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... 

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 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. 

In the close-coupled scheme, all produts and minerals are sent to the catalytic process. Heavy deposits of carbonaceous substances are inevitable. Heavy products of preasphaltene should be converted as much as possible in the primary stage. The detailed characterization of all products, including orgnometallics, suggests ways to convert or stabilize the poison precursors in the primary stage. 

Preasphaltene, a heavy liquid product, is usually rich in polar functional groups, as they are sometimes called preasphaltol (76). Such groups still contain cations that limit their solubility and their removal from the catalyst surface. 

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. 

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