(20a) Changes in Asphaltene Structure and Stability during Hydrotreating

This tutorial covers our experience within the area of asphaltene alteration during hydrotreating of residues to remove sulfur (HDS), nitrogen (HDN) and metals (HDM) either as a final upgrading, a coker pretreatment or as an FCC pretreatment process. Aspects of various reactor schemes will be described. The aim is to give an understanding of the relationship between structure, function, phase stability and formation of organic heavy sludge and coke that may impair other processes as well as storage facilities. It was previously shown that the sludge is intimately linked to the remaining asphaltenes in the process. Data in support of this will be reviewed, and the balance between hydrogenation and cracking documented. This balance will eventually be dominated by cracking as the process temperature is increased to meet HDS specifications. Cracking reactions will increase the molecular incompatibility between asphaltic material and the product maltene fraction, which will become more and more aliphatic. Stability tests have been carried out combined with thorough chemical analyses. It is well known that fractionation of asphaltenes may give a much better idea of the variations within the material. However, this method is used only infrequently. The present work includes both heptane-toluene extraction with increasing heptane content and extraction with n-methyl-pyrrolidone (NMP), and a correlation is made with the development of stability. NMP has been shown to generate a non-fluorescent insoluble fraction. The content of this change with process severity. The classical instability approach only takes hydrocarbon structure changes into account. It will be shown that molecular interactions through functional groups may be an important stabilizing mechanism inhibiting the asphaltene destabilization.