The role of recycle oil in direct coal liquefaction process development Page: 4 of 7
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tended to improve coal conversion and reduce SRC sulfur, but increased gas make
at the expense of recycle solvent and SRC yield.7
In addition, because the distillate yield in the SCR process was low (typically,
less then 5 wt % MAF coal), the replacement rate of the recycle solvent was low,
and changes in solvent composition over time were difficult to assess. Because
it was run at relatively constant conditions for long periods of time, and
because of its size, the 6 TPD Wilsonville pilot plant became an excellent source
of coal liquefaction data and samples for assessing the longer term effects of
coal liquefaction on recycle oil quality. In 1977 and 1978, we obtained three
relatively large and representative samples of the recycle distillate from
Wilsonville for use in bench-scale liquefaction research. Some of the solvents
were catalytically hydrogenated in a bench scale unit. These samples were the
basis for an extensive characterization effort,8 which included 1H-NMR and 19F-
NMR (for phenol determination following derivatization), GC/MS, FIMS, reverse
phase liquid chromatography, and empirical tests of solvent quality.
As the distillate recycle solvent in the SRC-I process evolved (Tables 2 and 3),
it increased in total hydrogen content, but was lower in molecular weight, more
aliphatic, and more phenolic. The practical consequence, as indicated by the
microautoclave solvent quality tests, was that it lost hydrogen donating ability.
The underlying structural changes were revealed by the NMR and FIMS data. The
overall decrease in aromaticity was totally at the expense of the condensed
aromatic structures; uncondensed aromatic hydrogen actually increased. The
increase in aliphatic hydrogen appeared in both cyclic and aliphatic regions
initially, but as the solvent further evolved, the cyclic aliphatic hydrogen
decreased. The apparent loss of hydrogen donor activity under kinetic control
(KIN test) was associated with the decrease in condensed aromatic hydrogen. The
decrease in conversions at the EQ conditions, designed to measure donor hydrogen
content, was associated with the ratio of cyclic to alkyl aliphatic hydrogen.
FIMS analysis (Figures 1 and 2) showed that catalytic hydrogenation of the more
aromatic solvent (8/77 sample) converted aromatics to hydroaromatics and improved
solvent quality. Although solvent evolution increased hydrogen content (and
alkyl tetralin) by an amount similar to catalytic hydrogenation, it decreased the
concentration the aromatics and the corresponding hydroaromatics.
SELECTIVE RECYCLE AS AN IMPROVED LIQUEFACTION OPTION
The research on the evolution of the SRC distillate solvent clearly indicated the
importance of higher molecular weight hydroaromatics as hydrogen donor solvent
components. However, the low distillate yield in the SRC process provided few
options for improving the situation, leading to the conclusion that recycle of
vacuum bottoms, or a vacuum-bottoms component, would be necessary to maintain
solvent quality8. This concept was tested by separating the SRC into "light" and
"heavy" components and using the light SRC (LSRC) as a component of the recycle
solvent in bench scale and microautoclave liquefaction experiments.9 In the
microautoclave experiments, the LSRC was added to the Wilsonville solvent sample
during 4/78. The addition of LSRC improved solvent quality at the "Kinetic"
conditions (Table 4), but decreased conversion at the "Equilibrium" conditions
indicating that it contained active hydrogen donors, but not in large concentra-
tion. The improvement seen at the EQ conditions under hydrogen pressure were
somewhat surprising and the degree of improvement was remarkable. These results
clearly indicated that this non-distillate oil was capable of facilitating gas
phase hydrogen utilization for coal conversion in the absence of an added
catalyst.
THE ROLE OF PARAFFINS IN SOLVENT QUALITY
Not all solvent quality effects can be ascribed to the activity and concentration
of hydrogen donors. There has been a tendency to think in terms of "average"
structures in describing coal and coal products. However, coal liquids are much
more heterogeneous than an average structure might suggest. One feature of
solvent quality that the FIMS data failed to reveal was the concentration of
straight-chain and branched paraffinic components in recycle oils; FIMS is
relatively insensitive to paraffins. In one case, the recycle distillate from
a Wilsonville ITSL run with subbituminous coal produced a 47% wax yield upon
ketone dewaxing; 12 wt% of the recycle distillate consisted of n-paraffins.0
Simple physical removal of this wax fraction increased the solvent quality in the
EQ microautoclave test from 71% to 87%.
SINGLE STAGE CATALYTIC LIQUEFACTION
The H-Coal process employs a single ebullated-bed reactor to convert coal to
distillate products. In PDU and pilot plant development, a relatively high
reaction temperature (825-840 *F) and resid recycle were used to achieve high
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Burke, F. P. The role of recycle oil in direct coal liquefaction process development, report, August 1, 1995; United States. (https://digital.library.unt.edu/ark:/67531/metadc618414/m1/4/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.