Viscosity of coal liquids

Sternberg et al. (7) showed that the presence of asphaltenes in coal-derived oils caused a marked increase in the viscosity. This group also showed that these asphaltenes were acid-base complexes and that hydrogen bonding occurs between the acidic and basic components of asphaltenes (9, 10). Recent work (J3, 11) on coal liquefaction bottoms has shown the importance of hydrogen bonding on the viscosity of coal liquids. 

The high viscosity at ambient temperature of coal liquids derived from hydrogenation processes has been related to the asphaltene (toluene-soluble, pentane-insoluble) and preasphaltene (toluene-insoluble, pyridine-soluble) fractions (1-5). Although the effect of preasphaltene concentration on the viscosity of coal liquids is dramatic, the increase caused by asphaltene materials has been attributed to hydrogen-bonding (6) and acid-base salt... 

Influence of Hydrogen Bonding on the Viscosity of Coal Liquids... 

To illustrate these points, we take up in more detail the relationship between the chemical processing history of coal-derived liquids and the properties of the asphaltenes. Variation of their properties is related in turn to the viscosity of the liquids containing the asphaltenes. 

Asphaltene content bears directly on the physical properties of the liquid product. Viscosity is of particular interest because of the importance of this parameter to operation of liquefaction plants and as a measure of the extent of liquefaction. The correlation between asphaltene content and the viscosity of the liquid has been a subject of a number of investigations (23-27). The logarithm of the viscosity ratio, In 7j/rj0 (where i and y0 are the viscosities of the solution and solvent, respectively) was found to be a linear function of concentration when asphaltene was redissolved in the pentane-soluble oil isolated from a coal-derived liquid (24). The slopes of these lines, termed the logarithmic viscosity numbers, are a measure of the contribution to the viscosity of a solution attributable to asphaltene. By comparison of logarithmic viscosity numbers of asphaltenes and their acidic and basic subfractions, it was determined that intermolecular association, which is especially strong between the acid and base subfractions, is responsible for a significant portion of the viscosity of these solutions. 

We gratefully acknowledge the contributions of R. B. LaCount and R. P. Noceti to the study of the viscosity of coal-derived liquids, and S. Friedman for helpful discussions. 

Current evidence indicates that coal-derived asphaltene constituents are a collection of predominantly one-to-four ring condensed aromatic systems that contain basic and nonbasic nitrogen constituents as well as oxygen (acidic and etheric) functions. These functionalities play a role in intramolecular relationships within the asphaltene fraction and also with the other constituents of the coal-derived liquid. This latter effect influences the viscosity of the liquid. Thus, coal-derived asphaltene constituents are an extremely complex solubility class by virtue of their thermal derivation from coal. 

The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the microstmcture and surface properties of these porous media. The interfacial properties of complex liquid agents—mixtures of polymers and surfactants—are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the composition and stmcture of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstmcture and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in synthetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the desired catalytic activity and separation selectivity. 

In other fuel markets, coal liquids can be more competitive. Industrial boilers presently are not amenable to stack gas scrubbing. The same is true of smaller utility plants. In particular, peak load units require a clean, storable liquid fuel as an alternative to natural gas. However, the high viscosity of primary coal liquefaction products is undesirable for many of these applications. Also, their residual sulfur and nitrogen contents may be excessive as emission standards become more stringent. 

The solvent-refined coal (SRC) reactor is normally operated at 850 °F and 2,000 lbf in 2 pressure. Creosote oil is used as solvent In a typical operation, coal particles of approximately 74 jum diameter with a solvent/coal ratio of 2 are used. The slurry is passed through a 2.5-in-i.d., 4-ft-tall reactor. Assuming that coal specific gravity = 1.35, oil specific gravity = 0.9, surface tension of oil under reaction conditions = 5 dyne cm" , viscosity of oil under reaction conditions = 0.7 cP, static slurry height in bed =10 cm, surface tension of liquid (surface tension of water (ow) = 72 dyne cm -... 

One way to get around the problem is to use liquid CO2 instead of water as the transport medium. One advantage of liquid CO2 is that it is less viscous than water. Friction in the pipeline would be lower, so less energy would be needed to transport a given amount of coal. Also there is little if any interaction between powdered coal and liquid carbon dioxide. Because of the lower viscosity and nonreactivity of liquid carbon dioxide (compared to water), slurries can carry more coal. That means additional energy savings and also means that a smaller pipelines could provide the same coal throughput. 

Mineral matter has been known to enhance the conversion of coal to liquid products (1,2,3). Addition of pyrite, pyrrhotite, and liquefaction residues ( ) to coal has been shown to affect the coal conversion yields and the viscosity of the products (5.). Of all the minerals present in coal, pyrite (and marcasite) are the most important for coal utilization, especially in direct coal liquefaction (1,5). However, one has to remember that under coal liquefaction conditions pyrite rapidly transforms to a nonstoichiometric iron sulfide Fe S(0 x 0.125). It is noted that the sulfur formed as a result of the decomposition of pyrite is able to extract hydrogen from poor donor solvents. The stoichiometry of the pyrrhotite formed from FeSp depends strongly on the partial pressure of H S. 

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