Asphaltenes preasphaltenes

The heavy-end portions (usually called heavy fractions) of bitumen (e.g. asphaltenes, preasphaltenes) can exist both in a random oriented particle aggregate form or in an ordered micelle form, peptized with resin molecules (16.17). In their natural state, asphaltenes exists in an oil-external (Winsor s terminology) or reversed micelle. The polar groups are oriented toward the center, which can be water, silica (or clay), or metals (V, Ni, Fe, etc.). The driving force of the polar groups... 

The recovery, regeneration, and repeated reuse of the active catalyst are of prime importance in substantially reducing the overall cost of coal liquefaction. The used catalysts usually remain in the bottoms products, which consist of nondistillable asphaltenes, preasphaltenes, unreacted coal, and minerals. The asphaltenes and preasphaltenes can be recycled with the catalyst in bottoms recycle processes. However, unreacted coal and minerals, if present in the recycle, dilute the catalyst and limit the amount of allowable bottoms recycle because they unnecessarily increase the slurry viscosity and corrosion problems. Hence, these useless components should be removed or at least reduced in concentration. If the catalyst is deactivated, reactivation becomes necessary before reuse. Thus, the design of means for catalyst regeneration and recycle is necessary for an effective coal liquefaction process. Several approaches to achieving these goals are discussed below. 

The percent of alkali-soluble acids varies somewhat, but not greatly, among asphaltenes, preasphaltenes, and total H-coal vacuum bottoms, but the percent of precipitable bases varies markedly. Assuming that phosphotungstic... 

Samples of each of the coal derived materials were reacted separately in the presence of several catalysts in a 70 ml batch autoclave using a 1 1 slurry of tetralinrmaterial at 425 C with an initial hydrogen pressure of 6 MPa for 1 hour at reaction temperature. The products from these reactions were separated into oils, asphaltenes, preasphaltene and THF insolubles. 

For general characterization of nitrogen compounds in a coal liquid sample, carbon, hydrogen, and other elements can be determined by conventional elemental analysis for the separated distillate, resins, asphaltenes, preasphaltenes, aromatics, n-paraffins, and nonparaffins. MS is useful for the analysis of saturated fractions [22] and carbon number distributions [23], especially for aromatics. Low-voltage MS (LV-MS) (10 eV) will only ionize aromatics instead of paraffins, since aromatics have lower ionization potentials. 

It is essential to state that the heavy fractions such as asphaltene and preasphaltene do contain large numbers of polar molecules (23.24). These polar molecules behave exactly as surfactants or amphiphiles (asphaltene usually contains a long-chain substituent (25)). We again have to emphasize that it is almost not possible to create a colloidal micelle from pure hydrocarbon and water without any surfactant. Hence, we conclude to say that asphaltene or asphaltene-like molecules (as-phaltics) will participate in a manner according to membrane-mimetic chemistry. 

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. 

Fig. 6. Influence of solvent/coal ratio on the liquefaction yields with different donors. Oil + asphaltene (4HF1), (8HAn) oil A (4HF1), O (8HAn) (3) gas (C) preasphaltene ( ) residue.

Fig. 11. Two-stage liquefaction using autoclave for both stages. J Gas toil L asphaltene [i ] preasphaltene residue, (a) First stage 350°C-20 min (N2 No catalyst) (b) a + 380°C-20 min (H2, FeS2 catalyst) (c) a + 380°C-40 min (H2, FeS2 catalyst) (d) a + 400°C-20 min (H2, FeS2 catalyst) (e) a + 400°C-40 min (H2, FeS2 catalyst) (f) single stage 400°C-40 min (H2, FeS2 catalyst).

The chemistry of coal liquefaction is not very well understood, even after more than two decades of research into the kinetics and mechanism of the process. There have been a number of models for conversion proposed, most of them focused on the several liquefaction products, including preasphaltenes, asphaltenes, oils, and gases. A survey of some of the models has been presented (1 ), and a common feature among them is the multiplicity of paths connecting all of the components. 

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. 

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