Asphaltene/toluene solutions

The solubility of asphaltenes is highly dependent on the medium in which they are placed (81). The presence of dissolved asphaltenes in crude oil is mediated by a combination of crude aromaticity and petroleum resins that act to solvate asphaltene aggregates. Adding an excess of aliphatic solvent, namely n-hep-tane, sufficiently reduces the solubility of asphaltenes in crude oil and causes precipitation. To perform subsequent film experiments these precipitated asphaltenes were then redissolved in toluene. As n-heptane was added to the asphaltene—toluene solutions. 

In the case of the hot-rod reactor experiments, the toluene solutions were combined and the toluene removed under reduced pressure. n-Hexane (250 ml) was added to the extract and it was allowed to stand for 24 hours with occasional shaking. The solution was filtered to leave a residue (asphaltene) and the hexane was removed from the filtrate under reduced pressure to give the oil. 

The steric repulsion mechanism is also difficult tc model in our system. A bitumen/toluene solution itself is a very complex system containing high molecular weight asphaltenes, natural surfactants, and ultrafine particles. These components are very likely to be adsorbed on the water/toluene interface. Due to this complexity, it is hard to model the adsorption layer with a single elastic modulus, as was done for the analysis of poly(ethylene oxide) adsorption layers on latexes (7). However, all steric forces resemble hard-wall interactions. They can be approximately modeled by high-order polynomial functions. We used a simple expression F steric c/h where h is the separation... 

First, traditional solvent methods were used to remove soluble impurities from precipitates (asphaltenes and preasphaltenes). The precipitates were then dissolved in toluene solution to remove the precipitated preasphaltenes. Finally, relatively pure asphaltenes were re-dissolved in toluene to obtain a 100 ppm concentration of asphaltene solution . 

Adsorption studies performed with the QCM-D technique showed multilayer or aggregate formation of asphaltenes at the silica surface. Asphaltenes were irreversibly adsorbed as a rigid film. Visual inspection of the resin-coated silica particles clearly showed that the initially hydrophilic silica (Aerosil 200 and Aerosil 7200) adsorbed considerably more resins than the hydrophobic silica (Aerosil 972). By analyzing the toluene solutions for remaining asphaltenes with near-infrared (NIR) spectroscopy, they could conclude that the initially hydrophilic particles had adsorbed considerably more asphaltenes than the hydrophobic particles. All the products exhibited less adsorption of resins than of asphaltenes and the influence of their adsorption was dramatic, especially on the wettability of the hydrophilic silica. Very hydrophilic particles like the unmodified 200 or very hydrophobic particles like the 972 preferred the water and oil phase, respectively. In contrast, the Aerosil 7200 seemed to be equally partitioned between the two phases, indicating intermediate wetting properties. Generally, the stabilization efficiency was enhanced by adsorption of crude oil components onto very hydrophilic or very hydrophobic silica. 

Three toluene solutions at different concentrations are titrated with a weak solvent such as isooctane. The weight of oil the volume of toluene (Tj), and the volume of isooctane titrant (V) are recorded at the flocculation point where asphaltenes just begin to precipitate for each solution. The flocculation ratio (FR) and dilution concentration (C) are calculated as follows (Heithaus, 1962) ... 

For the toluene extractions, the work-up procedure was as described previously (j> ). In the supercritical water experiments, most of the extract was insoluble in water, after cooling and lowering of the pressure, and precipitated out in the condenser and receiver from which it was collected by washing with acetone and then THF. The remainder of the extract was found in the aqueous suspension which was evaporated to dryness on a rotary evaporator and the residue extracted with acetone and THF. The solvents were removed under reduced pressure from the combined acetone and THF solutions to give the total extract. This was then extracted with hot toluene and the cooled solution filtered to give the preasphaltene fraction. After the toluene was removed under reduced pressure from the filtrate, the residue was re-dissolved in a small volume of toluene and a 20 fold excess of pentane added to precipitate the asphaltene which was filtered off. The pentane and toluene were then removed from the filtrate under reduced pressure to give the oil. For the NaOH extractions, the NaOH solutions were neutralised with HC1. The insoluble extract was washed with water and then extracted with THF. Removal of the THF gave the total extract. 

A qualitative survey of the ability of common acids in concentrated aqueous solution to precipitate readily filterable salts from solution of H-coal asphaltenes in toluene suggested perchloric acid as a particularly promising candidate for further examination. The salt that formed at the interface of the solutions was crisply granular, possibly crystalline, although nearly black, and was easily collected and washed. Experiments with this reagent showed that approximately one-third of an asphaltene sample could be precipitated. Because attempts to dry the salts to constant weight by heating led to... 

After the soxhlet extraction and drying of the extract are completed, the extract is dissolved in pentane first. An ultrasound bath can be used to improve solving. This solution of extract in pentane is filtered. After filtration, the extract is separated into two fractions pentane soluble fraction or filtrate and the remainder. The pentane soluble fraction is composed of compounds that have similar polarity as pentane or the liquid at the filtration temperature. The remainder contains the asphaltene fraction and precoke. This remainder is dissolved in toluene in the same manner as the extract was solved in pentane, and then filtered. The asphaltene fraction is the filtrate after filtration with toluene, and precoke is the remainder. The solvents (pentane and toluene) are removed from the filtrates by distillation at 40°C and normal pressure for pentane distillation, and 70 mbar for toluene distillation. 

In 1991, Andersen and Birdi (25) first reported a critical micelle concentration (erne) of asphaltenes in a mixture of n-aUcane and toluene, using a ealorimetric titration method. From their study, the aggregation process of asphaltic molecules in solutions was suggested as the following ... 

Figure 15 Emulsion stability as gaged by % water resolved following centrifugation (see Ref. 82 for details) versus % (v/v) toluene in heptane for 0.5% (w/w) San Joaquin Valley (SJV) asphaltenes and 0.5% (w/w) Alaska North Slope (ANS) asphaltenes dissolved in 30% (v/v) toluene in heptane and mixed with water in a 40 60 (v/v) proportion. Varying amoimts of resins from AH, ANS, SJV, and AB (Arab Berri) crude oils were added to the oleic solutions to give ratios (w/w) of resins to asphaltenes ranging from 0—6. In these experiments, it should be noted that a large range of R/A exists for which the emulsion stability is fairly invariant.

The time scale for the dynamic development of a cef in model asphaltene-heptane—toluene emulsions with water is illustrated in Fig. 19, in which solutions of Arab Heavy (AH) asphaltenes (0.5-1.0% w/w) in 40-50% toluene-in-heptol are emulsified with 30% water, and the cef is monitored as a fimction of time. As should be apparent, there is a characteristic time scale for the cef to rise markedly towards its steady-state value which varies with asphaltene concentration and solvation state of the asphaltenes. The time scale is most rapid at the limit of solubility (40% toluene) and with the higher concentration of asphaltenes (1% versus 0.5%). As concentration is reduced, the time required to reach the near-steady value decreases. Interestingly, it appears to be the same value, regardless of concentration (= 1.2 kV/cm). Also, as the toluene concentration is increased from 40 to 50%, the time scale increases and the long-term value of the cef decreases. This is consistent with a reduced driving force for adsorption of aggre-... 

Generally, asphalt can be fractionated into four important fractions saturates, aromatics, resins, and asphaltenes . The classic definition of fractions of asphalts is based on the solution properties of petroleum residuum in various solvents. A complete fractionation scheme is given in Figure 1 (1) the oil constituents are propane soluble (2) resins are n-pentane soluble but propane insoluble (3) asphaltenes are toluene soluble but n-pentane insoluble and (4) preasphaltenes are insoluble both in n-pentane and toluene. The fractionated part of oil is generally considered to be a combination of saturates and aromatics. The polarity of these four fractions increases from saturates, to aromatics, to resins, to asphdtenes. 

However, this was considered to be only a partial solution since the mass ranges of the asphaltene and preasphaltene of a coal tar [2] were practically identical, despite the evidence that their separation into fractions by solvent solubility (toluene soluble or insoluble) would produce fractions with very different upper mass limits, but with considerable overlap of mass ranges. Accordingly, laser desorption (LD) ionization methods were investigated. 

Example 5.8 Effect of aromatics on asphaltene precipitation Some aromatic solvents have been used to inhibit the asphaltene precipitation in crudes (Cimino et al., 1995), although aromatics such as benzene and toluene in general may not be very effective for asphaltene-precipitation inhibition because high concentrations may be required. When an aromatic such as benzene or toluene is mixed with a crude, the micellization model described in this chapter should be modified. The modification is necessary because as a result of crude/aromatic mixing, the aromatics mainly appear in the solvation shell and reduce the interfacial tension between the asphaltene-liquid core and liquid solution outside the core. Modify the formulation presented earlier for this purpose. 

By contrast, the specific components of crude oil that have been most closely associated with foam behavior reveal radically different chemistry to these simple hydrocarbon chain surfactants. As exemplified by the work of Poindexter et al. [4, 20], those components can be listed as asphaltenes, resins, and waxes. Of these, arguably asphaltenes are the most important. These components are derivatives of polycyclic aromatics, which are distinguished from other crude oil components by insolubility in short-chain n-alkanes such as n-heptane. They are, however, soluble in toluene. Resins are soluble in short-chain alkanes and are therefore usually extracted from crude oil by adsorption onto silica from solution. Both asphaltenes and resins can even each be present in crude oil at concentrations in excess of 15 wt.%. Such extremely high concentrations usually lead to crude oils of high density and high viscosity—so-called heavy crudes (see, e.g., reference [4]). 

Bauget et al. [24] have studied the surface activity of both an asphaltene and a resin at the air-toluene surface. The asphaltene is clearly surface active at that surface. However, the rate of reduction in air-toluene surface tension is extremely slow—even after 1 day equilibrium was not achieved at 20 wt.% asphaltene concentration. After that time, there was clear evidence of formation of a solid skin on the surface from concentrated solutions of the asphaltene. Bauget et al. [24] infer that the rate of adsorption is not diffusion controlled but rather is controlled by the formation of coalesced clusters in the surface. Addition of resin significantly increased the surface tension of asphaltene solutions and ameliorated the formation of the solid skin, implying solubilization of the asphaltene. 

The effect of asphaltene and resin on the surface tension of solvents has also been described by Poindexter and coworkers [20]. Here crude oil was simulated by mixtures of toluene and mineral oil. Volume ratios of mineral oil to toluene were 50 50, 60 40, and 70 30. In all cases, 1-3 wt.% asphaltene decreased the surface tension of the solvent, but by no more than 2 mN m. The decrease was more pronounced on increasing the proportion of mineral oil from 50 to 60 vol.%. In the case of both these 50 and 60 vol.% mineral oil solvents, the addition of asphaltenes increased both foamability by sparging and foam stability. Increasing the asphaltene concentration in both of these cases also reduced the surface tension until it became constant at a supposed critical nanoaggregate concentration, which Mullins [21] argues is analogous to the CMC of ordinary surfactant solutions. However, further increasing the proportion of mineral oil to 70 vol.% precipitated the asphaltene out of solution so that only a modest reduction in surface tension was observed, consistent with the concomitant reduction in activity. Unfortunately, Poindexter and coworkers [20] did not indicate whether their surface tension measurements were equilibrium values. 

The toluene + asphaltenes solutions were made solubilizing 0.5 g of asphaltene in 100 ml of toluene. First the interfadal tension was measured between toluene and water to have a default value and can then compare the results. The interfacial tension measurements of toluene+ asphaltenes solutions were determined in duplicate, so the values of the interfacial tension shown in Table 04 are the averages of duplicates determined. 

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