CA1234364A - Two-stage coal liquefaction process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process for liquefying coal by first forming a coal solvent slurry. 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 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 A gaseous stream com-prising the light products is withdrawn from the hydrothermal dissolving-stripping zone.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 is contacted in a reaction zone with a second hydrogen gas stream and an externally supplied hydro-cracking catalyst under hydrocracking conditions.

Description

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TWO-STAGE COAL LIQUEFACTION PROCESS

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.

~234~;4 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 ~234~

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 123~3~

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.

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 ~3~364 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 ~3~3~L

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 ~:3~

(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~

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~

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.

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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, ~3~

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~

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.

Claims (21)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for liquefying coal which comprises:
(a) forming a coal-solvent slurry by mixing subdivided coal with a solvent;
(b) in a hydrothermal dissolving-stripping zone substantially dissolving said coal in said solvent to form a mixture comprising solvent, dissolved coal, insoluble solids and light products, and simultaneously, stripping light products from said mixture by countercurrently con-tacting said mixture with a first hydrogen gas stream at an elevated temperature;
(c) withdrawing from said dissolving-stripping zone a gaseous stream comprising said light products;
(d) contacting at least a portion of said mixture comprising said insoluble solids in a hydrocracking reac-tion zone with a second hydrogen gas stream and an exter-nally supplied hydrocracking catalyst under hydrocracking conditions; and (e) withdrawing from said reaction zone an effluent stream having a normally liquid portion.

2. A process according to Claim 1 wherein said dissolving-stripping zone comprises at least one dissolving-stripping vessel containing internals which substantially reduce backmixing.

3. A process according to Claim 1 wherein the temperature in said dissolving-stripping zone is staged in a descending manner, such that the temperature is lower at the outlet end of the zone than at the inlet end of the zone.

4. A process according to Claim 3 wherein said staged descending temperature is obtained by the use of a plurality of dissolving-stripping vessels connected in series.

5. A process according to Claim 3 wherein said slurry is preheated and said staged descending temperature is obtained in a single vessel by the use of a said first hydrogen gas stream of lower temperature than said slurry and said vessel contains internals which at least partially eliminate backmixing.

6. A process according to Claim 1 wherein said first hydrogen gas stream has a lower flow rate than said second hydrogen gas stream.

7. A process according to Claim 1 wherein the hydrogen partial pressure in said hydrothermal dissolving-stripping zone is less than the hydrogen partial pressure in said hydrocracking reaction zone.

8. A process according to Claim 1 wherein said dissolving-stripping zone is free of externally supplied catalysts or contacted particles.

9. A process according to Claim 1 wherein the residence time of the slurry in said hydrothermal dis-solving-stripping zone is regulated by the use of a level control in a dissolving-stripping zone vessel.

10. A process according to Claim 1 wherein a mid-distillate fraction is stripped from said mixture and said withdrawn gaseous stream comprises a mid-distillate fraction.

11. A process according to Claim 10 wherein at least a portion of said mid-distillate is recycled as solvent.

12. A process according to Claim 1 wherein a recycle stream of said effluent stream is recycled as solvent to form said slurry.

13. A process according to Claim 1 wherein said portion of said mixture is passed from said dissolving-stripping zone to said reaction zone without an inter-vening solids separation step.

14. A process according to Claim 1 wherein a portion of said mixture comprising said insoluble solids is recycled to the dissolving-stripping zone.

15. A process according to Claim 1 wherein said solvent is crude petroleum oil.

16. A process according to Claim 1 wherein said solvent comprises a fraction of a crude petroleum oil boiling above 200°C.

17. A process according to Claim 1 wherein said solvent comprises a petroleum residium fraction.

18. A process according to Claim 1 wherein said solvent comprises an asphalted petroleum crude oil fraction boiling above 200°C.

19. A process according to Claim 1 wherein said solvent comprises a petroleum-derived solvent containing metal contaminants.

20. A process according to Claim 1 wherein the reaction zone temperature is from about 55°C to about 85°C
lower than the temperature of said dissolving-stripping step.

21, A process according to Claim 1 wherein the dissolving-stripping zone temperature is in the range of from about 400°C to about 480°C.