that asphaltenes are polynuclear aromatics and that they are the heaviest, most polar components of crude oils with the highest heteroatom content (e.g. nitrogen, oxygen and sulfur) and metals content (e.g. iron, nickel and vanadium) (1). However, since asphaltenes are a solubility fraction their yield and properties such as molar mass and density are sensitive to the technique employed to extract them from crude oils.

Solvent extraction methods are used to separate crude oils into several solubility fractions as shown in Figure 1. Typically, crude oils are separated into four fractions: saturates, aromatics, resins and asphaltenes (SARA). Since each fraction contains constituents of common solubility or adsorption properties, the constituents do not necessarily have similar size or structure. Nonetheless, constituents of a given class share some structural features. The saturates generally consist of naphthenes and paraffins. The aromatics, resins and asphaltenes appear to form a continuum of polynuclear aromatic species of increasing molar mass, polarity and heteroatom content. There is no clear distinction between asphaltenes and resins. Consequently, the amount of asphaltenes extracted from a crude oil depends on the type of solvent, the dilution ratio, contact time, and temperature (2).

Standard procedures have been developed in an effort to obtain consistent asphaltene fractions. Most extractions are carried out at room temperature. In some procedures (3), higher temperatures are employed and lower asphaltene yields can be expected since asphaltene solubility in hydrocarbon solvents increases with temperature (4).

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