Adsorption of asphaltenes

Barranco et. al. (1999) hypothesized that film formation results from the adsorption of asphaltenes at the coal tar-silica interface. Barranco et. al. (1999) further postulate that these asphaltene components are responsible for pH-dependent interfacial properties observed in coal tar-water-quartz systems. 

Initial deactivation is almost certainly due to adsorption of asphaltenes on acidic sites on the catalyst. Analysis of the catalyst shows that the initial deposits block smaller pores to cause up to a 50% loss of surface area. 

The initial heavy deposits of coke almost certainly originate mainly from asphaltenes in the feed. Preferential adsorption of asphaltene fractions have been observed on cobalt and molybdenum oxides [23] and sulphides [24] as well as on sulphided nickel-molybdenum... 

Reappraisal of catalyst deactivation during heavy end hydrotreating suggests that initial deactivation involves the adsorption of asphaltenes on acidic sites in the catalyst. This results in the blocking of smaller pores and loss of ca 50% of surface area. 

Catalyst can also be deactivated by adsorption of asphaltenes during the hydroprocessing of heavy crude oil residues. 

The schematic of Fig. 2 is obviously oversimplified nonetheless, there are several salient features illustrated which likely captiue the essence of asphal-tene-stabilized films. First, siuface adsorption of asphaltene molecules is probably driven by hydration of polar functional groups in the aromatic core of an individual asphaltene molecule. Second, resin molecules probably serve to solvate primary aggregates (asphaltene micelles) in the bulk phase, but these resins are likely shed and do not appreciably participate in the actual stabilizing film. In fact, as we will show later, resins are totally unnecessary in the stabilization of asphaltenic films. A missing detail in Fig. 2 is the means whereby individual asphaltene molecules crosslink to form... 

The affinity of AC for the adsorption of asphaltenes from VR at 523 K and 10 MPa of H2 was dependent on the mesoporosity of AC. Among three AC studied, the one possessing the greatest mesoporosity was the most efficient. The same AC was the best support for the Fe/AC catalyst used for hydroprocessing the VR. This catalyst exhibited the highest activity for asphaltenes conversion. 

Acevedo S., et al.. Adsorption of asphaltenes and resins on organic and inorganic substrates and their correlation with precipitation problems in production well tubing. Fuel, 74(4), 595-598 (1995). 

We therefore conclude that there is some evidence that certain components of crude oils, including asphaltenes and resins, can adsorb at the air-liquid surface of non-polar hydrocarbon liquids. However, the molecular origin of such surface activity is far from clear, especially in those cases where significant polarity is present. Moreover, there appears to be little direct evidence that, for example, adsorption of asphaltenes and resins occurs at gas-crude oil surfaces. The main challenge therefore remains the difficult task of verifying that such surface activity is actually present in crude oils. 

Asphaltenes were extracted from West Syberian crude by standard method described in previous work (Balabin Syunyaev, 2008). The value 750 g/mol was chosen as average molecular mass of asphaltenes (Mullins et al., 2007). Adsorption of asphaltenes was studied at concentration 1 g/1 in benzene (density 0,88 g/cm, molar mass 78 g/mol). Four fractions of quartz sand, three fractions of dolomite and two fractions of mica were used as adsorbents. Quartz and dolomite particles are considered to be quasispherical particles. Mica models crumbling rock with plate-like particles. Adsorbents were provided by company "Batolit". Its parameters are shown in Table 2. The particle size distribution was evaluated using optical microscope (OPTITECH SME-F2). 

The adsorption of asphaltenes is practically irreversible. Significantly larger masses and molecule sizes of asphaltenes appear to be the reason. Diffusion of such molecules to solid surface is embarrassing. The mechanism of diffusion limited adsorption is realized (Syunyaev et al., 2009 Diamant Andelman, 1996). Gibbs energy values are more or less the same for surfaces of all investigated materials quartz, dolomite, and mica. It is known that quartz and dolomite are the main components of oil reservoir framework rocks. The porosity has no influence on kinetic parameters of adsorption. Asphaltenes adsorption at the surfaces of quartz and dolomite is the most active. 

Fig. 6. Polymolecular multilayered adsorption of asphaltenes at mica surface.

Ekholm P., Blomberg E., Claesson P., Auflem I.H., Sjoblom J. and Kornfeldt A. (2002). A Quartz Crystal Microbalance Study of the Adsorption of Asphaltenes and Resins onto a Hydrophilic Surface, Journal of Colloid and Interface Science, 247(2), p. 342-350. 

TatNIPI. (1988). Scientific report. Adsorption of Asphaltenes onto Quartz and it s Influence to Wettability. 

The adsorption of asphaltenes and other natural surfactants at the water/toluene interface has been studied here. The adsorption isotherms for the different systems studied (see Figure 5) show that there is a systematic decrease of the interfacial tension with the increase in the sample concentration until a concentration is reached after which the tension remains unchanged. This value corresponds to the saturation of the aggregates in the solution which coincides with the results reported by Acevedo et al. in 2005. The evidence presented in this work suggests that asphaltenes adsorb to the oil-water interface. 

Rheological Studies of Extra- Heavy Crude Oils and Asphaltenes - Role of the Dispersion Effect of Resins in the Adsorption of Asphaltenes at the Interface of Water-In-Crude Oil-Emulsions. Colloids and Surfaces A-Physicochemical and Engineering Aspects, 71, 65-71. 

Despite these drastic assumptions, the Langmuir equation is often applied in the areas where assumptions behind it do not hold (e.g. adsorption of asphaltenes, surfactants or polymers). 

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