TABLE OF CONTENTS Heavy oils and bitumens are characterized by the presence of large molecules, the asphaltenes, which among their complex molecules also present metal-containing moieties. Typical metals present in petroleum comprise various species (e.g., Ni, V, Fe, A1, Na, Ca, and Mg), which are particularly accumulated in the asphaltene fraction of crude oil [382 384].
Vanadium and nickel are generally the most abundant of the trace metals, are present to some extent as chelated porphyrin complexes, and have been linked to catalyst poisoning during the upgrading of heavy oils [385 387]. The concentrations of Fe, A1, Na, Ca, and Mg have also been reported to vary in deposits as a function of well depth [388] and among asphaltene fractions [383,384]. The metal-containing molecules are usually accompanied by sulfur, oxygen, and nitrogen. The metal content of petroleum becomes significant as the asphaltene content increases. Thus, heavy oils contain metals at levels below 1%, but enough for creating problems in petroleum refining and upgrading [389,390]. The metals have been associated with the porphyrin structures present in the heavy oils, though now it is known that porphyrins are only a fraction of the metal-containing molecules. Typical core structures of porphyrin molecules are shown in Fig. 35.
Metal moieties and in general, asphaltenes are responsible for the processability limitations faced by heavy crude oils and bitumens. Catalyst poisoning, polymerization, coke formation, precipitation, emulsification, foaming, etc., are some of these problems. The highly asphaltenic resources must be rigorously...
