Short contact time direct coal liquefaction using a novel batch reactor. Quarterly report, May 15, 1995--September 15, 1995 Page: 4 of 27
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regardless of whether the system is in hydrogen or nitrogen. However, the Fixed Carbon
increases at a very rapid rate above 408*C (Figure 5). This is indication of the retrograde
process which results in low liquid yields and the formation of tar and coke.
The induction period has now been shown to be a pseudo induction period and is
due to the simultaneous occurrence of two processes, a very rapid extraction and a slower
liquefaction of the coal structure. As the temperature increases the induction period
steadily becomes less pronounced as the rate of break down of the coal structure increases
and becomes closer in reaction rate to the extraction step itself. That the induction period
is not due to the build up of free radicals, as we first thought, is shown by ESR data
previously obtained which shows that the radical concentration actually decreases somewhat
during the liquefaction process.
2.1.2. Catalyzed Liquefactions
Figure 6 shows the conversion vs time curves for coal liquefaction at 390*C in the
presence of about 0.9% sulfided molybdenum naphthenate in tetralin in 1000 psi hydrogen
and without catalyst in tetralin and nitrogen. The rapid initial extraction is again observed
in the first minute in both the catalyzed and thermal runs, followed by an induction period.
However, the subsequent conversion is much faster for the catalyzed than the thermal
liquefaction. More importantly, the retrograde process is very significantly reduced in the
presence of catalyst and hydrogen as is shown by the Fixed Carbon content in the residue
as a function of time (Figure 7).
When the catalyzed liquefaction is run at higher temperature, the Fixed Carbon of
the residue is further reduced, indicating that the precursors of the retrograde processes are
being hydrogenated and stabilized during the catalyzed liquefaction (Figure 8). Liquid yields
are thereby significantly improved.
2.1.3. Effect of Solvent on Liquefaction Kinetics
It has been known for a long time that liquefaction yields are improved if a strong
hydrogen donor solvent is used. To demonstrate this in the SCTBR, liquefactions of Illinois
#6 coal were run in three solvents of varying hydrogen ability at two temperatures. The
results are shown in Table 1. These data suggest that a very strong donor solvent such as
tetrahydroquinoline gives higher conversion at a given reaction time than tetralin. Further,
less hydrogen is derived from the gas phase. A weak donor solvent such as methyl
naphthalene requires almost all of its hydrogen from externally supplied hydrogen.
2.1.4. Is Coal Liquefaction Mass Transfer Controlled?
The Argonne coal samples used in most of this work are relatively fine particles (-100
mesh). However to determine whether the liquefaction rates were limited by mass transfer,
another Illinois #6 obtained from Amoco which was much finer was tested. The mesh
analysis of the two coals is shown in Figure 9. Figure 10 shows a conversion vs time curve
at 394*C for the finer coal. Little if any difference can be seen in the conversion rates of
the two coals, suggesting that if the coal is finer than 100 mesh mass transport will not be3
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Klein, M. T.; Calkins, W. H. & Huang, He. Short contact time direct coal liquefaction using a novel batch reactor. Quarterly report, May 15, 1995--September 15, 1995, report, October 5, 1995; Newark, Delaware. (https://digital.library.unt.edu/ark:/67531/metadc672607/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.