Note: Descriptions are shown in the official language in which they were submitted.
~L~3~6~
01 -1-
TWO-STAGE COAL LIQUEFACTION PROCESS
05 BACKGROUND OF THE INVENTION
The present invention relates to processes for
the liquefaction of coal. In particular, it relates to
two-stage processes for the hydrothermal and hydrocatalytic
liquefaction of subdivided coal in a solvent slurry.
The production of liquid products by the high
temperature and pressure hydrogenation of a coal and sol-
vent slurry in the presence of a hydrogenation catalyst is
well known. The resulting coal liquid~ however, has a
high average molecular weight and a high viscosity. These
properties present considerable dificulty in any needed
subsequent processing, such as, fines removal and/or cata-
lytic hydrocracking. Rosenthal and Dahlberg found (U.S.
Patent 4,330,391) a two-stage process for the liquefaction
of coal in which a subdivided coal is substantially dis-
solved in a solvent in the presence of hydrogen at 750F
to 900F and in which the entire effluent from the dis-
solver stage (gases, liquids, and solids) may be passed
directly to a catalytic hydrocracking zone at a tempera-
ture below 800F and lower than the temperature in the
dissolving zone. This process provides, in high yield, a
product having an API gravity of at least -3. In one
embodiment, this process is known to the industry as the
"CCLP" which stands for Chevron Coal Liquefaction Process.
In a preferred embodiment of the CCLP, the
dissolver and the catalytic reactor are close-coupled.
Solids separation takes place downstream of the reactor.
Coal conversion and distillate yield are maximized. The
product viscosity is low, so solids separation is easier
and performed more flexibly. A consequence is higher
~; 35 severity dissolver operation, i.e., more cracked products
and light gases are produced in the hydrothermal dissolver
~ stage. Furthermore, the distillate species formed in the
; dissolver are further hydrogenated in the catalytic reactor,
and although this improves product quality, hydrogen con-
sumption is higher.
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01 -2-
The high temperature hydrothermal dissolver
which is characteristic of the CCLP, produces saturated
05 light products which can form an unstable mixture with the
remaining heavy uncracked materials which are thought to
be mostly aromatic and other unsaturates. The heavy
portion of a coal liquid contains asphaltenes which
require an aromatic medium for solubilization. There may
be insufficient solvency in the bulk of the material, or
co-solvency in the added solvent, to retain the uncracked
heavier asphaltenes in solution. The result may be phase
separation and precipitation of asphaltenes which would
tend to occur as the temperature is dropped between the
dissolver stage and the lower temperature hydrocracking
stage. U.S. Patent 4,330,393 teaches that in the Rosenthal-
Dahlberg process the small quantities of water and Cl to C4
gases produced in the dissolver are preferably removed before
the dissolver effluent enters the hydrocracking zone for the
purpose of increasing the hydrogen partial pressure in the
hydrocracking stage~ The physical structuring of the dis-
solving zone in U.S. Patent 4,330,393 is such that the slurry
may flow upwardly or downwardly in said zone. In the multi-
stage coal liquefaction process of U.S. Patent 4,110,192, it
has been found advantageous to vent most of the gases from
the dissolver zone whi~e co-currently passing hydrogen and
liquids into the dissolver zone and out of the dissolver zone
to the catalytic treatment zone.
The preferred embodiment of CCLP produces the
most hydrogenated product among all the major coal lique-
faction processes. The CCLP product is of higher hydrogen
content throughout the boiling range, and especially in
the mid-distillate range. While other coal liquefaction
processes reject heavy material and solids upstream of the
catalytic reactor (thereby reducing liquid yield), CCLP,
in its preferred embodiment, catalytically processes the
heavy material and solids for highest yield. Thus, CCLP
requires more hydrogen, which can be supplied by known
processes from natural gas or coal, at a price. The cost
40 of CCLP could be reduced without loss of benefit if
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01 _3_
(i) light products which consume hydrogen in the catalytic
reactors could be separated before the catalytic hydro-
05 cracking stage; (ii) some solids or heavy material rejec-
tion occurred before the catalytic stage; and (iii) milder
operating conditions were selected.
It would be advantageous if hydrogen utilization
efficiency could be improved in two-stage coal liquefaction
processes such as the CCLP by reducing the hydrogenation
of the mid-distillate fraction of the product of the two-
stage process. This could be accomplished if the lighter
fractions, including mid-distillates, could be continuously
removed from the dissolver stage.
It would also be advantageous if the light
products, including light saturated hydrocarbons found in
the dissolver stage of a two-stage coal liquefaction
process, could be continuously stripped away from the
remaining liquid together with water, carbon monoxide, and
other materials which cause instability and are deleterious
to the processes of the catalytic hydrocracking stage. By
this means the second stage would operate more efficiently
and the instability of the product towards asphaltene
precipitation would be overcome. This, and other
advantages, are achieved by the process of the present
invention.
SUMMARY OF THE_INVENTION
A process for li~uefying coal which comprises
forming a coal solvent slurry by mixing subdivided coal
with a solvent. In a hydrothermal dissolving-stripping zone
the coal is substantially dissolved in the solvent to form a
mixture comprising solvent, dissolved coal, insoluble solids,
and light products, while simultaneously the mixture is
stripped of substantial amounts of the light products by
3S contacting the slurry countercurrently with a first hydrogen
gas stream at elevated temperatures. A gaseous stream com-
prising the light products is withdrawn from the hydrothermal
dissolving~stripping zone. At least a substantial amount of
the insoluble solids in the remaining mixture is contacted in
a reaction zone with a second hydrogen gas stream and an
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01 _4_
externally supplied hydrocracking catalyst under hydro-
cracking conditions. An effluent stream having a normally
~5 liquid portion is withdrawn from the hydrocracking zone.
BRIEF DESCRIPTION OF THE DRAWING
The drawing is a block flow diagram of suitable
flow paths for use in practicing an embodiment of the
invention.
10DETAILED DESCRIPTION OF PREFERRED
EMBODIMENTS OF THE INVENTION
-
Referring to the drawing, in a preferred embodiment
of the present invention, comminuted coal is slurried with a
solvent in a mixing zone 10. The effluent slurry from
zone 10 passes via line 15 to a hydrothermal dissolving-
stripping zone 20 which it traverses in a generally downflow
manner in countercurrent contact with added hydrogen gas
entering the hydrothermal dissolving-stripping zone 20
through line 25. The slurry is heated to dissolve at least
about 50 weight percent of the coal in the presence of the
added hydrogen gas, thereby forming a mixture of solvent,
dissolved coal, insoluble solids, and light products. The
hydrogen gas traverses zone 20 in a generally upflow manner,
thereby stripping substantial amounts of the light products
from the mixture and conveying same out of the dissolving-
stripping zone via line 28. The mixture from zone 20 passes
via line 30 to zone 35 where it is cooled, if desired, to a
temperature lower than the temperature of the dissolver and
preferably about 55C to about 85C lower than the tempera-
ture of the dissolver. Optionally, some solids may beremoved from the mixture via line 36. The cooled mixture is
then conveyed by line 40 to hydrocracking zone 45 where it is
catalytically hydrocracked in the p~esence of hydrogen sup-
plied via line 38 to produce a relatively low viscosity
liquid product which may be readily separated from any
~ remaining coal residue.
;Referring to the drawing-in detail, subdivided
coal and a solvent are mixed in zone 10 to form a pumpable
slurry. The basic feedstock of the invention is a solid
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Ol _5_
particulate coal such as anthracite, bituminous coal, sub-
bituminous coal, lignite, or mixtures thereof. The
05 bituminous and sub-bituminous coals are particularly
preferred. It is also preferred that said coals be com-
minuted or ground to a particle size smaller than lO0 mesh,
Tvler Standard Sieve size, although large coal sizes may
be processed in this invention. The solvent used in
zone lO may be selected from the various solvents known to
the coal liquefaction art, and it may be process-derived.
Hydrogen-donor solvents are known in the coal
liquefaction art and comprise polycyclic aromatic hydro-
carbons such as tetrahydronaphthalene or dihydronaphthalene,
which are capable of being at least partially saturated.
After hydrogenation, these solvents can donate or transferthe acquired hydrogen to hydrogen-deficient dissolved coal
molecules.
In general, suitable solvents may be obtained
~ from numerous materials, but it is particularly preferred
to use crude petroleum or a 200C or higher-boiling petro-
leum fraction, such as a topped naphthenic crude or a
vacuum residua. Asphaltic or naphthenic crudes are
generally higher in aromatics and naphthenes in comparison
to paraffinic based crudes. As a result, such crudes are
preferable over the paraffinic crudes for use as solvents
in the present invention. Such crudes are also usually
higher in sulfurl nitrogen, and metals than paraffinic
crudes and thus create problems in refining processes.
The process of the present invention, however, is capable
of tolerating the higher metals content in the hydro-
cracking zone without prior demetalation or pretreatment
precautions. A substantial portion of the metals of the
crude are bound to or deposit upon the coal residue
suspended in the liquid feedstock and thus do not deposit
on the cracking catalyst.
While it is understood that suitable solvents
can be obtained from many different sources, it is also
preferred to use a solvent obtained from the process, or
particularly, a portion of the 400F and higher boiling
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01 -6-
fraction obtained from fractionation of the hydrocracking
zone effluent.
05 The subdivided coal is mixed with a solvent in a
solvent to coal weight ratio of from about 1:2 to about
50:1, preferably from about 1:2 to about 5:1 and more
preferably from about 1:1 to about 2:1. The slurry from
zone 10 may be heated by conventional means (not shown)
such as process heat exchangers, steam coils, or fired
heaters. The slurry is fed or pumped through line 15 to a
hydrothermal dissolving-stripping zone 20 comprising one
or more dissolver-strippers wherein the slurry is heated,
with added hydrogen, to a temperature in the range of
about 400~C to 480C (750F to 900F), preferably about
425C to 455C (800F to 850F) for a length of time to
substantially dissolve the coal. At least 50 weight per-
cent and more preferably greater than 70 weight percent,
and most preferably more than 90 weight percent of the
coal, on a moisture-free and ash-free basis, is dissolved
in zone 20, thereby forming a mixture of solvent, dissolved
coal, insoluble solids, and light products. ~uch light
products include acid gases, such as carbon monoxide,
light saturated hydrocarbons, such as methane, ethane,
butane, and the lighter fractions of hydrocarbonaceous
oils, including those which are generally known as mid-
distillates, i.e., having normal boiling points up to
about 370C (700F).
It is usually essential that the slurry be heated
to at least about 400C (750F) to obtain 50 weight percent
dissolution of the coal. Furthermore, it is usually required
;~ that the slurry not be heated to temperatures above 480C
(900F) in order to prevent excessive thermal cracking which
could substantially reduce the overall yield of normally
liquid product.
The hydrothermal dissolving-stripping zone 20
basically comprises one or more elongated vessels, prefer-
ably free of added external catalysts or contact materials,
which are designed so that in at least one vessel of said
zone slurry flows downwardly while hydrogen gas flows
01 _7_ ~23436~ 1936-1638
upwardly in countercurrent contact with the heated slurry,
and the mixture resulting from the hydrothermal dissolu-
05 tion of coal in solvent. More generally, the vessel used
for continuous contacting of hydrogen gas and the mixture
can be a tower filled with solid packing rnaterial, or an
empty tower into which the mixture may be sprayed and
through which the gas flows, or a tower which contains a
number of bubble-cap sieve or valve-type plates, but the
gas and the mixture flow in substantially countercurrent
contact with each other to obtain the greatest concentra-
tion driving force and therefore the greatest rate of
desorption, i.e., stripping. Design factors in this unit
operation are dealt with in "Chemical Engineers Handbook",
Perry and Chilton, 5th Edition, McGraw-Hill, Sections 4,
14, and lR
The hydrothermal dissolving-stripping zone 20
~ may comprise one or more dissolving vessels in which slurry
and added hydrogen move countercurrently or co-currently, but
it is essential that it comprises at least one dissolving-
stripping vessel in which the hydrothermal product mixture of
the coal-solvent slurry flows countercurrently to a hydrogen
gas stream. The dissolving-stripping vessel may be operated
as a liquid-full vessel with level control to ensure that the
vessel operates with a liquid mixture to a certain level
thereby regulating the residence time of the mixture in the
hydrothermal zone. Level control is exemplified by Perry and
Chilton, su~ra, Section 22. The latter o~eratm~ configuration is
~referr~d un~er conditions where substantial backmi~ing is not detri-
mental to the process and its products. Preferably, the
dissolving-stripping vessel is operated as a continuous
staged reactor of the vertical type (Perry and Chilton,
supra, page 4-21) by the use of the aforementioned reactoL
internals. The latter operating configuration is ?referred
under conditions requiring minimum backmixing.
The yield structure of products obtained from
the hydrothermal dissolving-stripping zone 20 is improved
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01 -8-
(i.e., less light normal gaseous products are produced) if
the vessels comprising zone 20 are temperature staged in
05 series, i.e., going from a higher temperature vessel near
the inward to zone 20 at line 15 to a lower temperature
vessel near the outlet of zone 20 at line 30, with all the
temperatures in zone 20 still within the aforementioned
range. By dropping the temperature toward the outlet of
zone 20 the mixture is not only prepared for the preferred
lower temperature subsequent stage in zone 35 and zone 45,
but the dissolving and cracking reactions are turned down.
Temperature control along a series of dissolver vessels is
easily achieved by intermediate cooling between vessels by
lS means of heat exchange or quench gas injection. Similar
benefits are obtainable in a single vessel dissolving-
stripping zone 20 by the use of the aforementioned
continuous staged reactor. With back mixing eliminated or
reduced in a staged reactor, a descending temperature
profile is obtained in the dissolving-stripping vessel by
the use of, for example, a downflowing preheated coal-
solvent slurry 15 and an upflowing hydrogen quench gas
stream 25~ In an alternative embodiment, hydrogen gas is
injected into the vertically elongated dissolving-
stripping vessel at several positions along the verticallength of the vessel. In yet another embodiment, cooling
stage 35 may not be necessary to achieve the lower temper-
ature preferred for hydrocracking stage 45, within the
ranges of temperatures specified, when such a temperature
staged dissolving-stripping zone is used.
Depending on operating conditions, and the
aforementioned design factors which are within the knowledge
of those skilled in the art, the counterflowing hydrogen
gas entering through line 25 and comprising fresh and
recycle hydrogen, will strip the mixture more or lçss
deeply as to the amount of the light products stripped and
the normal boiling points of the light products stripped
from the mixture. It is preferred that substantially all
gases, i.e., materials, having normal boiling points below
~0
~2~3fi~
01 -9-
about 0C (32F) be stripped from the hydrothermal dis-
solving-stripping zone 20 and removed via line 28. In
05 another embodiment, it is also preferred that substantial
amounts of all light materials having normal boiling
points below about 20C (70F) be stripped from the hydro-
thermal dissolving-stripping zone 20 and removed via
line 28. In other embodiments, it is preferred that sub-
stantial amounts of all materials having normal boilingpoints below about 35C to about 260C (100F to 500F) be
stripped from the hydrothermal dissolving-stripping zone 20
and removed via line 28. In one embodiment, substantial
amounts of mid-distillate, i.e., a fraction boiling below
about 260C (500F), is stripped, separated, and option-
ally recycled as solvent. Hydrogen should be separated
from the effluent stream 28 for recycle to the process.
The light hydrocarbon products in the effluent stream 28
should be fractionated and used directly, or, if necessary
for particular usages, subjected to further treatment.
Operating conditions in the hydrothermal
dissolving-stripping zone can vary widely, except for
temperature, in order to obtain at least 50 weight percent
dissolution of the coal. Other reaction conditions in the
hydrothermal dissolving-stripping zone include a residence
time of about 0.01 to 3.0 hours, preferably about 0.1 to
1.0 hours: a pressure of about 0 to 10,000 psig,
preferably about 1,500 to 5,000 psig, and more preferably
1,500 to 2,500 psig; a hydrogen gas rate of about 500 to
20,000 standard cubic feet per barrel of slurry,
preferably 500 to 10,000 standard cubic feet per barrel of
slurry and most preferably about 500 to 4,000 SCF/BBL; and
a slurry hourly space velocity of about 0.3 to 100 hr 1,
preferably about 1 to 10 hr l.
A remarkable advantage of the process of the
present invention is the decoupling of the hydrogen supply
to the dissolving and hydrocracking zones 20 and 45 while
the dissolving and hydrocracking stages may remain closely
coupled, if desired. Consequently, optimal hydrogen pres-
sure and gas rate rnay be provided to the hydrotherr~al
~3~36t~
01 -10--
dissolving-stripping zone 20, while simultaneously, a
different optimal hydrogen pressure and gas rate is pro-
05 vided in the catalytic hydrocracking zone 45. In general,the dissolver requires less hydrogen than the hydrocracking
zone. In the co-current hydrogen gas flow and liquid
process this flexibility is not practical. In general,
hydrogen gas flow rate should be higher in the hydrocracking
zone because of greater hydrogen consumption. By placing
a pump (not shown) in line 40 one may operate at a lower
hydrogen pressure in zone 20 and a higher pressure in
zone 45-
The dissolving zone will, in general, contain no
catalyst from any external source, although the mineralmatter contained in the coal may have some catalytic effect.
The mixture of solvent, dissolved coal and insoluble solids,
as well as any remaining light products, is preferably passed
via line 30 to a cooling zone 35. Cooling zone 35 will
typically contain a heat exchanger or similar means whereby
the effluent from dissolver 20 is cooled to a temperature
; below the temperature of the dissolving stage and at leastbelow 425C (800F). Some cooling in zone 35 may also be
effected by the addition of fresh cold hydrogen.
Optionally, some solids may be removed from zone 35 via
line 36.
The mixture of solvent, dissolved coal, insoluble
solids, and remaining products is fed through line 40 into
reactiGn zone 45 containing a hydrocracking catalyst.
Hydrogen comprising fresh and/or recycle hydrogen is fed via
line 38 into the hydrocracking zone 45 at the rate of about
4,000-50,000 SC~/BBL. In the hydrocracking reaction zone
hydrogenation and cracking occur simultaneously and the
higher molecular weight compounds are converted to lower
molecular weight compounds, the sulfur in sulfur-containing
compounds are converted to hydrogen sulfide, the nitrogen in
nitrogen-containing compounds are converted to ammonia, and
the oxygen in oxygen-containing compounds are converted to
water. Preferably, the catalytic reaction zone is a fixed
bed type, but an ebullating or moving bed may a]so be used.
`:
~23~369L
o 1 --1 1 -
The mixture of gases, liquids, and insoluble solids prefer-
ably passes upwardly through the catalytic reaction zone, but
05 may also pass downwardly. Countercurrent or co-current move-
ment of the added hydrogen gas with respect to the liquid
flow is also optional.
The primary advantage of passing such a mixture
of gases, liquids, and insoluble solids upwardly through
10 the fixed bed of particulate catalysts is that the prob-
ability of plugging is reduced. Downflow operation can
cause particles in the reactor feed to breach interstices
between stationary catalyst particles. Upflow operation,
on the other hand, results in opposing forces on the par-
15 ticles, i.e., the gravitational forces and the forces
exerted by the flowing liquid. These opposing forces tend
to reduce the probability of bridging. In addition, the
gravitational force tends to dislodge localized plugs
which may form.
A particularly desirable method of operating the
process is for the fixed catalyst bed to be operated in an
upflow mode, with the lower portion of the catalyst in the
bed being removed as the catalyst becomes fouled. Fresh
catalyst can be added to the top of the fixed bed to
25 replace the catalyst which is removed from the bottom.
This addition and removal of catalyst can take place
periodically or in a continuous or semi-continuous manner.
Continuous catalyst replacement according to this inven-
tion is carried out at such a low rate that the catalyst
30 bed is properly described as a fixed bed.
The catalyst used in the hydrocracking zone may
be any of the well known, commercially available hydro-
cracking catalysts. A suitable catalyst for use in the
hydrocracking reaction stage comprises a hydrogenation
35 component and a cracking component. Preferably the hydro-
genation component is supported on a refractory cracking
base. Suitable bases include, for example, silica,
alumina, or composites of two or more refractory oxides
such as silica-alumina, silica-magnesia, silica-zirconia,
~ alumina-boria, silica-titania, silica-zirconia-titania,
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01 -12-
acid-treated clays, and the like. Acidic metal phosphates
such as alumina phosphate may also be used. Preferred
05 cracking bases comprise alumina and composites of silica
and alumina. Suitable hydrogenation components are
selected from Group VI-B metals, Group VIII metals, and
their oxides, or mixtures thereof. Particularly useful
are cobalt-molybdenum, nickel-molybdenum, or nickel-
tungsten on silica-alumina or alumina supports.
Hydrocracking zone 45 comprises one or more hydrocracking
reactor vessels containing one or more of the aforemen-
tioned catalysts in any combination.
It is preferred to maintain the temperature in
the hydrocracking zone below 425C (800F), preferably in
the range of 340C to 425C (645F to 800F), and more
preferably 340C to 400C (645F to 750F), to prevent
catalyst fouling. The temperature in the hydrocracking
zone should be preferably maintained below the temperature
in the dissolving zone by about 55C to about 85C. Other
hydrocracking conditions include a pressure from 500 to
5,000 psig, preferably 1,000 to 3,000 psig, and more
preferably 1,500 to 2,500 psig; a hydrogen gas rate of
2,000 to 20,000 standard cubic feet per barrel of slurry,
preferably 3,000 to 10,000 standard cubic feet per barrel
of slurry; and a slurry hourly space velocity in the range
of from 0.1 to 24 0, preferably 0.2 to 0.5.
The product effluent 50 from reaction zone 45 is
separated in separation zone 55 into a gaseous fraction 60
comprising light oils boiling below about 150C to about
~60C (300F to 500F), preerably below 200C (400F) and
normally gaseous components such as hydrogen, carbon
monoxide, carbon dioxide, hydrogen sulf i~2, and the Cl to
- C4 hydrocarbons. Preferably, the hydrogen is separated
from the other gaseous components and recycled. Liquids-
solid fraction 65 is fed to a solids separation zone 70
wherein the stream is separated into a solids-lean stream
and a solids-rich stream. The insoluble solids are
separated by conventional means, for example, hydroclones,
filtration, centrifugation, and gravity settlingr or any
~23~3~
01 -13-
combination of these. Preferably, the insoluble solids
are separated by gravity settling which is a particularly
05 added advantage of the present invention since the
effluent from the hydrocracking reaction zone has a
particularly low viscosity and a high API gravity of at
least -3. The high API gravity of the effluent allows
rapid separation of the solids by gravity settling such
that 50 weight percent and generally 90 weight percent of
the solids can be rapidly separated in a gravity
settler. Preferably, the insoluble solids are removed by
gravity settling at an elevated temperature in the range
of 100C to 400C (200F to 800F). Separation of the
lS solids at an elevated temperature and pressure is
particularly desirable. The solids-lean product stream
produced in zone 70, or any fraction thereof, may be
recycled to the mixing zone 10 to provide additional
solvent.
The process of the present invention produces
extremely clean, normally liquid products. The normally
liquid products, that is, all of the product fractions
boiling above C4 have an unusually low specific gravity; a
low sulfur content of less than 0.2 weight percent; and a
low nitrogen content of less than 0.5 weight percent.
