Asphaltenes reactions

In the presence of a high pressure of hydrogen, tin metal facilitates the stabilization of radicals formed by initial coal depolymerization leading to the formation of preasphaltenes and asphaltenes (reactions 5 and 6). The stronger pressure dependence of the tin catalyzed reactions compared with other systems investigated here may be linked... 

Neurock, M., Libatani, C., and Klein, M. T., Modeling asphaltene reaction pathways Intinsic chemistry, AIChE Symp. Ser., Fundamentals Resid. Upgrading 85(273), 7-14 (1989). 

On account of the tendency of the asphaltene core to polycondensation, the increase in molecular weight at residence time over 15 minutes and a reaction temperature of 400°C can be explained by polycondensation of aromatic cores of asphaltenes (reaction (9.2)). Polycondensation reactions affect coke formation. This is noticed at 40 minutes residence time. By the thermal treatment of Bitumen 200, approximately 1 wt.% coke (based on water free feed) was formed. The fact that coke formation begins at the temperature of 400°C only if asphaltenes attain a very high molecular weight in comparison to higher temperature levels is caused by the formation of a stable steric colloid with the resins. This stable colloid is not cracked at the temperature of 400°C as deeply as at higher temperature levels. The formation of such a colloid is represented in Figure 9.3 ... 

Based on the knowledge about the possible reactions of asphaltenes during thermal processing of pure bitumen, we can suggest the following general scheme of asphaltene reactions during thermal treatment (9.7) ... 

Evaluation of Possibilities of Various Asphaltene Reactions Based on Thermodynamics... 

Without doubt, the most important asphaltene reactions, in the current context, are thermal reactions and the manner in which they relate to thermal and catalytic refining options. It is known that asphaltenes will produce significant quantities of distillable liquids in thermal processes, but the production of significant quantities of coke has always been an issue (I). 

Savage PE, Klein MT (1987) Asphaltene reaction pathways. 2. Pyrolysis of n-pentadecylbeneze. Ind Eng Chem Res 26 488-494... 

Savage, P.E., Klein, M.T., Kukes S.G. 1988. Asphaltene reaction pathways. El Effect of reaction environment. Energy Fuels 2 619-628. 

Other reactions that occur during hydrocracking are the fragmentation followed by hydrogenation (hydrogenolysis) of the complex asphaltenes and heterocyclic compounds normally present in the feeds. 

Cap Gas. Both crude and asphaltene-free oil were used to determine the consequences of low-temperature oxidation. It was found that the oxygen content in an artificial gas cap was completely consumed by chemical reactions (i.e., oxidation, condensation, and water formation) before the asphaltene content had reached equilibrium. 

Preliminary work showed that first order reaction models are adequate for the description of these phenomena even though the actual reaction mechanisms are extremely complex and hence difficult to determine. This simplification is a desired feature of the models since such simple models are to be used in numerical simulators of in situ combustion processes. The bitumen is divided into five major pseudo-components coke (COK), asphaltene (ASP), heavy oil (HO), light oil (LO) and gas (GAS). These pseudo-components were lumped together as needed to produce two, three and four component models. Two, three and four-component models were considered to describe these complicated reactions (Hanson and Ka-logerakis, 1984). 

For crude oils C and D, some lighter hydrocarbons are formed during the cracking reactions but the composition of the 210 fraction is hardly modified. In particular, it can be noticed that the asphaltene contents of both of the recovered oils remain high. 

Figure 12 clearly shows the effect of iron sulfide content of the coal on total conversion and liquid product yield during hydrogenation. The conversion increased from about 52 per cent to 70 per cent using the hot-rod reactor with no added catalyst. The yield of toluene soluble product (oil plus asphaltene) increased from about 30 to 44 per cent with total sulfur increase from 1 to 6.5 per cent. Thus it would appear that iron sulfide can act catalytically in the dry hydrogenation reaction as well as in slurried reactions (15). 

Formation of asphaltenes during solubilization of low-rank bituminous coals has been attributed to cleavage of open ether-bridges (6). But while the presence of such configurations in high- and medium-rank bituminous coals is well established (7), their existence in less mature coals remains to be demonstrated. From reactions of low-rank bituminous coals with sodium in liquid ammonia or potassium in tetrahydrofuran, it has, in fact, been concluded that open ether-bonds are absent (8) or only present in negligible concentrations (9). 

A question then arises as to whether the CSD recovery is being limited by the preasphaltene content produced from direct products of coal liquefaction or whether by low liquefaction severity a more thermally sensitive product is produced resulting in retrogressive reactions of liquefaction products to "post-asphaltenes." There is some indication that "virgin" preasphaltenes, primary products of coal dissolution, are more easily recovered via CSD as shown in Table VII however, "postasphaltenes" made from thermal regressive reactions are not. 

Many studies on direct liquefaction of coal have been carried out since the 1910 s, and the effects of kinds of coal, pasting oil and catalyst, moisture, ash, temperature, hydrogen pressure, stirring and heating-up rate of paste on coal conversion, asphaltene and oil yields have been also investigated by many workers. However, few kinetic studies on their effects to reaction rate have been reported. 

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... 

In this paper we have looked firstly at the effect that the catalyst concentration, secondly at the effect that the reactor temperature and finally at the effect that the residence time at temperature have on the chemical structure of the oils (hexane soluble product) produced on hydropyrolysis (dry hydrogenation) of a high volatile bituminous coal. Generally, the hydropyrolysis conditions used in this study resulted in oil yields that were considerably higher than the asphaltene yields and this study has been limited to the effects that the three reaction conditions have on the chemical nature of the oils produced. 

Product distribution For many years high pressure hydrogenation reaction has been dealt with as a consecutive reaction with asphaltene as the intermediate (4,5,6). Further it has been pointed out that Py-1, O2 likewise shows the behavior of intermediates. (See Figure 1) (3). 

Residue HDP is complicated by the quality of the feed high nitrogen concentration, asphaltenes and metals are the complicating factors. A large number of parallel and simultaneous reactions occur, both thermal and catalytic. Besides contributing to conversion, the thermal reactions contribute to coke formation, as well. 

The effect of conversion on the structure of an asphaltene molecule has been reported to depend on the operating conditions and on the presence or not of a catalyst. The effect of thermal processing reaction of a vacuum residue resulted in the selective cracking of the aliphatic or naphthenic side chains of the molecule, leaving the highly condensed aromatic core structure almost intact (see Fig. 16) [116]. 

Michael, G. Al-Siri, M. Khan, Z. H., and Ah, F. A., Differences in Average Chemical Structures of Asphaltene Fractions Separated From Feed and Product Oils of a Mild Thermal Processing Reaction. Energy Fuels, 2005. 19 pp. 1598-1605. 

Callejas, M. A., and Martmez, M. T., Hydroprocessing of a Maya Residue. 1. Intrinsic Kinetics of Asphaltene Removal Reactions. Energy Fuels, 2000. 14 pp. 1304—1308. 

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