Crude asphaltenes

The high viscosity of heavy crude oils is essentially due to the high levels of asphaltene content. Asphaltene is the highest MW component of crude oil, is a friable, amorphous dark solid, which is colloidally dispersed, in the oily portion of the crude. Asphaltenes are considered to be heavily condensed aromatic molecules with aliphatic side chains and with high heteroatom content (S, N, and O) as well as high-metal content. The asphaltene fraction is physically defined as that fraction insoluble in n-alkanes, but soluble in toluene and is the most polar fraction of oil. 

Figure 2. Hydroconversion of Cold Lake crude asphaltenes with 200 ppm molybdenum at 400°C and 6 MPa Ht asphaltenes (A) maltenes (%) coke (JC model (-).

To provide raw material for this comparative study of untreated and heat-treated oils, asphaltenes from Cold Lake crude (crude asphaltenes) and from Cold Lake vacuum residuum (residuum asphaltenes) were prepared by n-heptane precipitation as described in the Experimental section. The Cold Lake residuum fraction was prepared by Imperial Oil Enterprises, Ltd. at Sarnia, Ontario, Canada. The distillation history of this bottoms fraction indicates that the pot material was subjected to temperatures as high as 314-318°C during atmospheric and vacuum distillation. The length of time at 300°C or higher was about two hours. This is well in excess of what would be experienced in a pipestill and should have provided ample time for any decomposition. It should be noted, however, that since it was possible to maintain the system vacuum at 0.35 mm, the maximum temperature experienced by the residuum was not quite as high as it might be during refinery distillation (e.g. ca 350°C). 

Figure 3. IR spectrum of Cold Lake crude asphaltenes...

ESR Parameters for Cold Lake Asphaltenes Parameter Crude Asphaltenes Residuum Asphaltenes... 

The thermal history of these particular residuum asphaltenes is much more severe in terms of heating time than would ordinarily be the case for a refinery product from a pipestill since, in the present instance, a pot distillation was used. It therefore seems likely that refinery asphaltenes should be even less different from their respective crude asphaltenes than in this investigation, assuming that pipestill temperatures would be kept below the decomposition temperatures for the asphaltenes,... 

Since the Cold Lake crude used in this investigation has been exposed to the temperature of the pressuized steam used in the oil production, one cannot be certain that some thermal changes had not already occurred in the crude oil. To study this possibility the properties of cold bailed (i.e. recovered without steam injection) Cold Lake crude asphaltenes are being investigated by many of these same techniques and will be described in a future report. 

Figure I Model of asphaltene particle and measured molecular weight distribution of asphaltenes in heavy Venezuelan crude. Asphaltene molecular size distribution computed by configurational diffusion model (from Spry and Sawyer [I40p.

Trejo, F., Rana, M.S., Ancheyta, J. 2010. Thermogravimetric determination of coke from asphaltenes, resins and sediments and coking kinetics of heavy crude asphaltenes. Catal. Today 150 272-278. 

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