A heat transfer technique for the detection of the onset of asphaltene precipitation is presented and its versatility demonstrated. The technique is able to evaluate the effectiveness of asphaltene precipitation inhibitors, and solvents used to resuspend a deposit. It can also be used to study the reversibility of asphaltene precipitation.
A number of chemical species were tested for their ability to delay the onset a/precipitation. The nonionic surfactant nonyl phenol was the most effective chemical tested. Toluene and xylenes were shown to resuspend an-heptane precipitated deposit, and it was found that xylenes were, in general, more effective. Temperature traces indicate that the redissolution of precipitated asphaltenes upon n-heptane removal is reversible but incomplete. It is possible that the dissolution process may have proceeded further had longer dissolution times been allowed.
Asphaltene flocculation and precipitation remains a problem to petroleum production, refining, and upgrading operations. During production, asphaltenes may precipitate in the reservoir, well, or separation equipment leading to reduced production. In refining and upgrading, asphaltene flocculation and precipitation is acknowledged as the first step in the formation of coke, an unwanted product.
The asphaltene fraction is defined as the material of a petroleum sample that is soluble in toluene and insoluble in the solvents n-pentane or n-heptane at a dilution of 40 millilitre solvent per millilitre petroleum. Being a solubility class, the asphaltenes compose a large number of different molecules. Which are usually very difficult to separate. The methods used to isolate asphaltenes, and the influence of various parameters such as solvent type and contact time have been presented by Speight et al. 1. Asphaltenes are the highest molecular weight species, contain the largest weight percent of the undesirable heteroatoms nitrogen, sulphur, and oxygen, and are the most aromatic fraction of a petroleum (Speighr2).
When considering the stability of petroleum, it is known that a balance between the three pseudo-components: oil, resins, and asphaltenes is required. The resins fraction is responsible for keeping the asphaltenes in colloidal suspension, as was emphasized by Pfeiffer and Saal3. The resins peptize the asphaltenes by acting as a transition between the aromatic polar asphaltenes and the non-polar aliphatic oils, thus preventing asphaltene flocculation and subsequent precipitation. The importance of the resins has been further emphasized by others, such as Nellensteyn4, Swanson5, Koots and Speight6, and Andersen7.
In refining and upgrading processes, such as visbreaking, coking, and hydrocracking, where temperatures are typically in the order of 380 °C to 500 °C, reactions take place that cause the composition of the feed to change. In many of these processes, the net effect is that resins are depleted and the amount of asphaltenes increases (Lepage et al8, and Storm et al9, Wiehe10). To prevent coking in these processes, which is a result of asphaltene condensation reactions, asphaltene flocculation and precipitation must be prevented. Feeds to these processes typically have 20 or more weight percent asphaltenes.
