Two-stage coal liquefaction is improved by separating a light fraction from the first (dissolving) stage effluent, hydrogenating that fraction and reblending the hydrogenated light fraction with the material passed from the first stage to the second stage reactor operating at higher temperature than the first stage.

FIELD OF THE INVENTION 
The invention concerns improvement in solvent refining of coal whereby 
components of coal suitable for fuel are extracted from comminuted coal by 
a solvent and recovered as a low melting point mixture of reduced sulfur 
and mineral matter content adapted to use as fuel in conventional 
furnaces. In the type of operation to which the invention is directed, the 
solvent is derived from the product extract and applied to the raw coal 
feed. 
BACKGROUND OF THE INVENTION 
The present emphasis on the conversion of coal to substitute solid and 
liquid fuels has led to several alternative processes which are now being 
considered. The end use of the resultant converted coal will primarily 
determine the degree of conversion that must be accomplished and the 
quality of the desired product. The optimal use of the coal will depend on 
the specific application. 
Among the many processes presently being considered is the solvent refining 
of coal (SRC) in which coal is treated at an elevated temperature in the 
presence of a hydrogen donor solvent and hydrogen gas in order to remove 
the mineral matter, lower the sulfur content of the coal, and to convert 
it into a low melting solid which can be solubilized in simple organic 
solvents. This SRC can also be upgraded through catalytic hydrogenation to 
produce a liquid of higher quality. These two processes are of concern to 
the present invention. 
Little is known at present as to the exact mechanisms by which the coal is 
transformed into soluble form, or of the detailed chemical structure of 
the soluble product or even the parent coal. It is known that many coals 
are easily solubilized and for others solubilization is more difficult. 
Some correlations have been made between the rank of the coal and ease of 
solubilization and product yield. A somewhat better correlation has been 
found with the petrography of the coal. Little is known about the 
relationships to product quality. 
The initially dissolved coal (SRC) may have utility as a substitute clean 
fuel or boiler fuel; however, for substitute fuels of higher quality, 
specifications on viscosity, melting point, ash, hydrogen, and sulfur 
contents are much more stringent. Attempts to meet these specifications by 
operating the SRC process more severely have met with many difficulties 
such as low liquid yields, high hydrogen consumption, difficulty of 
separating unreached residue, and excessive char formation, which often 
completely plugs process transfer lines and reactors. 
Alternative methods of improving specifications through catalytic 
hydrogenation are also difficult. The problems which arise are threefold. 
(1) SRC components are susceptible to further condensation and may deposit 
as coke on catalysts used for their conversion, (2) they can also foul the 
catalysts by physical blockage as their size approaches the pore size of 
conventional catalysts, and (3) they may contain metal contaminants, and 
their highly polar nature (particularly nitrogenous and sulfur compounds) 
can lead to selective chemisorption, and thus poison the catalysts. 
The precise chemical nature of the SRC is still unknown; generally its 
composition is discussed in terms of solubility. Several classifications 
are commonly used. These include oils which are hexane or pentane soluble, 
asphaltenes which are benzene soluble, and pyridine soluble-benzene 
insoluble materials. Of these the asphaltenes and pyridine soluble-benzene 
insoluble materials are believed to be responsible for high viscosity, 
solvent incompatability, and processing difficulties. Little is known 
about the pyridine soluble-benzene insoluble materials. These have been 
referred to as "pre-asphaltenes" which implies that asphaltenes are 
derived from them; however, this has yet to be established. 
More information is available on the nature of asphaltenes. It is common 
experience that coal liquids contain large quantities of materials known 
as asphaltenes. In fact, it has even been suggested that the formation of 
asphaltenes is a necessary step in the liquefaction of coal. 
The term asphaltene is a rather nebulous and all-inclusive classification 
of organic materials for which a detailed chemical and physical 
identification is quite difficult, and has not yet been accomplished. 
This classification generally refers to high molecular weight compounds, 
boiling above 650.degree. F., which are soluble in benzene and insoluble 
in a light paraffinic hydrocarbon (e.g., pentane). Usually no distinction 
is made regarding polarity, as the term has been used customarily in the 
characterization of heavy petroleum fractions (resids, etc.) where the 
amount of highly polar materials is small. However, in coal liquids this 
may not necessarily be the case due to the high degree of functionality of 
coal itself. Thus, coal liquids of low molecular weight may still be 
"asphaltenes". There is considerable variation in the molecular weight of 
solubilized coals which arises from differences in the parent coals, or 
different solvent or solvent-reactant systems at the same temperature of 
reaction. This could well be related to colloidal properties of coal 
liquids. It is well documented that asphaltenes found in heavy petroleum 
fractions are colloidal in nature. 
Some comments on the chemical nature of coal asphaltenes have recently been 
made. Asphaltenes from Synthoil Process liquids were separated into a 
basic fraction (containing oxygen only as ether or ring oxygen and basic 
nitrogen as in pyridine) and an acidic fraction (containing phenolic OH 
and nitrogen as in pyrrole). The two fractions were found to have very 
different properties. The basic fraction could be hydrotreated only with 
difficulty, while the acid fraction underwent facile hydrotreating. This 
is consistent with reported data on the influence of nitrogen heterocycles 
on conventional hydroprocessing. 
Based on these results an acid-base pair structure for asphaltenes was 
proposed and this structure was extrapolated to that of coal itself. This 
structure is quite different from the more amphoteric nature of coal which 
has been proposed previously. 
Mechanisms have been proposed for the noncatalyzed formation of asphaltenes 
from coal. In this work it was concluded that asphaltenes were a necessary 
product of coal liquefaction and that oils were derived from asphaltenes. 
The more polar pyridine soluble materials were not investigated and were 
assumed to be equivalent to unreacted coal. The maximum yield of 
asphaltenes was found, however, to be a function of the conditions of coal 
conversion; hydrogen donor solvents greatly reduced the propensity for 
formation of asphaltenes at low conversion. In addition, it was not 
determined whether the asphaltene fractions resulting from different 
conditions were of the same chemical and/or physical nature. Thus, 
asphaltenes may be inherent constituents of coal products or they could 
well be the result of either thermal or catalytic transformations of more 
polar materials. 
In considering what may be involved in the formation of asphaltenes during 
coal solubilization or conversion, it may be instructive to consider what 
is known of coal structure. Coal is a rather complicated network of 
polymeric organic species, the bulk of which is porous in the natural 
form; the pore system varies from coal to coal. Depending upon the 
specific nature of the porous structure of each coal, its chemical 
constituents, and the reaction conditions, the rate of diffusion and mass 
transport of organic molecules through the pores could have a strong 
effect on the rates of dissolution, hydrogen transfer, and hydrogenation 
and hydrocracking reactions, and thus on the ultimate yield of soluble 
product. 
As the rank of coal becomes higher, an increasing number of colloidal size 
aggregates (20-50 A) can be observed by X-ray scattering and diffraction. 
If, in the early stages of the dissolution of coal these colloidal 
aggregates dissociate to some degree and go into solution, the molecular 
weight of the lowest unit appears to be consistent with the lowest 
molecular weights observed in solublized coals (.about.500 MW). This 
comparison may be coincidental, however. Unfortunately, in order to 
dissolve coal it is generally found that temperatures in excess of 
300.degree. C. are necessary. It is also known that coal begins to 
pyrolize and evolve volatile matter at temperatures as low as 250.degree. 
C. (depending on rank), and by 350.degree. C. considerable material has 
evolved. This strongly suggests that extensive internal rearrangement of 
the coal occurs during the dissolution process. Rearrangement can include 
hydrogen migration to produce highly condensed aromatic rings as well as 
further association of small colloidal aggregates or condensation of 
reactive species. Major physical changes in the pore system of the solid 
coal have also been reported. 
This rearrangement could possibly be responsible for some of the very high 
molecular weights (.about.3000 MW) observed with some solvents. No 
detailed relationships of solvent type and/or reaction condition to the 
molecular weight distribution of solubilized coal has yet been 
established. Similarly, the possibility of reversible molecular weight 
changes, due to recondensation causing increased molecular weights at 
various temperatures, has not been investigated thoroughly. 
An alternative route to high molecular weight is through the catalytic 
influence of inorganic coal minerals which are present in the processing 
of coal. It is known that some coals are more reactive than others, 
producing higher yields of liquid products at shorter residence times. It 
is believed that this is due to the fact that the initial coal products 
are reactive and condense to char unless proper reaction conditions are 
established. This further condensation could well be a catalytic 
phenomenon induced by intrinsic coal minerals. 
Another more subtle consequence of certain inorganic constituents is their 
influence on the physical properties of pyrolytic coal chars, and thus on 
the diffusional properties imposed on reactive intermediates. The volume 
of char has been observed to vary by a factor of four or more, with little 
change in weight, by varying the type of inorganic contaminants in a given 
bituminous coking coal. The pore system of the resultant chars must be 
vastly different and changes of this type magnitude in the physical 
structure of the coal or char could greatly influence mass transport of 
intermediates produced within the pore system. Mass transfer limitation 
during the pyrolysis and hydrogasification of some coals at high 
temperatures has recently been established. This study showed that for 
some coals, reactive primary products are formed which can recombine to 
produce char if the conditions are not properly adjusted. The criticality 
was found to be the rate of diffusion of the reactive species out of the 
coal relative to its rate of conversion to char. 
At lower temperatures, the rates of reaction are, of course, slower and 
thus less susceptible to mass transport limitations. However, the 
imposition of a liquid phase, commonly used in liquefaction processes, may 
greatly enhance diffusional restrictions. Recent model studies conducted 
in aqueous systems, have shown that restriction of diffusion through 
porous structures with pore radii ranging from 45 A to 300 A for even 
relatively small solute molecules is very significant. 
At the present stage of the art, the accumulated information is largely 
empirical, with little basis for sound extrapolation to predict detailed 
nature of solvent and processing conditions for optimum yield and quality 
of solvent refined coal. It is recognized that the poorly understood 
asphaltenes are probable sources of many of the problems encountered, e.g. 
formation of char at processing conditions conducive to efficient 
separation of mineral matter (ash) and sulfur from desired product at high 
yield. 
In the process of converting coal to a low sulfur, low melting solid by use 
of recycled product fractions as solvent, several reaction steps occur. 
Generally coal is admixed with a suitable solvent recycle stream and 
hydrogen and the slurry is passed through a preheater to raise the 
reactants to a desired reaction temperature. For bituminous coal, the coal 
is substantially dissolved by the time it exits the preheater. 
Sub-bituminous coals can be dissolved but care must be exercised not to 
raise the temperature too high and thus promote charring. 
The products exiting from the preheater are then transferred to a larger 
backmixed reactor where further conversion takes place to lower the 
heteroatom content of the dissolved coal to specification sulfur content 
and melting point. The geometry of this reactor is such that the linear 
flow rate through it is not sufficient to discharge a substantial quantity 
of particulate matter of a desired size. Thus the reactor volume becomes 
filled (at steady state) up to about 40 vol % by solids which are produced 
from the coal. These solids have been shown to be catalytic for the 
removal of heteroatoms and the introduction of hydrogen into the coal 
products and solvent. The products exiting the reactor are initially 
separated by flash distillation, which depressurizes the stream and 
removes gases and light organic liquids. The products are further 
separated (filtration, centrifugation, solvent precipitation, etc.) and 
the filtrate is distilled to recover solvent range material (for recycle) 
and the final product SRC. 
SUMMARY OF THE INVENTION 
We have found that in two-stage coal liquefaction schemes, various factors 
in solvent composition are important. Advantage can be realized by their 
proper use and control. 
The extent of solvent hydrogenation affects SRC solubility in solvents. 
Thus, hydrogen-poor solvents are better physical solvents, especially in 
the first stage. Phenols having 10 or more carbons can be hydrogen donors; 
phenols in solvents can condense with SRC's, especially in the first 
stage, but the condensation can be reversed and the phenols can be 
recovered again, especially in the second stage. The rate of solvent 
rehydrogenation may be the controlling factor in the rate at which coal 
can be processed (coal residence time in system). 
According to the invention, hydrogenation of a portion of the solvent 
between the stages takes advantage of these factors as follows. After the 
slurry leaves the first-stage reactor, the gases and lower-boiling 
materials up to and including about C.sub.14 compounds (.about.275.degree. 
C.) are flashed off and passed through a catalytic hydrogenator. In this 
step, naphthalene and its homologs are converted to tetralin and its 
homologs, and phenols having a single aromatic ring are destroyed. This 
stream is then sent to the second stage along with the majority of the 
solvent that had not been flashed off. Thus, the solvent to the second 
stage has reduced light phenols, increased hydro-aromatics, and still 
contains the heavier phenols that are hydrogen donors. There are thus two 
advantages. First, the solvent is an excellent donor, and second, the 
solvent is less phenolic and so the SRC will be less phenolic, will 
consume less hydrogen in its upgrading, and will be more compatible with 
highly-upgraded or petroleum stocks. An important point is that the 
solvent initially entering the second stage has sufficient donor ability 
to achieve SRC upgrading by hydrogen transfer reactions and does not have 
to be regenerated in the second stage. Thus, the residence time in the 
second stage can be shorter. The hydrogenated solvent is needed only in 
the second stage and hydrogenation is done just before this stage. On exit 
from the second stage, the solvent can be considerably depleted in 
hydrogen so long as depletion is not so severe that char formation occurs 
near the end of the second stage. This hydrogen-poor solvent is suitable 
for recycle to the first stage where hydrogen donor capacity requirements 
are minimal. Furthermore, this solvent is more aromatic and phenolic 
(phenols are produced in SRC upgrading, partly by reversal of the 
condensation that occurred in the first stage), and so a better physical 
solvent for initially-solubilized coal products formed in the first stage. 
This scheme can be coupled with several variations of the procedure for 
solids removal. An important role of the coal mineral matter in the SRC 
process is catalysis of solvent rehydrogenation. This is not required 
according to the invention. Therefore, solids can be removed entirely 
between the stages by any of the known techniques (centrifugation, 
settling, filtration, anti-solvent precipitation, etc.). Optionally, the 
flash to remove light material for catalytic hydrogenation can be done 
before or after the separation. This can help control factors important to 
the optimal operation of the various separation techniques (percent 
solids, viscosity, total slurry volume, solvent polarity, etc.). Another 
option, again depending upon the separation technique used, is to return 
the rehydrogenated solvent to the system before the solids separation step 
.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
The process of this invention can even be conducted without the atmosphere 
of hydrogen pressure normally used in processes for solvent refining of 
coal with a solvent derived at least in part from the product. For that 
reason, solid residues of ash components, unreacted coal, iron sulfides, 
coke and the like may be separated at any desired stage of the process as 
will appear from the detailed discussion below. This added flexibility is 
achieved in a process sequence affording increased efficiency in 
utilization of hydrogen and increased throughput (or decreased reactor 
size). In processes of the prior art, the solids are retained in the 
reaction mixture for catalytic effect in hydrogenation of chemical 
species, such as naphthalene, which become hydrogen donors, e.g. tetralin, 
on hydrogenation to suppress formation of char by transfer of hydrogen to 
polymerizable fragments formed in dissolution of coal. 
The flow sheet of FIG. 1 can be considered with reference to solvent 
refining of Monterey Mine, Illinois #6, a typical bituminous coal. 
Inspection data on that coal are shown in Table I. 
TABLE I 
______________________________________ 
Name of Coal Illinois #6 
Mine 
Location 
State Illinois 
County Macoupin 
Seam 6 
Name of Mine Monterey 
Proximate 
Analysis* 
% Moisture (as rec.) 
12.81 
% Ash (as rec.) 9.43 
% Volatile Matter 
41.73 
% Fixed Carbon 47.45 
BTU (as rec.) 10930. 
BTU 12536. 
Free Swelling Index 
Ultimate 
Analysis* 
% C 69.72 
% H 4.98 
% O** 8.20 
% N 1.08 
% S (total) 5.14 
% S (pyritic) 2.26 
% S (organic) 2.70 
% S (sulfate) 0.18 
% Cl 0.06 
% Ash 10.82 
______________________________________ 
*All analyses are given on a dry weight basis unless otherwise stated. 
**By difference 
Petrographic Analysis 
Pseu- Sem- Mas- Granu- 
Vit- do- i- sive lar 
ri- vitri- Exi- Fusi- 
fusi- 
Micri- 
Micri- 
Resi- To- 
nite nite nite nite nite nite nite nite tal 
______________________________________ 
89 3 1 1 1 2 2 1 100 
Mean Maximum Reflectance in Oil (564 nm): 0.47% 
______________________________________ 
For processing in accordance with the invention, the coal of Table I will 
be ground to pass 100-200 mesh standard screen, maximum particle size of 
about 0.15-0.07 mm. The comminuted coal will be admitted to the process at 
line 10 for admixture with approximately 1-6 parts by weight of a 
hydrogen-poor solvent derived in the process and recycled by line 11. The 
mixture passes to a first stage low temperature dissolver 12 where it is 
maintained at a temperature of about 400.degree.-460.degree. C. for a 
residence time of about 1-10 minutes. The solvent at this first stage will 
be rich in potent solvents such as polycyclic aromatics, phenols and the 
like which rapidly dissolve soluble components of the coal. In addition, 
other transformations will take place, such as alkylation of phenols by 
coal fragments. The slurry from first stage dissolver 12 will be passed to 
flash separator 13 where the pressure is reduced to a level to vaporize 
components up to and including hydrocarbons having 14 carbon atoms, i.e. 
atmospheric boiling points of about 275.degree. C. and lower. Suitable 
conditions for flash separator 12 may be 150-450 pounds per square inch 
gauge (psig) and 350.degree.-460.degree. C. 
Overhead from flash separator 13 is conducted to catalytic converter 14 
where it is admixed with hydrogen and contacted with a hydrogenation 
catalyst such as cobalt/molybdenum on alumina under conditions to remove 
single ring phenols by conversion to hydrocarbons and to generate hydrogen 
donors by hydrogenation of polycyclics, e.g. naphthalene to tetralin. 
Suitable conditions are 5-50 standard cubic feet of hydrogen per pound of 
distillate from flash separator 13, pressure of 500-2500 psig and 
temperature of 260.degree.14 400.degree. C. The product is light solvent 
rich in hydrogen as hydrogen donor compounds and depleted in monocyclic 
phenols which is passed by line 15 for use in the process according to a 
manner presently to be described. 
The liquid fraction from flash separator 13 is transferred to second stage 
reactor 16 which operates at a temperature equal to above that of 
dissolver 12, say 400.degree.-480.degree. C. and 500-3000 psig. An 
alternative to direct transfer which can offer significant advantage is to 
separate solids from the dissolved coal between stages in solids separator 
17. Because further solids separations are feasible, the operation of 
separator 17 may be relatively inefficient, such as a simple settling 
chamber of low residence time, say 15-300 seconds. Depending on factors 
important to optimal operation of the various separation techniques 
(percent solids, viscosity, total slurry volume, solvent polarity, etc.), 
the flash separation may be conducted in flash separator 18 subsequent to 
solids separation instead of, or in addition to action of flash separator 
13. On like considerations, hydrogenated light solvent from reactor 14 may 
be added in whole or part to the slurry entering solids separator 17, as 
indicated by broken line 19. 
Depending on efficiency of separation in separator 17, if used, solids may 
be withdrawn from the system by line 20, or a slurry may be taken off to 
be discharged as such at line 21 or settled (or centrifuged or filtered) 
in separator 22 with return of clarified liquid to the inlet of first 
stage dissolver 12. 
The effluent of first stage dissolver 12 from which a light fraction has 
been removed by flash separator 13 or 18 and containing more or less 
solids, depending whether solids separator 17 is employed and at what 
efficiency, will now be introduced to second stage reactor 16 where it is 
admixed with hydrogen rich solvent from line 15. In reactor 16, the 
process of producing solvent refined coal is completed by conventional 
reactions, but under conditions superior to those previously proposed. 
Reactor 16 may be maintained at 400.degree.-480.degree. C. and 500-3000 
psig of H.sub.2 for a residence time of about 5-120 minutes. To the extent 
coal fragments have not previously equilibrated as to hydrogen content, 
that reaction will now be completed in the presence of hydrogen 
"shuttling" agents like polycyclic phenols, naphthalenes, anthracenes and 
substitution products thereof which accept protons from hydrogen rich 
fragments and confer the same on hydrogen poor fragments. Fragments which 
have alkylated phenols at an earlier stage will reappear by dealkylation 
under an environment which inhibits polymerization of these potential char 
precursors because of the concentration of hydrogen donors. 
The hydrogen donors of relatively low molecular weight derived from 
hydrogenation in reactor 14 will function in reactor 16 to supply labile 
hydrogen where needed to stabilize SRC components and are thus themselves 
converted to the hydrogen-poor counterparts which have the high solvent 
power needed in the first stage low temperature dissolver 12. Those 
solvent species together with the high solvent power monocyclic phenols 
derived from the coal constitute important components of recycle solvent 
taken off the effluent of reactor 16 in separator 23 which also has the 
function of removing any solids present for discharge by line 24. 
The recycle solvent will be a fraction from the total effluent adequate in 
amount to satisfy needs of dissolver 12 and boiling generally below about 
500.degree. C. Before transfer to line 11, the recycle solvent is 
stabilized by removal of normally gaseous components boiling below about 
35.degree.-40.degree. C. which are discharged by a conduit not shown for 
use as fuel, chemical feed stock and the like, all in manner conventional 
in the art. 
As will be apparent to those skilled in this art, the treatment parameters 
will vary depending on nature of the coal, desired end use of the SRC, 
means available for transport of SRC and the like. In general, the 
recycled solvent will have a boiling range above about 30.degree. C. and 
not higher than 500.degree. C., preferably 180.degree. C. to about 
460.degree. C. and will be supplied at a weight ratio to coal between 1 
and 6. Conditions in the first stage dissolver will be temperatures of 
about 400.degree. C. to about 460.degree. C. and pressures between 500 and 
3000 psig. Flash separator 13 or 18 will be operated at temperature and 
pressure to vaporize material boiling below about 300.degree. C., 
preferably below about 275.degree. C., it being recognized that flash 
distillation is relatively inefficient, taking overhead some portion of 
components boiling above the "cut point" and leaving some portion of the 
lighter components dissolved in the liquid phase. The second stage 
generally operates at temperatures between 400.degree. C. and 480.degree. 
C., preferably between about 420.degree. C. and 460.degree. C. under a 
pressure of say 500 to 3000 psig. 
In practicing preferred embodiments of the invention, there is little or no 
mineral solids content of the material in reactor 16 to catalyze 
hydrogenation of components which could thereupon function as hydrogen 
donors. Hydrogen, if present, is therefore partly a diluent occupying 
reactor space. Although use of diluents is considered to be within the 
scope of the invention, it is therefore within the scope of this invention 
to operate without addition of elemental hydrogen. 
One reason for removing solids in the two-stage process described above is 
to avoid their acting as surfaces and possibly catalysts for char 
formation. According to the present invention, this effect is reduced 
because the solvent is hydrogen-rich. Therefore, solids separator 17 is 
run at an inexpensive reduced efficiency; or, optionally, it may act on 
only a portion of the stream, the remainder of the solids being removed in 
separator 23. Thus, more of the undissolved coal, which is a portion of 
the solids, might be dissolved in reactor 16. A solids-rich slurry 
withdrawn from separator 13 can be recycled to the first stage reactor 
where additional dissolution can take place. A portion of this slurry can 
be removed in order to remove solids from the system, as must be 
accomplished, or there can be another separator 22 for further solids 
removal. Only separator 23 need be highly efficient to produce an ash-free 
SRC product. This separator is the easiest to run at high efficiency 
because the solids content, solvent viscosity, and SRC polarity and 
molecular weight are all lowest at this point. The slurry optionally 
removed after separator 13, and the solids removed from any and all 
separators, can be burned for process heat or used in hydrogen generation. 
External catalytic rehydrogenation of process solvent is known, but not 
between stages in a two-stage process and treating only the lower boiling 
range. The concept of using cheap, inefficient separators for most of the 
solids removal, the optional addition of rehydrogenated recycle solvent 
before the solids separation (which, for instance, would improve the 
operation of a settler by reducing solvent viscosity), and the fact that 
C.sub.10 + phenols may be hydrogen donors are all unique to the present 
invention. 
The invention thus improves coal liquefaction by alleviating the problems 
associated with hydrogen depletion of solvents, increasing the efficiency 
of hydrogen utilization, increasing throughput (or decreasing second-stage 
reactor size), improving complete solids separation where required, and 
allowing inefficient solids separation where appropriate.