Asphaltenes diffusion

A major deficiency in the asphaltene diffusion studies results from the limitations of the experimental conditions employed. Measurements at room temperature and in clean solvents can potentially alter the effective size of the diffusing species relative to that in hydrotreating catalysts at reaction conditions. 

Other techniques such as X-ray diffusion or small angle neutron diffusion are also used in attempts to describe the size and form of asphaltenes in crude oil. It is generally believed that asphaltenes have the approximate form of very flat ellipsoids whose thicknesses are on the order of one nanometer and diameters of several dozen nanometers. 

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... 

Despite claims by Spry and Sawyer (1975) of analytical measurements verifying asphaltene molecular sizes in the 100 A range at ambient conditions, it is unlikely that molecules this bulky exist at reaction conditions. The good predictive capability of the model may therefore result from a compensation effect. Electrostatic and adsorption interactions between solute molecules and the pore walls not explicitly accounted for with the purely geometric partition coefficient may result in the diffusing molecules appearing larger than they are at reaction conditions. 

Contact time between the hydrocarbon and the feedstock also plays an important role in asphaltene separation (Figure 3-12). Yields of the asphaltenes reach a maximum after approximately 8 hr, which may be ascribed to the time required for the asphaltene particles to agglomerate into particles of a filterable size as well as the diffusion-controlled nature of the process. Heavier feedstocks also need time for the hydrocarbon to penetrate their mass. 

The presence of solids further complicates the performance requirements for a demulsifier. Emulsions stabilized by fine particles can usually be broken if the wettability of the particles is reversed. Inorganic particles, such as iron sulfides or clay minerals, can be made water-wet, causing them to leave the interface and diffuse into the water phase, or they can be made oil-wet so that they leave the interface and diffuse into the oil phase [68]. Organic particles, such as paraffins and asphaltenes, can be removed from interfaces by dissolution [461,463,466]. 

This calibration does not assume that the n-alkanes and polystyrenes are typical of residual molecules. However, they do provide well-defined size standards in the elution time range of interest. No assumptions can be made concerning the shapes of the asphaltene or maltene molecules. Therefore, the GPC size calculated is defined as the critical molecular dimension, which determines if the asphaltene or maltene molecule will diffuse into the pores of the GPC packing. This size is assumed to be related to the size parameter that determines the molecular diffusion into hydrotreating catalyst pores. 

Both asphaltene and maltene molecular size distributions were compared with the pore size distribution of a small pore desulfurization catalyst. Figure 4 shows the Kuwait maltene and asphaltene size distributions along with the catalyst pore size distribution. Most of the maltene molecules are small enough to diffuse into the catalytic pores. In contrast, the Kuwait asphaltenes have a... 

The GPC size data also were used to monitor the effect of various processes on the asphaltene molecular size distributions. Only the asphaltenes were studied in these experiments since they are the hardest to process, and they cause the most diffusion and coking problems. One of the processes studied was visbreaking, that is, noncatalytic thermal processing. Previous workers had examined the thermal decomposition of an Athabasca asphaltene... 

Catalyst A with a small pore size rapidly lost its activity due to plugging of the catalyst pores from SOR to EOR. This type of catalyst is not strongly deactivated by coke deposition, because asphaltenic compounds, which easily make coke on the catalyst surface, are not allowed to diffuse into the catalyst pores. However, this catalyst is easily deactivated through pore plugging by metal deposition. 

Pore diffusion effects can be considered in terms of a theoretical equation (ref, 6) which has been shown to describe the diffusion of petroleum asphaltene molecules,... 

If one cannot diffuse the asphaltenes to the catalyst, why not diffuse the catalyst to the asphaltenes Dispersed catalysts also can be continuously added in sufficiently low enough amounts (i.e., 100 ppm) to consider them throwaway catalysts with the carbonaceous by-product. However, economics usually dictate some form of catalyst recycle to minimize catalyst cost. Nevertheless, by designing the reactor to maximize the solubility of the converted asphaltenes, the conversion of vacuum resids to gas and volatile liquids can be above 95% with greater than 85% volatile liquids. However, the last 5-10% conversion may not be worth the cost of hydrogen and reactor volume to produce hydrocarbon gases and very aromatic liquids from this incremental conversion. The answer depends on the value and use of the unconverted carbonaceous liquid by-product. 

One additional feature occurs when this catalyst is used with extremely heavy residua feedstocks. With proper preparational techniques, bimodal pore distributions are produced in which constrictions at the openings inhibit diffusion of large asphaltenic, coke producing molecules, whereas smaller sulfur compounds have easy access. This effectively reduces excessive coking which otherwise occurs. ... 

Slurries frequently involve a wide range of particle sizes that include submicron particles in the Brownian diffusional region. When cakes are deposited, the finest particles may diffuse into the filtrate and continuously clog the supporting medium, leading to increasing values of R. Tiller and Leu (1984) showed that clogging was a major problem in the removal of ash and asphaltenes from liquefied coal. 

The hydrotreating catalyst (Haldor Topsoe tk-551) consisted of 2.8% Ni, 10.1% Mo and 7.2% P supported on alumina. The 1/16 inch extrudates were crushed and sieved to 40-60 mesh size particles and calcined at 500° C for 16 hours- Catalyst samples were sulfided at 400c 0 in a separate flow reactor with 10% HjS in H for two hours and rapidly transferred to a high pressure, stirred autoclave containing vacuum gas oil (VGO) or a mixture of VGO in mineral oil. The VGO had a corrected boiling range of 265-515° C and a density (15° C) of 0.924 g/cm3, and contained 2.86% S, 740 ppm N, and 200 ppm asphaltenes. Catalysts were coked for 2-3 days at 375 cC in the different concentrations of VGo with added CS, in order to obtain different amounts of coke. Two additional run were made in which the sulfided catalyst was treated with a mixture of n-butylamine or N-phenylcarbazole in hexadecane at 300° C, Coked catalysts were Soxhlet extracted with xylene, followed by an acetone wash, dried at 110° C, and subsequently resulfided in order to assure removal of oil and weakly adsorbed species. Coronene uptakes and diffusivities of... 

Figure I Model of asphaltene particle and measured molecular weight distribution of asphaltenes in heavy Venezuelan crude. Asphaltene molecular size distribution computed by configurational diffusion model (from Spry and Sawyer [I40p.

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