Asphaltenes rates

Colloidal State. The principal outcome of many of the composition studies has been the delineation of the asphalt system as a colloidal system at ambient or normal service conditions. This particular concept was proposed in 1924 and described the system as an oil medium in which the asphaltene fraction was dispersed. The transition from a coUoid to a Newtonian Hquid is dependent on temperature, hardness, shear rate, chemical nature, etc. At normal service temperatures asphalt is viscoelastic, and viscous at higher temperatures. The disperse phase is a micelle composed of the molecular species that make up the asphaltenes and the higher molecular weight aromatic components of the petrolenes or the maltenes (ie, the nonasphaltene components). Complete peptization of the micelle seems probable if the system contains sufficient aromatic constituents, in relation to the concentration of asphaltenes, to allow the asphaltenes to remain in the dispersed phase. 

Asphaltene deposition during oil production and processing is a very serious problem in many areas throughout the world. In certain oil fields (12,13,14) there have been wells that, especially at the start of production, would completely cease flowing in a matter of a few days after an initial production rate of up to 3,000 BPD. The economic implications of this problem are tremendous considering the fact that a problem well workover cost could get as high as a quarter of a million... 

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. 

Hydrogenation of bituminous coal was found more difficult than that of brown coal, chiefly because the primary products of hydrogenation of bituminous coal have a higher percentage of asphaltenes (material soluble in benzene but insoluble in w-hexane). The slower rate of destructive hydrogenation of asphaltenes is reflected in the much lower throughput of bituminous coal per unit volume of reactor. 

Residuum desulfurization kinetics are generally not first order. Figure 5 illustrates this with a first-order plot for desulfurization of Arabian light residuum. On this type of plot a first-order reaction would yield a straight line with a slope corresponding to the reaction rate constant. The over-all desulfurization reaction is not therefore first order and can in fact be represented by second-order kinetics. However, the figure shows that it may also be considered as the sum of two competing first-order reactions. The rates of desulfurization of the oil and asphaltene fractions are reasonably well represented as first-order reactions whose... 

Vanadyl salen is readily converted at 100°C with H2S in the absence of a catalyst to a vanadium sulfide and a free organic ligand (or decomposition products). Vanadyl phthalocyanine is more stable with respect to ring attack and demetallation. Rates relative to catalytic reactions have not been measured. If VO-salen is an appropriate model of vanadium binding in asphaltenes, asphaltenic metals are more readily converted to sulfides under hydrotreating conditions than the porphyrinic metals. This suggests... 

Spry and Sawyer (1975) developed a model using the principles of configurational diffusion to describe the rates of demetallation of a Venezuelan heavy crude for a variety of CoMo/A1203 catalysts with pores up to 1000 A. This model assumes that intraparticle diffusion is rate limiting. Catalyst performance was related through an effectiveness factor to the intrinsic activity. Asphaltene metal compound diffusivity as a function of pore size was represented by... 

In the present context, the deposition of coke on a desulfurization catalyst will seriously affect catalyst activity with a marked decrease in the rate of desulfurization (Chapter 5). In fact, it has been noted that even with a deasphalted feedstock, i.e., a heavy feedstock from which the asphaltenes have previously been removed, the accumulation of carbonaceous deposits on the catalyst is still substantial. It has been suggested that this deposition of carbonaceous material is due to the condensation reactions that are an integral part of any thermal (even hydrocracking) process in which heavy feedstocks are involved. 

It appears that the high molecular weight species originally present in the feedstock (or formed during the process) are not sufficiently mobile (or are too strongly adsorbed by the catalyst) to be saturated by the hydrogenation components and, hence, continue to condense and eventually degrade to coke. These deposits deactivate the catalyst sites and eventually interfere with the hydrodesulfurization process. Thus, the deposition of coke and, hence, the rate of catalyst deactivation, is subject to variations in the asphaltene (and resins) content of the feedstock as well as the adsorptive properties of the catalyst for the heavier molecules. 

Nevertheless, the development of general kinetic data for the hydrodesulfurization of different feedstocks is complicated by the presence of a large number of sulfur compounds each of which may react at a different rate because of structural differences as well as differences in molecular weight. This may be reflected in the appearance of a complicated kinetic picture for hydrodesulfurization in which the kinetics is not, apparently, first order (Scott and Bridge, 1971). The overall desulfurization reaction may be satisfied by a second-order kinetic expression when it can, in fact, also be considered as two competing first-order reactions. These reactions are (1) the removal of nonasphaltene sulfur and (2) the removal of asphaltene sulfur. It is the sum of these reactions that gives the second-order kinetic relationship. 

Because of their high molecular weight and complexity, the asphaltenes remain an unknown entity in the hydrodesulfurization process. There are indications that, with respect to some residua and heavy oils, removal of the asphaltenes prior to the hydrodesulfurization step brings out a several fold increase in the rate of hydrodesulfurization and that, with these particular residua (or heavy oils), the asphaltenes must actually inhibit hydrodesulfurization. As a result of the behavior of the asphaltenes, there have been several attempts to focus attention on the asphaltenes during hydrodesulfurization studies. The other fractions of a... 

In the process (Figure 9-37), the residue feed is slurried with a small amount of finely powdered additive and mixed with hydrogen and recycle gas prior to preheating. The feed mixture is routed to the liquid phase reactors. The reactors are operated in an up-flow mode and arranged in series. In a once through operation conversion rates of >95% are achieved. Typically the reaction takes place at temperatures between 440 and 480°C and pressures between 150 and 250 bar. Substantial conversion of asphaltenes, desulfurization and denitrogenation takes place at high levels of residue conversion. Temperature is controlled by a recycle gas quench system. 

Figure 4. The effect of asphaltene source on the global kinetic rate parameters for the pathways illustrated in Figure 3 (7).

Catalytic tests have been performed in a 500 ml stainless steel batch reactor under hydrogen pressure using 50 g of presulfided catalyst and 125 g of Safanyia atmospheric residue (SAR), The SAR feed had a specific gravity of 0,977 and contained 4.1 wt % S, 0.25 wt % N, 25 wt ppm Ni, 81 wt ppm V and 15.5 wt % C7-asphaltens, A set of used catalysts (symbol P) has been obtained by varying the pressure between 2 to 15 MPa at reaction temperature of 390 °C, contact time of 1 h and hydrogen flow rate of 30 1/h. Further experimental details are reported elsewhere (30). 

The results of the depolymerization studies such as metal reductions and low temperature solvolysis have shown that the mechanism and the rate of degradation of asphaltene depend to a large extent on the chemical environment. 

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