GB2148937A - Two-stage petroleum residuum hydroconversion using a countercurrent gas-liquid first stage - Google Patents

Abstract

In a method of hydroprocessing a petroleum residuum fraction, there is formed in a hydrothermal treating-stripping zone (10) a mixture which comprises petroleum residuum and its hydrothermal reaction products. Simultaneously, substantial amounts of light products are stripped from the mixture by countercurrent contact with a hydrogen gas stream at elevated temperatures. A gaseous stream comprising the light products is withdrawn and the remaining heavier effluent is contacted in a catalytic reaction zone (35) with a second hydrogen gas stream and externally supplied hydrocracking catalyst at temperatures generally lower than the temperature of the hydrothermal dissolving-stripping zone, a second effluent stream (50) being withdrawn from the hydrocracking zone (35). <IMAGE>

Description

SPECIFICATION Two-stage petroleum residuum hydroconversion using a countercurrent gas-liquid first stage This invention relates to the hydroconversion of heavy hydrocarbonaceous fractions or residua. In particular, it relates to a two-stage process for the hydrothermal and hydrocatalytic conversion of petroleum residue.

The production of liquid products by the high temperature and pressure hydrogenation of heavy hydrocarbonaceous fractions or residua derived from petroleum, oil shale, tår sands or coal is well known. The resulting liquids, however, are often inefficiently obtained with high consumption of hydrogen and catalyst and excess production of lighter, normally gaseous products.

in a preferred embodiment of a two-stage process for the hydroconversion of heavy hydrocarbons, the heavy material is first heated and subjected to a high severity hydrothermal treatment, preferably without added catalyst or contact materials, in the presence of hydrogen at 400"C to 480"C, i.e., 750"F to 900oF, then the entire effluent from the first stage (gases, liquids and solids) may be passed directly to a catalytic hydrocracking zone, preferably at a temperature below 800"F (427"C) and lower than the temperature in the hydrothermal zone. In a preferred embodiment of that process, the hydrothermal zone and the catalytic reactor are close-coupled. Residuum conversion and distillate yield are maximized.A consequence of higher severity hydrothermal stage operation is that more cracked products and light gases are produced in the hydrothermal stage. Furthermore, the distillate species formed in the hydrothermal stage are further hydrogenated in the catalytic reactor, and although this improves product quality, hydrogen consumption is higher. The high temperature hydroth ermal stage produces saturated light products which can form an unstable mixture with the remaining heavy uncracked materials, which are thought to be aromatic, and other unsaturates. The heavy portion of residue is mostly asphaltenes which require an aromatic medium for solubilization. There may be insufficient solvency in the bulk of the material to retain the uncracked heavier asphaltenes in solution.The result is phase separation and precipitation of asphaltenes which tends to occur as the temperature is dropped between the hydrothermal stage and the lower temperature hydrocracking stage.

In the case of coal liquefaction with added petroleum residuum, U.S. Patent No.

4,330,393 teaches that the small quantities of water and C1 to C4 gases produced in the hydrothermal stage are preferably removed before the effluent enters the hydrocracking zone for the purpose of increasing the hydrogen partial pressure in the hydrocracking stage. The physical structuring of the hydrothermal zone in U.S. Patent No. 4,330,393 is such that the coal/residuum slurry may flow upwardly or downwardly in said zone. In the multistage coal liquefaction process of U.S.

Patent No. 4,110,192, it has been found advantageous to vent most of the gases from the dissolver zone while co-currently passing hydrogen and liquids into the dissolver zone and out of the dissolver zone to the catalytic treatment zone.

In general, two-stage (hydrothermal-hydrocracking) treatments require more hydrogen, which can be supplied by known processes from natural gas or coal at a price. The cost could be reduced without loss of benefit if (i) light products which consume hydrogen in the catalytic reactors could be separated before the catalytic hydrocracking stage; (ii) some heavy material rejection occurred before the catalytic stage; and (iii) milder operating conditions were selected.

It would be advantageous if hydrogen utilization efficiency could be improved in twostage residuum hydro-conversion processes 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, optionally including mid-distillates, could be continuously removed from the dissolver stage product. It would also be advantageous if the light products, including light saturated hydrocarbons found in the hydrothermal stage of a two-stage residuum hydroconversion 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.

Thus in accordance with the present invention there is provided a method of hydroprocessing a petroleum residuum fraction. In a hydrothermal treating-stripping zone, there is formed a mixture comprising said petroleum residuum fraction and hydrothermal reaction products, and simultaneously light products are stripped from said mixture by contacting said mixture counter-currently with a first hydrogen gas stream at elevated temperatures.

A gaseous stream comprising the light products is withdrawn from the hydrothermal treating-stripping zone and a heavier effluent stream is left behind. At least a portion of the heavier effluent stream is contacted in a reaction zone with a second hydrogen gas stream and an externally supplied hydrocracking catalyst under hydrocracking conditions which in dude a temperature lower than the temperature of the hydrothermal treating-stripping zone. A second effluent stream is withdrawn from the hydrocracking zone.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing which is a block flow diagram of suitable flow paths for use in practising a preferred embodiment of the invention.

Referring to the drawing, a heated (pumpable) petroleum residuum fraction enters via line 1 into the hydrothermal treating-stripping zone 10 which it traverses in a generally downflow manner in countercurrent contact with added hydrogen gas entering the hydrothermal treating-stripping zone 10 through line 5. The residuum is heated and hydrotreated in the presence of the added hydrogen gas, thereby forming a mixture of petroleum, residuum and hydrothermal reaction products.

The hydrogen gas traverses zone 10 in a generally upflow manner, thereby stripping substantial amounts of the lighter products from the mixture and conveying same out of the treating-stripping zone via line 1 5. The mixture from zone 10 passes via line 20 to zone 25 where it is cooled, if necessary, to a temperature lower than the temperature of the hydrothermal stage and preferably up to about 85"C lower than the temperature of the hydrothermal stage. The cooled mixture is then conveyed by line 30 to hydrocracking zone 35 where it is catalytically hydrocracked in the presence of hydrogen supplied via line 40 to produce a relatively low-viscosity liquid product which may be readily separated from any solid residue.

Referring to the drawing in more detail, a petroleum residuum fraction may be mixed with a solvent to form a pumpable slurry or the residuum fraction may be heated by conventional means to form a pumpable liquid.

The basic feedstock of the invention is a petroleum residuum fraction, heavy oil, or hydrocarbonaceous material derived from petroleum, coal, oil shale, tar sands, bitumen, lignite, or mixtures thereof. The petroleum residua derived from heavy crudes such as Maya (Mexico), Bet (Offshore California), Hondo (Offshore California), and Pt. Arguello (Offshore California), are particularly preferred.

A solvent used in preparation of the feedstock may be selected from the various solvents known to the art, and it may be processderived.

The process of the present invention, however, is capable of tolerating a higher metals content in the hydrocracking zone without prior demetalation or pretreating precautions if a small amount, up to about 5 weight percent, of comminuted coal is slurried with the feedstock. A substantial portion of the metals of the crude are bound to or deposited upon the coal residue suspended in the liquid feedstock and thus do not deposit on the cracking catalyst.

The feedstock oil is fed or pumped through line 1 to a hydrothermal treating-stripping zone 10 comprising one or more treater-strippers wherein the oil is heated, with added hydrogen, to a temperature generally in the range from 400"C to 480'C (750oF to 900"F), preferably 425"C to 4554C (800"F to 850"F) for a length of time to react a portion of the feedstock, thereby forming a mixture of feedstock and hydrothermal reaction products, including light products.Such lighter products include acid gases, such as carbon monoxide; light saturated hydrocarbons, such as methane, ethane and butane; and the lighter fractions of hydrocarbonaceous oils, optionally including those which are generally known as mid-distillates, i.e., having normal boiling points up to about 370"C (700"F).

It is preferred that the feedstock be heated to at least 400"C (750"F) to obtain sufficient conversion in the hydrothermal zone. Furthermore, it is usually preferred that the feedstock not be heated to temperatures above about 480"C (900"F) in order to prevent excessive thermal cracking which could substantially reduce the overall yield of normally liquid product.

The hydrothermal treating-stripping zone 10 basically comprises one or more elongated vessels, preferably free of added external catalysts or contact materials, which are designed so that in at least one vessel of said zone, the feedstock flows downwardly while hydrogen gas flows upwardly in countercurrent contact with the heated feed, and the mixture resulting from the hydrothermal reaction. More generally, the vessel used for continuous contacting of hydrogen gas and the feed can be a tower filled with solid packing material, or an empty tower into which the feed may be sprayed and through which the gas flows, or a tower which contains a number of bubblecap sieve or valve-type plates, but the gas and the feed flow in substantially countercurrent contact with each other to obtain the greatest concentration 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 18.

The hydrothermal treating-stripping zone 10 may comprise one or more treating vessels in which feed and added hydrogen move countercurrently or co-currently, but it is essential that it comprises at least one treating-stripping vessel in which the hydrothermal product mixture of feedstock and reaction product flows countercurrently to a hydrogen gas stream..

The treating-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, supra, Section 22. The latter operating configuration is preferred under conditions where substantial backmixing is not detrimental to the process and its products. Preferably, the treating-stripping vessel is operated as a continuous staged reactor of the vertical type (Perry and Chilton, supra, Section 4, page 21) by the use of the aforementioned reactor internals. The latter operating configuration is preferred under conditions requiring minimum backmixing.

The yield structure of products obtained from the hydrothermal treating-stripping zone 10 is improved (i.e., less light, normally gaseous products are produced) if the vessels comprising zone 10 are temperature staged in series, i.e., going from a higher temperature vessel near the inward of zone 10 at line 1 to a lower temperature vessel near the outlet of zone 10 at line 20, with all the temperatures in zone 10 still within the aforementioned range. By dropping the temperature toward the outlet of zone 10, the mixture is not only prepared for the lower temperature next stage in zone 25 and zone 35, but the cracking reactions are turned down. Temperature control along a series of vessels is easily achieved by intermediate cooling between vessels by means of heat exchange or quench gas injection.Similar benefits are obtainable in a single vessel treating-stripping zone 10 by the use of the aforementioned continuous staged reactor.

With backmixing eliminated or reduced in a staged reactor, a descending temperature profile is obtained in the treating-stripping vessel by the use of, for example, a downflowing preheated feedstock 1 and an upflowing hydrogen quench gas stream 5. In an alternative embodiment, hydrogen gas is injected into the vertically elongated treating-stripping vessel at several positions along the vertical length of the vessel. In yet another embodiment, cooling stage 25 may not be necessary to achieve the lower temperature necessary for hydrocracking stage 35 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 5 and comprising fresh and recycle hydrogen, will strip the mixture more or less 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 optionally preferred that substantial amounts of mid-distillates having normal boiling points below about 260"C (500"F) be stripped from the hydrothermal treating-stripping zone 10 and removed via line 15.It is preferred that substantial amounts of all light hydrocarbons having normal boiling points below about 20"C (70"F) be stripped from the hydrothermal treating-stripping zone 10 and removed via line 1 5. It is most preferred that substantially all gases having boiling points below about 0 C (32"F) be stripped from the hydrothermal treating-stripping zone 10 and removed via line 1 5. Hydrogen should be separated from the effluent stream 1 5 for recycle to the process. The light hydrocarbon products in the effluent stream should be fractionated and used directly or, if necessary, for particular usages, subjected to further treatment.

Operating conditions in the hydrothermal treating-stripping zone can vary widely, except for temperature. Other reaction conditions in the hydrothermal treating-stripping zone include a residence time of 0.01 to 3.0 hours, preferably 0.1 to 1.0 hours; a pressure of O to 10,000 psig (to 702 kg/cm2), preferably 1,500 to 5,000 psig (104 to 350 kg/cm2), and more preferably 1,500 to 2,500 psig (104 to 1 75 kg/cm2); a hydrogen gas rate of O to 20,000 standard cubic feet per barrel of feed, preferably 3,000 to 10,000 standard cubic feet per barrel of feed; and a feed hourly space velocity of about 0.3 to 100 hr-1, preferably about 1 to 10 hr-1.

A remarkable advantage of the process of the present invention is the decoupling of the hydrogen supply to the treating and hydrocracking stages 10 and 35 while the treating and hydrocracking stages may remain closely coupled, if desired. Consequently, optimal hydrogen pressure and gas rate may be provided to the hydrothermal treating-stripping zone 10 while, simultaneously, a different optimal hydrogen pressure and gas rate is provided in the catalytic hydrocracking zone 35. 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 30, one may operate at a lower hydrogen pressure in zone 10 and a higher pressure in zone 35.

The treating zone will, in general, contain no catalyst from any external source, although the mineral matter contained in added coal, if any, may have some catalytic effect. The mixture of heavier products and unconverted feed, as well as any remaining lighter products, is passed via line 20 to a cooling zone 25. Cooling zone 25 will typically contain a heat exchanger or similar means whereby the effluent from treater 10 is cooled to a temperature below the temperature of the treating stage and at least below 425"C (800"F).

Some cooling in zone 25 may also be effected by the addition of fresh cold hydrogen, if desired. Cooling zone 25 is an optional feature of this embodiment and may not be necessary to effect the temperature gradient between zone 10 and zone 35. Optionally, zone 25 comprises a flash unit to remove light solvent fractions from the remaining light products in the heavier effluent stream. The light solvent fractions may be burned to provide process heat.

The mixture of heavier reaction products and feed and remaining light products is fed through line 30 into reaction zone 35 containing a hydrocracking catalyst. Hydrogen comprising fresh and/or recycle hydrogen is fed via line 40 into the hydrocracking zone 35. 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 the sulfur-containing compounds are converted to hydrogen sulfide.

the nitrogen in the nitrogen-containing compounds are converted to ammonia, and the oxygen in the oxygen-containing compounds are converted to water. Preferably, the catalytic reaction zone is a fixed bed type, but an ebullating or moving bed may also be used.

The mixture of heavier reaction products and unconverted feed preferably passes upwardly through the catalytic reaction zone, but may also pass downwardly. Counter-current or cocurrent movement of the added hydrogen gas with respect to the liquid flow is also optional.

The primary advantage of passing such a mixture upwardly through the fixed bed of particulate catalysts is that the probability of plugging is reduced.

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 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 invention is carried out at such a low rate that the catalyst bed is properly described as a fixed bed.

The catalyst used in the hydrocracking zone may be any of the well known, commercially available hydrocracking catalysts. A suitable catalyst for use in the hydrocracking reaction stage comprises a hydrogenation component and a cracking component. Preferably the hydrogenation 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, acid-treated clays, and the like. Acidic metal phosphates such as alumina phosphate may also be used. Preferred cracking bases comprise alumina and composites of silica and alumina. Suitable hydrogenation components are selected from Group Vl-B metals, Group VIII metals, and their oxides, or mixtures thereof.Particularly useful are cobalt-moiybdenum, nickel-molybdenum, or nickel-tungsten on silica-alumina or alumina supports.

Hydrocracking zone 35 comprises one or more hydrocracking reactor vessels containing one or more of the aforementioned catalysts in any combination.

It is preferred to maintain the temperature in the hydrocracking zone below 425"C (800 F), preferably in the range of 340"C to 425"C (645"F to 800"F), and more preferably 340'C to 400"C (645"F to 750"F), to prevent catalyst fouling.The temperature in the hydrocracking zone should be preferably maintained below the temperature in the hydrothermal zone by from 55"C to 85"C. Other hydrocracking conditions include a pressure from 500 to 5,000 psig (34 to 350 kg/cm2), preferably 1,000 to 3,000 psig (70 to 210 kg/cm2), and more preferably 1,500 to 2,500 psig (104 to 175 kg/cm2); 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 spaced velocity in the range of from 0.1 to 2.0, preferably 0.2 to 0.5.

The product effluent 50 from reaction zone 35 is separated in separation zone 55 into a gaseous fraction 60 comprising light oils boiling generally below 150"C to 260"C (300"F to 500"F), preferably below 200"C (400"F), and normally gaseous components such as hydrogen, carbon monoxide, carbon dioxide, hydrogen sulfide, and the C, to C4 hydrocarbons. Preferably, the hydrogen is separated from the other gaseous components and recycled. Liquid fraction 65 is available for fractionation or further processing.

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 (8)

1. A method of hydroprocessing a petroleum residuum fraction which comprises: (a) in a hydrothermal treating-stripping zone, forming a mixture comprising said petroleum residuum fraction and hydrothermal reaction products, and simultaneously stripping light products from said mixture by contacting said mixture countercurrently with a first hydrogen gas stream at an elevated temperature; (b) withdrawing from said hydrothermal treating-stripping zone a gaseous stream comprising said light products and leaving a heavier effluent stream; (c) contacting at least a portion of said heavier effluent stream in a catalytic reaction zone with a second hydrogen gas stream and an externally supplied hydrocracking catalyst under hydrocracking conditions, including a temperature lower than the temperature of said hydrothermal treating-stripping zone; and (d) withdrawing from said catalytic reaction zone a second effluent stream.

2. A method according to Claim 1, wherein said hydrothermal treating-stripping zone comprises at least one treating-stripping vessel containing internals which substantially reduce backmixing.

3. A method according to Claim 1 or 2, wherein said treating-stripping zone is free of externally supplied catalyst or contact particles.

4. A method according to Claim 1, 2 or 3, wherein the hydrogen partial pressure in said hydrothermal treating-stripping zone is less than the hydrogen pressure in said reaction zone.

5. A method according to Claim 1, 2, 3 or 4, wherein the reaction zone temperature is from 55 to 85"C lower than the temperature of said hydrothermal treating-stripping zone.

6. A method according to Claim 1, 2, 3, 4 or 5, wherein the treating-stripping zone temperature is from 400 to 480"C.

7. A method according to any preceding claim, wherein the temperature in said treating-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.

8. A method of hydroprocessing a petroleum residuum fraction, substantially as herein before described with reference to the accompanying drawing.