Asphaltene incompatibility

Asphaltene Incompatibility/Dropout in Marine and Residual Fuel Oils... 

With respect to fuels utilized as heating fuels for industrial furnaces, or as motor fuels for large diesel engines such as those in ships or power generation sets, the characteristics of primary importance are viscosity, sulfur content and the content of extremely heavy materials (asphaltenes) whose combustion can cause high emissions of particulates which are incompatible with antipollution legislation. 

On the w hole. these analyses show that gas oil steam cracking produces fewer light produas than the treatment of naphtha, and more heavy products which display a higher aromatics content. Hence the Cj- 200 C cut boosts the BTX (Benzene, Toluene, Xylenes) (majority benzene concentration. Similarly, fuel oil (fraction above 200 C) displays a more pronounced aromatic character. This feature makes it incompatible with straight-run distillation fuel oils. The mixture causes the deposition of asphaltenes and other... 

It was concluded that definition of asphaltenes based only on solubility is not a satisfactory criterion and that the behavior of asphaltenes in chromatographic separations is incompatible with such structures where the polymer units are interconnected predominantly by a-bonds. The asphaltenes are a complex state of aggregation best represented by the stacked cluster structure (micelle), which, however, cannot explain some of the GPC behavior of very dilute asphaltene solutions. 

Asphaltene-type deposition may, however, result from the mixing of fuels of different origin and treatment, each of which may be perfectly satisfactory when used alone. For example, straight-run fuel oils from the same crude oil are normally stable and mutually compatible whereas fuel oils produced from thermal cracking and visbreaking operations that may be stable can be unstable or incompatible if blended with straight-run fuels and vice versa (ASTM D-1661). 

During the 1990s there was a marked increase in problems arising from complex aromatic asphaltenic species in fuel and their incompatibility in predominantly paraffinic lubricants. This marine black paint phenomenon led to progressively darker and more tenacious surface coatings, especially in rocker boxes and crankcases. Lubricant suppliers resolved this by altering the balance of detergents used in trunk piston oils. 

Understanding the nature of the higher molecular weight constituents 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 in the resin fraction but have received 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 necessary constituent and that by their presence they prevent incompatibility (3,4). This is only one of several factors that influence dispersibility or compatibility, and others will be noted, in turn, throughout the chapter. 

Solubility and Incompatibility. Asphaltenes are, in fact, insoluble in the oil fraction (3, 4) and the likelihood that asphaltene dispersion is mainly attributable to the resins is possible. 

The presence of hydrogen changes the nature of the products, especially the coke yield by preventing the buildup of precursors that are incompatible in the liquid medium and form coke. But precisely how asphaltenes react with the catalysts is open to much more speculation. 

Incompatibility is still possible when asphaltenes interact with catalysts, especially acidic support catalysts, through the functional groups, for example, the basic nitrogen species just as they interact with adsorbents. The possibility exists for interaction of the asphaltene with the catalyst through the agency of a single functional group in which the... 

After treatment, it is also desirable to have no accumulations at the layer between the oil and the water which forms a middle phase or interface pad. A large middle phase (interface pad) would have to undergo treatment separately as it tends to collect solids, emulsifiers, asphaltenes, and other assorted surface-active impurities. On flic one hand, an interface pad presents no problem as the resolved water is drawn out from the bottom of a setting tank, or the pad may act as a filter for the mass Iransfer of sohds into the oil phase. This interface can be removed and treated separately, after oil dehydration. On flie other hand, a large pad creates slop oil and presents its own separation and disposal problems. For this reason the user is cautioned against overdosing in treatment. The choice of chemicals used is thus crucial to avoid overdosing or incompatibilities. 

Asphaltene constituents in coal liquids are important for several reasons (1) they contribute to many problems in the processing of crude oil (2) they contribute to instabilities in processing (3) they lead to excessive viscosity, contributing to pumping expense, clogging, and slow processing (4) the presence of asphaltene constituents can lead to incompatibility with some solvents used in petroleum processing and (5) asphaltene constituents lead to coke formation under some circumstances. 

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. 

Reservoir fluid sample analyses should be reviewed or conducted if analyses do not yet exist. Produced fluids can cause damage through deposition of wax or asphaltenes from oil or through scale formed from produced brine. Also, while well assessment continues, it may be found that fluids incompatible with the reservoir fluids, inducing organic deposition or scale formation, may have been introduced sometime in the well s past. 

Compatibility is the degree to which two or more crude oils of different origin can be mixed without causing flocculation (precipitation) of asphaltene or wax deposition. As a general rule, crude oils of the same type are always compatible. However, even within the same group or type of crude oil, incompatibility can occur. In such cases, the prediction of incompatibility problans requires considerable experimental work. 

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