Paso 2: Diagnóstico de la situación actual y fijar objetivos

2.1 BACKGROUND TO HEAVY FUEL OILS 2.1.1 Processing

In refining crude petroleum, a variety of processes are available for converting the crudes Into more valuable products. For example, materials boiling below about 673 K may be recovered by atmospheric distillation, whereas materials boiling up to 900 K or higher are recovered by vacuum distillation. The residuum heavy fuel oil generally contains high concentrations of high molecular weight organic compounds with sulphur, nitrogen, oxygen, metals and other non-hydrogen species, as well as high molecular weight hydrocarbons. Including condensed-ring aromatics. Nowadays further processing Is carried out on the fuel to increase the proportion of lighter hydrocarbons. This can be achieved by thermal cracking which thermally decomposes (723 K to 1023 IQ, under pressure (65 barg), large hydrocarbon molecules to smaller molecules. Under these conditions, the main reactions occurring are carbon-carlx)n bond fission and dehydrogenation. In contrast, viscosity breaking, or visbreaking, is a mild cracking operation used to reduce the viscosity of straight run residues. By reducing the viscosity of the residue, visbreaking reduces the amount of diluent required for blending a fuel oil to specification. Visbreaking conditions range from 3 to 20 barg pressure and temperatures of 723 - 773 K. In addition to the major product, fuel oil, gas oil and gasoline are produced. The severity of the visbreaking process is limited by coke formation in the unit, and by the unstable chemical/physical nature of the resultant residues in terms of subsequent fuel oil blending. Visbreaking is the preferred route for crude oil processing in Europe. In the United States legislation for combustion of heavy fuel oil, particularly with a high sulphur content is severe. The preferred process in the US is coking which is a severe form of thermal cracking, designed to convert residual feed stocks completely into gas, naphtha (gasoline component), gas oil and coke.

2.1.2 Composition

It is not possible to define the exact composition of any individual crude oil, although similar families of hydrocarbons (alicyclics, cyclics and aromatics) are present in each, together with organic compounds of many elements. The composition of crude oil and residual fuel oil have been reviewed by Spears and Whitehead (1970) and Whitehead (1981) respectively. Heavy fuel oil is basically a colloidal system, consisting of asphaltene micelles dispersed in a lower

with an adsorbed covering sheath of high molecular weight aromatic resins as a stabilising solvating layer. Asphaltenes in heavy fuel oil are generally considered to be highly aromatic materials of fairly high molecular weight (10^ to 10^). They are brown to black amorphous solids containing, in addition to caiton and hydrogen, nitrogen, sulphur and oxygen (Girdler 1965). Further away from the centre of the micelle there is a gradual transition to the less aromatic oily dispersion medium. The stabilising micelle will not normally precipitate until a solvent (eg n-heptane) is added which will reduce the peptising power of the dispersion medium and leave the asphaltenes free to agglomerate and precipitate. In fact solvent fractionation (Institute of Petroleum test IP 143/77) is used to define asphaltenes In an arbitrary way. This test separates the residue, using n-heptane, into an insoluble asphaltene portion and a soluble maltene portion, which can be further fractionated into resins and oils. Resins and oils are required for asphaltenes to dissolve in the distillate portion of a crude oil providing a transition between the polar (asphaltene) and the relatively non-polar (oil) fractions in heavy fuel oil thus preventing the assembly of polar aggregates that would be non-dispersible in the oil. The problem is complicated by the fact that once asphaltenes are removed from their original environment their structure, or perhaps the arrangement of various groups collectively called asphaltenes, change. Thus observed asphaltene structures vary with source and precipitating agent. Several values for the molecular weight of asphaltenes have been reported, and since asphaltenes associate strongly even in dilute solutions the range is wide. Separation by gel permeation chromatography suggest values of 7-30 kg/mol, while the lower limit of 1-2 kg/mol is found for Alberta asphaltenes disassociated in nitrobenzene.

X-ray diffraction studies (Yen et al 1961) have investigated the structural parameters of various asphaltenes. Studies of rates of oxidation have indicated that the heteroatoms oxygen, sulphur and nitrogen occur largely in chemically stable configurations, probably rings. Speight (1975) has shown that the concentration of heteroatoms in structural group entities falls with increasing molecular weight of the fraction. Nitrogen and sulphur do not predominantly exist in functional groups but gain stability during cracking by incorporation into ring systems. On the other hand, oxygen appears mostly in thermally-labile configurations and is eliminated during pyrolysis. Infra-red absorption evidence points to the presence of naphthenic groups as well as aromatic and paraffinic, and some structural investigations have been carried out by NMR. Functional groups within the asphaltene structure include phenols, indols, carboxylic acids, sulphoxides, amides, pyrazines and pyridines. The presence of stable free radicals has been shown by electron spin resonance studies to be localised in the aromatic groupings.

It can be seen that the structure of an asphaltene constitutes a complex problem and many attempts have been made to elucidate a definitive configuration. One of the most complete pictures of the asphaltene structure has been discussed by Yen (1974), see Figure 2.1.

Aa/^s/V \A / W ^ / V V ^ O H 8-1.6 n HOOC /^ 'V N o 0.5-0 6nm METAL 1.6-2.0 nm |

Zig-zag lines represent saturated cartx>n sheets or naphthenic rings; ^ the straight lines represent the edge of flat sheets of condensed aromatic rings (Yen 1974)