Suspensions: Fundamentals and Applications in the Petroleum Industry

8 Asphaltenes in Crude Oil and Bitumen: Structure and Dispersion

Downloaded by MICHIGAN STATE UNIV on November 28, 2013 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0251.ch008

James G. Speight Western Research Institute, 365 North 9th Street, Laramie, WY 82070-3380

CRUDE

PETROLEUM

IS

A

MIXTURE

OF

COMPOUNDS

boiling

at

different

temperatures that can be separated into a variety of generic fractions by distillation and by fractionation ( J ) . In fact, such methods provide a better sense of the overall composition of petroleum and the behavioral characteristics. However, petroleum from different sources exhibits different characteristics, and the behavioral characteristics are often difficult to define with any degree of precision. As anticipated and inasmuch as there is a wide variation in the properties of petroleum, the proportions in which the different constituents occur will also vary widely (1-3). T h u s , some crude oils have higher proportions o f the lower boiling constituents, whereas others (such as bitumen, also referred to as natural asphalt) have higher proportions of the higher boiling constituents (often called the "asphaltic components" or " r e s i d u u m " ) . It is these higher boiling constituents that often lead to problems during recovery and refining operations. Petroleum can be considered to be a delicately balanced system insofar as the different fractions are compatible, provided there are no significant disturbances or changes made to the system. Such changes are (1) the alteration of the natural occurrences of the different fractions; (2) the chemical or physical alteration of the constituents as might occur during refining, especially changes that might be brought on by thermal processes; and (3) alteration o f the polar group distribution as might occur during oxidation (i.e., asphalt manufacture) or the elimination o f polar functions during processing. In addition, the sudden exposure of petroleum to air, as might occur during the initial stages of recovery operations, or the release of dissolved gases when a reservoir is first penetrated can also cause disturbances to the system. 0065-2393/96/0251-0377$13.25/0 © 1996 A m e r i c a n C h e m i c a l Society

In Suspensions: Fundamentals and Applications in the Petroleum Industry; Schramm, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


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SUSPENSIONS: F U N D A M E N T A L S & APPLICATIONS IN P E T R O L E U M INDUSTRY

A l l of the aforementioned occurrences disturb the petroleum system. However, when such disturbances occur, it is the higher molecular weight constituents that are most seriously affected. This can lead to incompatibility, w h i c h is variously referred to as precipitation, sludge formation, and the separation of coke precursors, depending on the circumstances. Thus, the dispersibility of the higher molecular weight constituents becomes an issue that needs to have attention. One of the ways by which this issue can be understood is to be aware of the chemical and physical character of the higher molecular weight constituents. B y such means, the issue of dispersibility, and the attending issue of incom patibility, can be understood and even predicted. The luxury of predictability is a valuable asset in understanding the behavior of petroleum during refining and recovery operations, and no where is it more helpful than i n understanding the behavior of heavy oil and bitumen, which have greater proportions of the higher molecular weight constituents than conventional petroleum. Because of this (in part or in total), more problems exist in recovery and refining operations. Understanding the nature of the higher molecular weight constit uents is a first step in understanding dispersibility and incompatibility. In this chapter, the current understanding of the most complex (in terms of molecular weight and polarity) constituents of heavy oil and bitumen is reviewed. This fraction is often referred to as "asphaltenes" (Figure 1). Other complex entities also exist i n the resin fraction but have re ceived considerably less attention. At this point, it is noteworthy that without the resin fraction, asphaltenes are generally nondispersible in the remainder of petroleum, thereby indicating that the resins are, under ambient conditions, a nec essary constituent and that by their presence they prevent incompati bility (3,4). This is only one of several factors that influence dispersibility or compatibility, and others w i l l be noted, in turn, throughout the chapter.

Separation Determination of the asphaltene fraction of petroleum has been inves tigated for most of this century (5-11), and therefore the art is not new. However, it is now generally accepted that asphaltenes are, by definition, a solubility class that is precipitated from petroleum, heavy o i l , and bitumen by the addition of an excess of liquid hydrocarbon (II). T h e procedure not only dictates asphaltene y i e l d but can also dictate the " q u a l i t y " of the fraction (12-16). In fact, the very method of asphaltene separation is a prime example of the disturbance of the system by the addition of an external agent. Thus, during deasphalting, the dispersi bility (or compatibility) of the asphaltenes in the system is changed. T h e



8.

SPEIGHT

Asphaltenes in Crude Oil and Bitumen

379

Figure 1. Asphaltenes are also classed as polar aromatic compounds and are an extension of the petroleum continuum. result is the separation (precipitation) of an asphaltene fraction. Various l i q u i d solvents (or more correctly, nonsolvents) have been used for as phaltene separation. T h e most common liquids are low-boiling hydro carbon liquids, w i t h nonhydrocarbon liquids also receiving some, but much less, attention. A recommended procedure for reproducibility is to ensure " s t a b l e " asphaltene yields from heavy oils. It is necessary to use >30 m L hydrocarbon/g feedstock; pentane or heptane, w h i c h are the preferable l i q u i d hydrocarbons, although volatility constraints and stability of asphaltenetype are tending to favor the use of η-heptane; and 8- to 10-h contact time, which is the preferable period (II). H y d r o c a r b o n S o l v e n t s . Different feedstocks have different amounts of asphaltenes (Table I), w h i c h can influence properties. This is particularly true of different feedstocks from any one crude oil (Figure 2) in w h i c h the nonvolatile asphaltenes are concentrated in the residue or in the propane asphalt.


380

SUSPENSIONS: F U N D A M E N T A L S & APPLICATIONS IN P E T R O L E U M INDUSTRY

Table I.

Ranges for Asphaltene Occurrence in Different Feedstocks

Substance

Asphaltenes

Petroleum H e a v y oil Residue

Suspensions: Fundamentals and Applications in the Petroleum Industry

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