CA1238005A - Multi-stage catalytic process for high conversion of heavy hydrocarbon liquid feedstocks - Google Patents

Abstract

ABSTRACT OF DISCLOSURE

A process for high hydroconversion of heavy liquid hydrocarbons, such as petroleum resid feedstocks containing at least about 25 V % material boiling above about 975°F+, using multi-stage catalytic reactions at elevated temperature and pressure to produce increased yields of lower boiling hydrocarbon liquids and gas. In the process, the feedstock is fed in liquid phase with hydrogen at elevated temperature and pressure conditions into a staged catalytic reactor system, preferably containing ebullated catalyst beds. from which a vacuum bottoms fraction having a nominal boiling point above about 800°F is recycled to the last-stage reactor rather than to the first-stage, to improve conversion of the 975°F+ fraction to lower boiling liquid products. If desired, three stages of catalytic reaction can be used with the vacuum bottoms material fraction being recycled to the third stage reactor.

Description

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OF HEAVY HYDROCARBON LIQUID FEEDSTOCKS

BACKGROUND OF INVENIION
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This invention pertains to a process for high percentage catalytic conversion of heavy hydrocarbon liquid feedstocks using multiple stage catalytic reactors connected in series flow arrangement. It pertains particularly to such a multi-staye hydroconversion process for heavy petroleum feedstocks .
containing at least about 25 Y ~ boiling above about 975F
and in which d vacuum bottoms material fraction is recycled to the last stage catalytic reactor for achieving increased hydroconversion of the feed to produce lower boiling hydro-~;~ carbon liquid and gas products.
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The percent hydroconversion of a hydrocarbon feedstock in once-through operations in a single catalytic reactor to produce lower boiling hydrocarbon liquids and gases is usually limited to only about 75 V ~. For achieving higher conversions of petroleum residua, a heavy resid fraction is recycled to the reactor, and in two-stage reactor systems the resid fraction has been recycled to the first stage reactor.
However, it has been found that in such a two-stage catalytic operation on hydrocarbon liquid feedstocks7 conversion in the .
~ firs~ stage reactor generally does not exceed about 60 V ~
; and the addition of ~a heavy recycle liquid fraction to the first stage reactor is not very effective because of the relatively low resid concentration change which occurs therein, in combination with a reduced total residence time of the resid fraction at reactor conditions.
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The catalytic hydrogenation and conversion oF petroleum residua in a single~stage ebullated bed catalytic reactor with recycle of a vacuum bottoms fraction to the reactor is well known, having been previously disclosed in U.S. Patents No. 2,987,465 to Johanson and U.S. 3,412,010 to Alpert, et al. U. S. Patent 3,184,402 to Kozlowski, et al, discloses a two-stage catalytic hydrocracking process with intermediate fractionation and some recycle of a distillation bottoms fraction to either a first or second catalytic cracking zone.
Also, U.S. Patent 3,775,293 to Watkins, discloses a two-stage desulfurization process with recycle of some heavy oil ; fraction boiling above diesel fuel oil to a second stage fixed bed type reactor. However, these processes recycle relatively low boiling materials, and further process impro-vements are needed to achieve higher conversion of heavy higher boiling hydrocarbon liquid feedstocks, such as petro-leum residua nominally boiling above about 800~F. It has now unexpectedly been found that the effect of recycling a heavy liquid fraction on the overall conversion of a heavy¦
hydrocarbon feedstock will be more beneficial if the heavy liquid fraction is recycled to the last stage reactor of a multi-stage catalytic reactor system.
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SUMMARY OF INVENTION

The present invention provides a multi-stage catalytic process for high percentage hydroconversicn of heavy hydro-carbon liquid feedstocks containing at least about 25 V ~
,material normally boiling above about 975F to produce lower boiling hydrocarbon liquid and gas products. In the process, the liquid feed material is catalytically reacted in the ~:3~

liquid phase with hydrogen at elevated temperatures and pressure cond;tions in a multi-stage catalytic reactor system, wherein a heavy vacuum bottoms fraction having a nominal cut point above about 800F, and preferably above about 850F, is recycled to the last-stage ebullated catalyst bed reactor of a multi-stage reactor system, in order to improve the conversion of the feedstock and particularly the 975F~ material fraction and thereby increase the yields of C4-975F liquid products.

More specifically, the,process according to the in~ention comprises feeding a heavy hydrocarbon liquid feedstock with hydrogen into a first stage catalytic reactor containing a particulate catalyst, the reactor being maintained at 780-850F temperature, 1000-3000 psi hydrogen partial pressure, and 0.20-2.0 Yf/Hr/Vr (volume fresh feed per hour per volume of reactor) overall space velocity for partially hydroconverting the feedstoc~ to produce an effluent material; passing said partially hydroconverted effluent material to all subsequent stage catalytic reactors including a last stage ebullated bed catalytic reactor maintained at 780-850F temperature, 1000-3000 psi hydrogen partial pressure, and 0.20~2.0 Vf/Hr/Vr overall space velocity and further hydroconverting the material to produce hydrocarbon gases and lower boiling liquid fractions; removing said hydrocarbon gases and liquid fractions from said last stage reactor, and separating said hydrocarbon gases from said liquid fractions a~d withdrawing the liquid fractions;
distilling said liquid fractions to produce medium boiling hydrocarbon liquid products having normal boiling ranges of about 400-750F, 750-975F and a vacuum bottoms material having a nominal cut point above about 975F; and recycling :

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at least a portion of said vacuum bottoms material to said last stage ebullated bed catalytic reactor to produce increased yields of said medium boiling hydrocarbon liquid products.

The present invention is useful for hydroconversion of petroleum residua) bit~men from tar sands and raw shale oil.
The process can use three catalytic reactors in series, however, it is preferably used for two ebullated bed cataly-tic reactors connected in series arrangement wlth recycle of the heavy vacuum bottoms fraction to the second stage reac-.
torO Although this process can be used with any type cata-lytic reactor system, ebullated bed catalytic type reactors are, usually preferred particularly for liquid feedstocks containing appreciable solids particles and/or metals com-pounds.

hdvantages o~ the present invention in which at least a portion of the heavy hydrocarbon liquid fraction is recycled .
to the second or last stage catalytic reactor include higher hydroconversion of the feedstock to provide increased production of hydrocarbon distillate fraction products.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 are graphs showing the comparative effects of recycling a vacuum bottoms fraction to either the first or last stage catalytic~ reactor on percent hydroconversion of the feedstock versus results from once-through operations.

FIG. 3 is a schematic ,diagram of three catalytic hydro-conversion processing arrangements showing how the reactor ' .residence time for the resid fraction i.s affected in similar ~;~ 3~

two-stage operations for once-through, heavy liquid recycle to khe first-stage reactor, and heavy liquid recycle to the second-stage reactor.

FIG. 4 is a schematic diagra~ of a two-stage cata1ytic hydroconversion process for heavy hydrocarbon liquid feedstocks using recycle of a vacuu~ bottoms fraction to the last stage reactor according to the invention.

DESCRIPTION OF INVENTION

In a pre~erred embodiment of the present invention, a heavy hydrocarbon liquid feedstock such as petroleum residua is pressurized, heated and introduced with hydrogen gas into ;a first stage catalytic reactor of a two-stage catalytic reactor system. The first stage reactor can be either a fixed bed type catalytic reactor or an ebullated bed cataly-tic reactor, with the ebullated bed type reactor being pre-ferred particularly for -feedstocks containing more than about 300 ppm total metals content, which are usually mainly vanadium and nickel compounds. The first stage reactor zone conditions are maintained at 780-850F temperature, 1000-3000 psi hydrogen partial pressure, and 0.2-2.0 Yf/Hr/Vr overall space velocity to achieve partial catalytic hydroconversion o~ the feedstock. Preferred reaction conditions are 790-840F temperature, 1200-2800 psi hydrogen partial pressure and 0.25-1.8 Yf/Hr/Yr overall space velocity.
, From the first stage reactor, the resulting gas and partially converted liquid material is passed with additional hydrogen ~o a second stage ebullated bed catalytic reactor for further hydroconversion reactions~ The second stage reac~or can be maintained at ~substantially the salne !

conditions as in the first stage reactor, depending on the feed-stock being processed and ~he percentage conversion desired. If the feedstock contains total metals such as vanadium and nickel exceeding about 300 ppm, then a demetallization type catalyst, i.e., an inexpensive catalyst material whlch is capable of removing vanadium and nickel preferentially over sulfur, will be preferably used in the first stage ebullated bed reactor because of its lower cost, and a hydroconversion or desulfurization type catalyst used in the second stage ebullated bed reactor. The demetallization type catalyst can be of particulate naturally-occurring bauxite or aluminum oxide material which may contain minor amounts of other metal oxides such as iron or molybdenum, such as is known in the art. The hydroconversion or desulfurization type catalyst contains active metal oxides of metals selected from the group consisting of cadmium, chromium, cobalt, iron, molybdenum, nickel, tin, tungsten and mixtures thereof deposited on a support material selected from the group of alumina, silica, and combinations thereof.
From the second stage reactor, the effluent material is phase separated and the resulting liquid fraction is passed at lower pressure to distlllation steps. The resulting vacuum bottoms fraction is recycled into the second stage ebullated bed catalytic reactor, where the concentration of resid in the reactor is much lower than in the first stage, thus maximizing the effect o~ the heavy recycled liquid on the percent conversion of the feedstock.
~ ccording to the present invention, it has now been found that although the effect of recycling a vacuum bottoms fraction on conversion of the heavy hydrocarbon feedstock at conversion levels lower than about 60 % is nil, as indicated in FIG. 1, at conversions higher than about 60 V %, the effect of vacuum bottoms recycle on the percent conversion is more beneficial when the recycle fraction is fed to the last stage reactor of a multi-stage catalytic reactor system, as is shown in FIG. 2.
The effect on the percent conversion of the heavy hydro-carbon feedstock caused by recycle of a vacuum bo-ttoms ma~erial to a single stage reactor is shown in FIG. 1, in ~2 ~

which percent conversion results with vacuum bottoms recycle to the single stage reactor has been plotted against single stage conversion achieved without any such recycle. As seen from the graph, Line A, at below about 60% conversion the effect of heavy liquid recycle to the reactor on the percent conversion is nil, because the relatively low resid con-centration change which occurs in the reactor cannot offset the reduced residence time of the resid at reactor con-ditions~ However~ as the percent conversion is increased~
such as by increasing the reaction severity, concentration of the resid fraction (usually 975~F+) in the reactor liquid decreases. Thus, the change in concentration of the resid fractlon in the reactor caused by the addition of a vacuum bottoms fraction to the reactor is larger than that existing for low conversion conditions. Even though there ls a lower resid residence time than the once-through type operation, this recycle arrangement results in increased percent con-version of the feedstock. This result is shown bY the data in FIG. 1, through whic~ Line B has been drawn.

In a two-stage catalytic reactor operation, conversion in the first stage reactor is usually less than about 60 V ~.
Therefore. recycle of heavy liquid to the first stage reactor is effective in i~proving resid conversion only above about 60% conversion, as is indicated by FIG. 2, Line B. However, as mentioned above~ it has been found that conversion can be further improved by recycling the heavy liquid or resid fraction to the second stage instead of to the first stage reactor, as shown in FIG. 2, Line C. This conversion increase occurs even though resid recycle to the first stage reactor produces a higher resid concentration in both the ..
first and second stages, whereas when the resid is recycled .

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to the second stage reackor, only that reactor benefits from a higher resid concentration therein. Overall conversion is ne~ertheless higher with recycle of the resid fraction to the second stage only as compared with recycle to the first stage, because the increase in resid residence time more than compensates for reduced concentration benefi-ts. Therefore, the net result provides a higher level of conversion when the resid fraction is recycled to the second or last stage reactor, than when it is recycled to the first stage reactor.
By way o~ explanation for this recycle relationships the , .
percent hydroconvers;on of the heavy liquid fraction in the feedstock is influenced mainly by three factors, i.e. reactor temperature, the concentration of the heavy liquid or resid fraction in the reactor, and the resid residence time in the reactor. An increase in the level o~ any of these Factors will increase the conversion of the resid to varying degrees.
When a concentrated portion of unconverted heavy liquid is recycled to a catalytic reactor. the concentration of this resid material in the reactor is increased. However, the residence time for the resid will actually decrease, where residence time is deflned as the volumetric fraction of resid in the total reactor liquid feed times the reactor volume divided by the total liquid volumetric feed rate to the reactor. This decrease in resid residence time results from feeding more resid per unit time through the same reactor volume.

To demonstrate these changes in resid residence times, three flow arrangements for two-stage catalytic reactor hydroconversion operations are presented in FIG. 3, showing the three types of operations. In each arrangement, a con-sistent basis is used, i.e.. both reactors have -the same .. ..

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temperature and the same volume and the fresh feed rate to the reactor system is also the same for each. It is found that for the t~wo recycle arrangements, B and C, the resid residence times are both lower than for the once-through type operation, arrangement A. However9 for the case with recycle to the second stage per arrangement C L the resid residence time is higher than for recycle to the first stage per arrangement B.

A preferred embodiment of the present inventioh is shown by FIG. 4. Referring now to the FIG. 4 diagram, a heavy .. :
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petroleum feedstock such as Kuwait vacuum resid or Safaniya vacuum bottoms is provided at 10, pressurized bY pump 12 and passed through preheater 14 for heating the feed to at least about 500F, but not to a suf~iciently high temperature to cause any feedstream coking in the preheater. The heated feedstream at 15 is fed into the ~irst-stage upflow_ebullated bed catalytic reactor 20. Heated hydrogen is provided at 16 and also introduced into reactor 20. The reactor 20 contains an inlet flow distributor and catalyst support grid 21, so that the Feed liquid and gas will pass uniformly upwardly through the reactor 20 and will expand the catalyst bed 22 by at least about 10% and usually not more than about 50~ over its settled height, and place the catalyst in random motion in the liquid. This reactor is typical of that described in U. S. Patent No. Re. 25,770~ wherein a liquid phase reactlon occurs in the presence of a reactant gas and a particulate catalYst such that th~é catalyst bed is expanded.

The catalyst particles in bed 22 usually have a relative-ly narrow size range ~or uniform bed expansion under con-trolled upward liquid and gas flow conditions. While a useful catalyst particle size range is between 6 and 100 mesh 1~ .

(U.S. Sieve Series), the catalyst is preferably particles of 6 and 50 mesh size including extxudates of approximately 0.010-0.100 inch diameter. In the reactor, the density of the catalyst particles, the liquid upward flow rate, and the lifting of the upflowing liquid and hydrogen gas are important factors in the expansion and operation of the catalyst bed~ By control of the catalyst particle size and density and the upflowing liquid and ~as velocities and taking into account the viscosity of the liquid at the reactor operating conditions, the catalyst bed 22 is expanded to a desired extent to have an upper level or interface in the liquid as indicated at 22a.
The hydroconversion reaction in bed 22 is greatly facilitated by use of an effective catal,vst. The catalyst used is a typical hydrogenation catalyst containing activated metal oxides which are deposited on a support material of alumina, silica, and com-binations thereof. Fresh catalyst is periodically added directly into the reactor 20 through suitable inlet connection means 25 at a rate between about 0.10 and 2.0 lbs catalyst/barrel petroleum feed, and used catalyst is withdrawn through suitable withdrawal means 26 to maintain the desired catalyst inventory and activity in the reactor. For hydrocarbon feedstocks containing more than about 300 ppm total vanadium and nickel, a demetallization type catalyst should be used in the first stage reactor.
Recycle of reactor liquid from above the solids interface 22a to below a flow distributor 21 is usually necessary to establish a sufficient upflow liquid velocity to maintain the ~3~

catalyst in desired expanded random motion in the liquid and to facilitate effectivP reactions. Such liquid recycle is preferably ac~omplished by the use of a central downcomer conduit 18 which extends to a recycle pump 19 located below the flow distributor 21, to assure a positive and controlled upward flow o~ the liquid through the catalyst bed 22. The recycle of reactor liquid through internal conduit i8 has some mechanical advantages and tends to reduce the number of external high pressure connections needed in a hydrogenation reactor, however, liquid recycle upwardly through the catalyst bed can al ternati Ye1y be establi shed by a recycle conduit and pump located external to the reactor. In an ebullated bed reactor system, a vapor space 23 exists above the liquid level 23a.

Operability of the ebullated catalyst bed reactor system to assure good contact and uniform (iso-thermal) temperature therein to achieve the desired hydroconversion results depends not only on the random motion of the catalyst par-ticles in the liquid e~vironment resulting from the buoyant effect of the upflowing liquid and gas, but also requires the proper reaction conditions. With improper reaction con-ditions insufficient hydroconversion is achieved7 which results in a non-uniform distribution of liquld flow and usually resulting in excessive coke deposits on the catalyst.

For the hydroconversion of petroleum feedstocks using this invention, operating conditions needed in the reactor 20 are ~lithin the r~nges of 780-850F temperature, 1000-3000 psi, hydrogen partial pressure, and overal 1 space velocity of 0.2-2.0 V~/hr/Vr ( vol u~e fresh feed per hour per volume of reactor). Preferred reactor conditions dre 790-840F
temperaturet 1200-2800 psi, hydrogen partial pressure, and ~ 3~

overall space velocity of 0.25-1.8 Vf/hr/Yr. The feedstock hydroconversion achieved is at least about 60 V % for once through type operations.

From first stage reac~or 20, an overhead stream con-taining a mixture of both gas and liquid fractions is withdrawn at 27, and passed into the lower end of second stage reactor 30, containing an ebullated bed 32 of par-ticulate catalyst. This catalyst 32 is usually the same size as that used in reactor 20 but may also be somewhat larger and preferably has particle size of 0.030-0.070 inch effective diameter. Operation of this second stage ebullated bed reactor 30 is quite similar to that for reactor 20, with reactor liquid being recirculated through downcomer conduit 34 and pump 35 and then upwardly through flow clistributor 31 to assure uniform and controlled expanslon and ebullation of the catalyst bed 32. Suitable reaction conditions used in reactor 30 are 780-850F temperature, 1000~3000 psi hydrogen partial pressure, and 0.2-2.0 Yf/Hr/Vr overall space velocity. Preferred reaction conditions are 790-840F
temperature, 120Q-2800 psi hydrogen partial pressure, and 0.25-1.8 V~/Hr/Vr overall space velocity. Fresh catalyst is added and used catalyst withdrawn from reactor 30 as needed to maintain desired catalyst activity therein similarly as ~or reactor 20.

From reactor 30, an effluent stream 39 containing gas and lower boilin~ hydrocarbon liquid fractions is withdrawn and passed to high pressure phase separator 40. The resulting gas frac~ion 41 contains principally hydrogen which is recoYered in gas purification step 42. A vent gas containing mainly H2, C02, H2S, light hydrocarbon gases, and water is removed at 43.~- The recovered hydrogen at 44 is usual1y , .

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recycled bv compressor 44a through conduit 45, and reheated at heater 46 as needed, then is introduced into the bottom of reactor 20 along with make-up hydrogen at 45a as needed.
Also, a portion of hydrogen stream 45 is reheated at heater 47 as needed and introduced with effluent stream 27 into reactor 30.

From separator 40, the remainins liquid fraction 48 is pressure-reduced at 49 to a pressure below dbout 200 ps'ig7 and passed to fract;onation step'S0, from which is withdrawn a low pressure hydrocarbon vapor stream 51. This vapor .. ..
, stream is phase separated at 52 to provide low pressure pro-~ , , duct gas 53 and liquid stream 54 to provide reflux liquid to fractionator 50 and a naphtha product stream 55. A middle boiling range distillate liquid product stream is withdrawn at 56, and a heavy fuel oil' liquid stream Is withdrawn at 58.
, From fractionator 50, the heavy bot~oms oil stream 58 which usually has normal boiling temperature range of 650-975F, is reheated ,as needed in heater 59 and passed to vacuum distillation step 60. A vacuum gas oil product stream is withdrawn at 62, and a vacuum bottoms stream is withdrawn at 64. A portion 65 of the vacuum bottoms material having a nom;nal boiling or cut point above about 800F and preferably above about 850F is pressurized at 66, reheated at heater 67 as needed, and recycled to reactor 30 for further hydroconversion reactions therein, such as to achieYe 75-g5 V % conversion of the feedstock to lower boiling hydrocarbon materials. Depending on the percent conversion and products desired from the process, a~ least about S0 Y
and preferably all the vacuum bottoms material at 64 is recycled at 65 to réactor 30. The volume ratio of the recycled 975F+ material to the fresh feedstock should be -~2 3~

within a range of about 0.2-1.5. A heavy vacuum pitch material is withdrawn at ~8 for further processin~ or use as desired.

The process of the present invention will be fur-ther described by use of the following examples, which are illustrative onlv and should not be construed as limiting the scope of the invention.

The benefit to overall conversion of the feedstock and - . ~ .. .
, yields provided by the heavy liquid recycle ~rrangement of the present invention is summarized in Table 1 below. In this example, the vac,uum bottoms ~raction boiling above 800F
is recycled to the first stage reactor of a two-reactor system for Case 1, and to the second-stage reactor for Case .

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TABLE 1 i COMPARATIVE T'~O-STAGE HYDROCONYERSION OPERATIONS
-Feedstock Used Safaniya Vacuum Bottoms Gravity~ API 4 5 Sulfur 9 W % 5.6 Carbon, W ~ 83.9 Hydrogen 9 W % 10 . O
Vanadium, ppm 157 Nickel, ppm 49 Ramsbottom Carbon Residue, W ~ 22 975F+ Fraction~ V ~ of Feed 95 Case No. 1 2 Reactor Temperature, F 825 825 Catalyst Used Co-Moly on Alumina Overal1 Liquid Space Velocity, Yf/Hr/Yr 0.37 0.37 No. of Stages . . . - Two Two Vacuum Bottoms Fraction Recycle to Stages First Second Recycle Ratio, VvB/V Fresh Feed 0 5 0 5 975F+ Converslon~ LV ~ 82 88 Distillation Yield, V % of Feed C4-400~F ~phtha 22.0 25.0 400-650F Distillate 27.5 30.0 650-975F Heavy Fuel Oil 41.3 41.7 Totals 90.8 ~ T
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As seen from the Table I results, at the same operating conditions the conversion of the feedstock obtained by recycling the vacuum bottoms fraction to the second stage reactor is about 6~ higher than for its recycle to the first stage reactor. A comparable improvement is also shown for the distillate yields, where the total yield of the C4-975F
material is also about 5.9 V % greater when using the present invention.

The heavy liquid recycle arrangement of the present invention can also be advantageously extended to a three-. ~

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staye reactor system and operation with the same heavypetroleum feedstock as for Example 1 using a three-stage catalytic reaction process operated at the same reaction conditions in each stage. The vacuum bottoms recycle material is added to either the first stage reactor of the three-stage reactor system, the second stage reactor, or to the third-stage reactor. Comparative conversion results achieved by using recycle of the vacuum bottoms fraction to each stage reactor are shown in Table 2 below.

:- TABLE 2 '' -; . . . .................. . .
. CO~PARATIVE THREE-STAGE HYDROCONVERSION OPER_TIONS
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Case No. 3 4 5 Feedstock Properties Same as in Table 1 Reaction Conditions Same as in Table 1 No. of Stages Three Three Three Vacuum Bottoms Recycle To Stages- First _ SecondThird 975F+ Conversion, LY ~ 82 86 88 All other operational cpnditions are the same for all three cases.

From the above results, it ls seen that a significantly higher percentage convers~on of the 975F+ material is achieved, along with higher yields of 400-975F boiling range products with the present invention using recycle of vacuum bottoms fraction to the third stage reactor, rather than to a conventional single-stage reactor or to a second-stage catalytic reactor.

Although this invention has been described broadly and in terms of a preferred embodiment, it will be understood that modifications and variations of the process can be made and tha~ some steps can be used without others all within the .

spirit and scope oF the invention, which is defined by the .
following claims.

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Claims (16)

WE CLAIM:

1. A process for multi-stage catalytic high percentage conversion of heavy hydrocarbon liquid feedstocks to produce higher conversion and increased lower boiling hydrocarbon liquids and gases, said process comprising:

(a) feeding a hydrocarbon feedstock with hydrogen into a first stage catalytic reactor containing a par-ticulate catalyst, said reactor being maintained at 780-850°F temperature, 1000-3000 psig hydrogen par-tial pressure, and 0.20-2.0 Vf/Hr/Vr overall space velocity for partially hydroconverting said feedstock to produce an effluent material;

(b) passing said partially hydroconverted effluent material to all subsequent staged catalytic reactors including a last stage ebullated bed catalytic reactor maintained at 780-850°F temperature, 1000-3000 psi hydrogen partial pressure, and 0.20-2.0 Vf/Hr/Vr overall space velocity and further hydroconverting the material to produce hydrocarbon gases and lower boiling liquid fractions;

(c) removing said hydrocarbon gases and liquid fractions from said last stage reactor, and separating said hydrocarbon gases from said liquid fractions and withdrawing the liquid fractions, (d) distilling said liquid fractions to produce a medium boiling hydrocarbon liquid product having normal boiling range of 400-800°F and a vacuum bottoms material having a nominal boiling point above about 800°F; and (e) recycling at least a portion of said vacuum bottoms material directly to said last stage ebullated bed catalytic reactor to achieve at least about 75 V %
conversion of 975°F+ fraction to lower boiling material and produce increased yields of said medium boiling hydrocarbon liquid product.

2. A hydrocarbon conversion process according to claim 1, wherein said first stage reactor contains a fixed bed of particu-late catalyst.

3. A hydrocarbon conversion process according to claim 1, wherein said first stage reactor contains an ebullated bed of particulate hydrogenation catalyst.

4. A hydrocarbon conversion process according to claim 1, wherein said first stage reaction conditions are 790-840°F
temperature, 1200-2800 psig hydrogen partial pressure, and 0.25-1.8 Vf/Hr/Vr overall space velocity.

5. A hydrocarbon conversion process according to claim 1, wherein said last stage reaction conditions are 790-840°F temper-ature, 1200-2800 psig hydrogen partial pressure, and 0.25-1.8 Vf/Hr/Vr overall space velocity.

6. A hydrocarbon conversion process according to claim 1, wherein said recycled vacuum bottoms material has a nominal boiling point above about 850°F.

7. A hydrocarbon conversion process according to claim 1, wherein the volume ratio of vacuum bottoms recycle to said last stage to the fresh feedstock to said first stage is about 0.2-1.5.

8. A hydrocarbon conversion process according to claim 1, wherein the catalyst contains active metal oxides selected from the group consisting of oxides of cadmium, chromium, cobalt, iron, moly-bdenum, nickel tin, tungsten and mixtures thereof, deposited on a support material selected from the group of alumina, silica, and combinations thereof.

9. A hydrocarbon conversion process according to claim 1, wherein the catalyst in said first and said last stage catalytic reactor is cobalt-molybdenum on alumina support.

10. A hydrocarbon conversion process according to claim 1, wherein the catalyst in said first and said last stage catalytic reactor is nickel-molybdenum on alumina support.

11. A hydrocarbon conversion process according to claim 1, wherin the feedstock is petroleum residua.

12. A hydrocarbon conversion process according to claim 1, wherein the feedstock is bitumen derived from tar sands.

13. A hydrocarbon conversion process according to claim 1, wherein the feedstock is raw shale oil.

14. A hydrocarbon conversion process according to claim 1, wherein the reaction system contains two reactors.

15. A hydrocarbon conversion process according to claim 3, wherein said feedstock contains total metals exceeding about 300 ppm and a demetallization type catalyst comprising substan-tially particulate aluminum oxide is used in said first stage reactor.

16. A process for two-stage catalytic high percentage conversion of petroleum feedstocks to produce increased yields of lower boiling hydrocarbon liquids and gases, said process comprising:
(a) feeding a hydrocarbon feedstock with hydrogen into a first stage catalytic reactor containing an ebullated bed of particulate catalyst, said reactor being main-tained at 790-840°F temperature, and 0.25-1.8 Vf/Hr/Vr overall space velocity for partially hydroconverting said feedstock to produce an effluent material;

(b) passing said partially hydroconverted effluent material to a second stage ebullated bed catalytic reactor main-tained at 780-850°F temperature, 1000-3000 psig hydrogen partial pressure, and 0.20-2.0 Vf/Hr/Vr overall space velocity and further hydroconverting the material to produce hydrocarbon gases and lower boiling liquid fractions;
(c) removing said hydrocarbon gas and liquid fractions from said second stage reactor and separating said hydro-carbon gases from said liquid fractions and withdrawing the liquid fractions;
(d) distilling said liquid fractions to produce a medium boiling hydrocarbon liquid product having a normal boiling range of 400-850°F and also a vacuum bottoms material having a nominal boiling point above about 850°F; and (e) recycling at least a portion of said vacuum bottoms material directly to said second stage ebullated catalytic bed reactor to provide a recycle ratio of recycled material to fresh feed of about 0.2-1.5 to achieve at least about 75 V % conversion of 975°F+
fraction to lower boiling material and produce increased yields of said medium boiling hydrocarbon liquid product.