Properties of Asphaltenes Solubility, Molecular Weight, Aggregation

PROPERTIES OF ASPHALTENES (SOLUBILITY, MOLECULAR WEIGHT, AGGREGATION) 

Petroleum consists of four hydrocarbon-t3 es (saturates, aromatics, resins, and asphaltenes) that may be defined in terms of solubility, polarity, and MW. Of these structural t)q)es, asphaltenes have markedly adverse effects on the processability of petroleum and play a significant role in the physical properties of heavy oils and bitumen. Because of these effects, in this chapter, asphaltenes will be discussed in detail in terms of their properties, composition, and thermal chemistry during upgrading, as well as their influence on instability/incompatibility during the production, transportation and upgrading of petroleum. 

In terms of processability, aggregation of asphaltenes can hinder the conversion of heavy residues to lighter products and can enhance coke formation. Using smface tension measurements and rheological data, Sheu et al. and Storm et al. have shown that asphaltenes exhibit properties similar to colloids. These colloids exist in the heavy oil matrix in a micelle form. Since the mole fraction of resins is higher than asphaltenes in petroleum, micelles are richer in resins. Asphaltenes also exhibit properties of colloidal systems such as the critical micelle concentration (CMC) at which aggregates begin to form. 

Asphaltenes from Athabasca bitumen were first separated using n-pentane by Pasternak and Clark in 1951. In most refinery practices, the solvent of choice is n-heptane and asphaltenes are defined as materials soluble in toluene or a solvent having a solubility parameter in the 17.5-21.6 Mpa range. As the carbon number of the extracting solvent increases, the amount of asphaltenes that precipitate decreases. Fundamentally, it is important to note that during asphaltene precipitation by any solvent, smaller asphaltene molecules, as well as some maltene materials, co-precipitate because of 

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