Asphaltene reactivity

Based on 13C-NMR analysis, ultimate analysis and molecular weight determination (table 9.1), a model of the average molecule was built for native asphaltene (Fig. 9.4a) and asphaltene from visbreaking at 425°C and a residence time of 30 minutes (Fig. 9.4b). These figures show clearly that during the thermal treatment of crude oil residue, only the cracking of paraffinic chains can cause asphaltenes reactivity to yield coke. 

Oils C (Boscan), D (Cerro Negro), E (Athabasca) - from the class of asphaltenic-aromatic crude oils, have a high sulfur content. The first two oils coming from carbonate source rocks contain polar compounds consisting of very stable polycyclic aromatics. On the other hand, the last oil contains aromatics which are less condensed and more reactive. 

A most striking result from the work described above is that the composition of the bottoms product and residues from the dissolution reaction did not depend on the chemical structure of the original coal material only their relative quantities differed. This supports the view of a mechanism involving the stabilisation of reactive fragments rather than an asphaltene-intermediate mechanism. The formation of a carbon-rich condensed material as a residue of the reaction and the fact that hydrogen transfer occurred largely to specific parts of the coal further supports this view. 

The present authors studied the solvolytic liquefaction process ( ,7) from chemical viewpoints on the solvents and the coals in previous paper ( 5). The basic idea of this process is that coals can be liquefied under atmospheric pressure when a suitable solvent of high boiling point assures the ability of coal extraction or solvolytic reactivity. The solvent may be hopefully derived from the petroleum asphaltene because of its effective utilization. Fig. 1 of a previous paper (8) may indicate an essential nature of this process. The liquefaction activity of a solvent was revealed to depend not only on its dissolving ability but also on its reactivity for the liquefying reaction according to the nature of the coal. Fusible coals were liquefied at high yield by the aid of aromatic solvents. However, coals which are non-fusible at liquefaction temperature are scarcely... 

The recovery, regeneration, and repeated reuse of the active catalyst are of prime importance in substantially reducing the overall cost of coal liquefaction. The used catalysts usually remain in the bottoms products, which consist of nondistillable asphaltenes, preasphaltenes, unreacted coal, and minerals. The asphaltenes and preasphaltenes can be recycled with the catalyst in bottoms recycle processes. However, unreacted coal and minerals, if present in the recycle, dilute the catalyst and limit the amount of allowable bottoms recycle because they unnecessarily increase the slurry viscosity and corrosion problems. Hence, these useless components should be removed or at least reduced in concentration. If the catalyst is deactivated, reactivation becomes necessary before reuse. Thus, the design of means for catalyst regeneration and recycle is necessary for an effective coal liquefaction process. Several approaches to achieving these goals are discussed below. 

A spectrum of metal compound reactivities in petroleum could arise for several reasons. Nickel and vanadium exist in a diversity of chemical environments. These can be categorized into porphyrinic and non-porphyrinic species vanadyl and nonvanadyl or associated with large asphaltenic groups and small, isolated metal-containing molecules. Each can be characterized by unique intrinsic reactivity. Reaction inhibition which occurs between the asphaltenes and the nonasphaltenes, as well as between Ni and V species, can also contribute to reactivity distributions. The parallel reaction interpretation of the observed reaction order discrepancy is therefore compatible with the multicomponent nature of petroleum. Data obtained at low conversion could appear as first order and only at higher conversions would higher-order effects become obvious. The... 

Vanadyl and nickel reactivity differences resulting from the chemistry of the oxygen ligand on vanadium were discussed in Section IV,A,l,c. Enhanced V reactivity could also arise from molecular size constraints. Beuther and co-workers (Beuther and Schmid, 1963 Larson and Beuther, 1966) speculate that nickel concentrates in the interior of asphaltene micelles while vanadium concentrates on the exterior. Thus a combination of stronger adsorption due to the oxygen ligand and inhibition of Ni reaction, coupled with the exposed position at the periphery of the asphaltene, may all contribute to the enhanced vanadium reactivity relative to nickel. 

Table III extends the comparison of these resids with an emphasis on reactivity, asphaltene characteristics, compound types and the refractory forms of sulfur, such as benzothiophenes and asphaltenic sulfur.

Table I summarizes relevant data for the pyrolysis of a series of isolated asphaltene feedstocks. This table highlights the dependence of reaction product yields and selectivi-ties on the resid origin. Clearly the reactivity of each feed depends upon its source (7).

Thus, in the simultaneous liquefaction and primary catalytic hydrorefining of coal as practiced here, at the point of complete disappearance of hexane-insoluble asphaltenes very large proportions of the original coal remain as nonhydrocarbons in both the total filtered liquid product and in the distillate boiling below 1000°F. At this stage of hydrorefining, an additional input of ca. 2.2 wt % of hydrogen is required to convert all HCl-reactive, basic nonhydrocarbons to hydrocarbons and neutral nonhydrocarbons in the total filtered liquid product and about 1.7% in... 

The liquids require a hydrorefining step to stabilize their reactive properties, to reduce the asphaltenes and preasphaltenes, to reduce sulfur, nitrogen, and oxygen, and to make the liquids more distillable. The extent of hydrorefining depends on the end use of liquids—fuel oil or chemical feedstocks. The objective of this work is to evaluate the hydrorefining processibility of ORC flash pyrolysis coal tar as a part of the tar characterization task. Results of the initial phase of catalyst screening tests are reported in this chapter. 

Petroleum-derived asphaltene is less reactive to physical or chemical agents than is coal-derived asphaltene. 

The high polarity and low association of coal-derived asphaltenes can be used to explain the nature (hydrogen-bonding) and reactivity of coal conversion. 

To obtain a more clearly defined picture of these structural features and to establish the relationship between the chemical structure of asphaltene and its reactivity under a variety of conditions, the potential of chemical and thermal degradation reactions as diagnostic tools has been studied. The specific subject of this investigation was the high molecular weight, sulfur rich asphaltene from the Athabasca bitumen. 

Coal derived materials These products were obtained from our 1 kg h continuous reactor unit (7) as oils (X4 soluble) asphaltenes (tetralin soluble/X4 insoluble) preasphaltene (also known as asphaltol) (tetralin insolubles/tetrahydrofuran (THF) solubles) and THF insoluble materials for subsequent reactivity studies. 

The results in Table 1 show that for reactions at 425 C significant conversion of the preasphaltenes and the asphaltenes produced at 400 C to other products was possible. In particular for the preasphaltenes >95% interconversion occurred, while for the asphaltene the interconversion was >59%. A complete range of products was formed from high oil yields to repolymerized THF insoluble material. This reactivity underlines the inherent instability of these Intermediate products. The addition of a sulphided Ni/Mo catalyst led to 50% improvement in oil yields. 

Cation-Exchange Chromatography. The sample of acid-free asphaltene, dissolved in 98% cyclohexane-2 % benzene, was charged to the A-15 resin (60 g) that had been wet-packed in the column. Unreactive material was washed from the resin with cyclohexane for 12 hr. The reactive material (bases) was recovered from the resin in three subfractions. The first subfraction was removed with benzene. The second subfraction was removed with 60% benzene-40% methanol. The third subfraction was removed in a batch apparatus, using 54% benzene-36 % methanol-10 % isopropyl amine. 

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