NO136155B - PROCEDURES FOR THE CONVERSION OF AN ASPHALT-CONTAINING HYDROCARBON MATERIAL TO LOWER BOILING HYDROCARBON UNDER THE ACTION OF HYDROGEN AND A NON-STICHIOMETRIC VANADIUM SULFIDATE CATALF. - Google Patents

Description

The present invention relates to a process for converting asphaltene-containing hydrocarbon feed materials into lower-boiling hydrocarbon products. More particularly, the present invention relates to a combination process for a continuous transformation of bottom fractions from vacuum towers or towers operating at atmospheric pressure, crude oil residue, topped crude oils, coal oil extracts, crude oils extracted from tar sands and similar oils, which are referred to in petroleum refining technology as "black oils".

These black oils contain high-molecular sulfur compounds in very large quantities, and in addition large quantities of nitrogen-containing compounds, high-molecular organometallic complexes which essentially comprise compounds of nickel and vanadium, as well as asphalt material. Asphalt material is generally found to be complexly bound or linked to sulfur and to some extent to organometallic compounds. There is an abundant supply of such hydrocarbon-containing material, and most of this has a specific gravity greater than 1.0. Black oils are generally characterized by a boiling range indicating that 10 LV% (liquid volume %), and usually more, has a normal boiling point above a temperature of

about. 566°C.

The combination of processes according to the present invention relates in particular to a conversion of black oils into distillable hydrocarbon products. Specific examples of black oils which are illustrative of those to which the present invention is particularly applicable include water-bottom fraction products with a specific gravity of 1L.02L, and which contain 4.05 wt. asphaltenes as well as a vacuum residue with a specific <weight of A. O09, and which contains 3.0 wt% S, 430O ippm -nitr.ogen and with 20rO LV% distillation temperature of :568°C.. In the case of the present invention makes it possible for the first time to transform the majority of such material. This is in contrast to the experience one has so far when processing such material. Thus, according to the state of the art, such asphaltene-containing oils will not be able to be treated in a fixed-bed process without rapid degradation of efficiency.

One purpose of the present invention is to specify a more efficient process for a hydrogenating conversion of heavy hydrocarbon-containing materials containing asphaltenes.

A further purpose is to convert hydrocarbon-insoluble asphaltenes into hydrocarbon-soluble, low-boiling, normally liquid products.

A particular purpose is to achieve a continuous removal of contaminants from asphaltene-containing black oils by providing a combination process using a solid, support-free catalyst that is minimally deactivated.

The present invention therefore relates to a catalytic suspension process for the conversion of an asphaltene-containing hydrocarbon starting material in the presence of a carrier-free, non-stoichiometric vanadium sulphide catalyst in one conversion zone and where at least part of the catalyst is recycled to the conversion zone, which process is characterized by

(a) a recycle stream comprising a solvent-lean mixture of non-stoichiometric vanadium sulfide and asphaltenes removed from a solvent extraction zone, and

hydrogen is introduced into the transformation zone,

(b) the recirculation stream is reacted with hydrogen in the conversion . the zone under such conditions that the asphaltenes are converted to lower-boiling hydrocarbons, (c) a hydrogen-containing stream is separated from the conversion zone outlet, (d) the remaining, normally flowing portion of the conversion zone outlet, containing the vanadium sulphide, is introduced into the solvent extraction ion sohen in which said liquid portion is deasphalted to provide a bsolvent-rich, normally liquid phase which is removed from the process, and a solvent-lean mixture comprising non-stoichiometric vanadium sulfide and "asphaltenes" as said recycle stream, and (e) the asphaltene-containing hydrocarbon feedstock is introduced into the recycle system by mixing with at least one of the streams selected from among

{1) the recycled stream in step (a), and

(2) the normally floating share of the output from conversion

the zone.

0

In one embodiment, new feed material is supplied to the conversion zone reactor, and the reactor's effluent liquid is deasphalted, after which the solvent-lean fraction obtained by deasphalting is recycled to the reactor.

In another embodiment, the feed material to the process is first deasphalted, and the solvent-lean asphalt fraction is fed to the reactor, and the reactor effluent is recycled to the deasphalting zone.

Other embodiments relate primarily to the operating conditions and the selected solvents used in the solvent extraction.

The basic feature including the use of carrier-free, non-stoichiometric vanadium sulfide as a catalytic agent for the conversion of asphaltene-containing, hydrocarbon-containing black oils is stated in US patent no. 3,558,474. As indicated in this patent, non-stoichiometric vanadium sulfide exhibits an unusual degree of activity with respect to converting the asphalt while simultaneously eliminating significant amounts of sulfur-containing and nitrogen-containing compounds. In this patent nothing is stated about solvent deasphalting.

In all examples in US patent 3,558,474 very large amounts of non-stoichiometric vanadium sulphide precursors are used which are mixed with the feed material in the reaction zone. Example V in this patent shows the use of 832 g of vanadium tetrasulphide added to 1200 g of black oil. One skilled in the art will appreciate that it will not be commercially feasible to add such large amounts of a vanadium compound to black oil.

In an attempt to reduce the large amounts of vanadium compounds that were necessary to carry out the invention according to US patent 3,558,474, an attempt was made to recycle some of the catalyst and let this recycled catalyst be a substitute for the addition of vanadium tetrasulphide. The above-mentioned patent indicated a method for recycling the catalyst. The reactor effluent is fed to a "hot flash separator" (HFS) which is maintained at the reaction zone temperature, but at a lower pressure. The HFS liquid is recycled to combine with new hydrocarbon feedstock, and the vanadium catalyst from this slurry is recovered by filtration from trap tanks or a series of centrifuges.

Both direct recycling of the HFS liquid and recycling of the catalyst recovered from this liquid in a conventional manner were tested. In all cases, the activity of the catalyst decreased upon repeated use.

This was a confusing problem. The catalyst precursor was cleaved to non-stoichiometric vanadium sulfide. This catalyst worked very well the first time it was passed through the reactor, while its activity decreased when it was used again. Chemical analysis of the catalyst indicated that there was no change in composition. Furthermore, all process conditions were kept the same during the experiments.

It was postulated that a possible change in the physical form of the catalyst caused a reduction in its activity. Microscopic analysis of recycled catalyst confirmed that this was the problem. V-5'J recycling, the catalyst built up into larger and larger particles. There also seems to be a build-up of carbon-like material on the catalyst, because these factors apparently reduced the effective surface area of the catalyst, which consequently reduced the reactor's efficiency. Apparently, an almost colloidal dispersion of the catalyst is necessary to achieve effective scarring of black oils. Unfortunately, conventional methods of obtaining colloidal dispersions of the catalyst in black oils were not applicable to this process. A simple physical stirring of the catalyst in the oil left the agglomerated particles practically unchanged in size. Other mechanical means of achieving such a colloidal dispersion would be too expensive to acquire and operate, especially on a commercial scale.

At the same time, attempts were made to remove the carbonaceous material from the catalyst. Washing the catalyst with a "white oil" had little effect on the carbonaceous material.

Finally, the solution was found. It seemed to son that the asphaltenes not only removed some of the carbonaceous material, but that the asphaltenes also acted as a carrier material for the catalyst. Surprisingly, the asphaltenes absorb or adsorb the catalyst and thereby greatly increase its surface area. Furthermore, the asphaltenes act as a relatively good solvent for the carbonaceous material that could be seen on the catalyst. It is not yet known exactly why asphaltenes act so favorably on catalyst recycling, but it has been found that this improvement is real. It is now possible to operate a suspension process for the conversion of black oils economically if a deasphalting zone is operated in connection with the suspension process.

The deasphalting zone can be either "upstream" or "downstream" of the suspension reactor, depending on the feed material and operational variables. The terms "upstream" and "downstream" are used somewhat loosely, since for the catalyst there is always a flow from the suspension reactor to the deasphalting zone of the suspension reactor.

In one embodiment, the deasphalting process is upstream of the suspension process. The feed material to the suspension reactor consists of the asphaltene-rich, solvent-poor fraction from the deasphalting zone. The liquid outlet from the suspension reactor is returned to the inlet of the deasphalting zone. This flow chart results in a suspension reactor of minimal size, although this is offset to a certain extent by an increase in the size of the deasphalting zone, which takes care of both the new supply material and the liquid outlet from the suspension reactor. This stream is preferred when the feed material to the process is amenable to such treatment. Unfortunately, certain crude oils, such as Venezuelan crude oil, contain so much metals and asphaltic material that conventional deasphalting operations yield a product that is still unsuitable for further processing in conventional units.

Conventional fixed-bed units, for example desulphurisation units, can tolerate up to 50 ppm, and in some cases even more, of metals or vanadium in the feed material. If the feed material contains more metals than this, it cannot be treated directly over the solid layer of the desulphurisation catalyst. Deasphalting removes approx. two-thirds of the metals, so that black oils containing up to 150 ppm metals can be deasphalted to give an asphalt-free fraction suitable for further treatment.

Deasphalting alone will result in a large loss of liquid product, as most deasphalting processes remove almost one part by volume of heavy oil for each part by volume of asphalt that is removed'. The present invention, with the deasphalting zone upstream of the reactor, allows the production of an asphalt-free oil with a low metal content with approx. 100 LV% dividend. Furthermore, because recycled asphaltenes act to "steal" metals from the new feed material, the deasphalting zone will remove up to 3/4 of the metals in the feed material, as opposed to 2/3 as obtained with conventional units.

If the black oil feed materials contain very high amounts of metal, neither conventional deasphalting nor deasphalting followed by the suspension treatment of the asphalt fraction will produce an oil with a tolerable metal content. In these cases, it is necessary to pass the black oil through the suspension reactor before deasphalting in order to thereby utilize the metal-removing activity of the suspension process.

These results can be summarized as follows:

ppm metal i

supply

the material

Extension of the invention with the deasphalting zone upstream of the suspension reactor is still possible in certain cases when deasphalting alone will not be sufficient. The recirculating asphaltenic material from the suspension reactor acts in certain cases as a "thief" to remove some of the metals from the feedstock that would otherwise remain in the solvent-rich fraction from the deasphalting zone.

To treat feed materials with a very high metal content or to obtain a product with the lowest possible metal content, it is preferred to operate with the suspension reactor upstream of the deasphalting zone to achieve a better removal of metal. Thus, the crude fraction of the process will first be fed to the suspension reactor and then to the deasphalting zone. The solvent-rich phase from the asphalting step will have a very low vanadium content and can be treated without difficulty in conventional solid-state facilities. The embodiment of the present invention is still necessary because the catalyst must be reused. Deasphalting the effluent from the suspension reactor ensures that there will be a fine dispersion of the catalyst in the asphalt phase from. the deasphalting zone. This asphalt is then recycled to the inriloop of the suspension reactor.

A further economic advantage of this special suspension ion process consists in converting the metal present in the oil into a catalyst.

If other metals were used as catalyst, e.g. Mo, - and if it was not possible to convert the metals in the oil into catalyst, a larger proportion of the bottom fractions from the deasphalting zone had to be continuously diverted to a larger catalyst regeneration unit.

This catalyst regeneration unit would consist of a

large expensive inorganic metal refining unit where first residual oil had to be dissolved from the catalyst, and then the catalyst had to be freed of Ni, Fe and V, followed by regeneration.

When practicing the present invention, it is no longer necessary or desirable to achieve 100% conversion per passing through the asphaltenes. Because the asphaltenes are recycled, it is now possible to make do with a smaller conversion per review and still achieve almost 100% conversion of the asphaltenes fed to the system.

Although an operation to obtain almost 100% conversion per review is possible, and almost was obtained in one review on the basis of US patent no. 3,558,474, this should be avoided when carrying out the present invention. Conversion of 15 - 85% per review of the asphaltenes ensures that there will be sufficient asphalt material in the outlet from the suspension reactor to maintain the catalyst activity and at the same time prevent excessive amounts of asphaltenic material from being recycled. The exact conversion amount pir. review that may be desired in a given installation will depend on the characteristics of the black oil being treated and on the efficiency and type of deasphalting unit used.

Acknowledgment necessitates an acknowledgment of the fact that a number of different techniques are known in the art for achieving solvent deasphalting of asphaltene-containing hydrocarbon feedstock. For the sake of brevity, no attempt will be made to fully describe the state of the art in the area of solvent deasphalting.

Suitable liquid deasphalting solvents include liquefied, normally gaseous hydrocarbons such as propane, n-butane, isobutane or mixtures thereof, as well as ethane, ethylene, propane, propylene, n-butylene, isobutylene, pentane, isopentane and mixtures thereof.

In a particularly preferred solvent defatting operation, butane or panthane is used with a solvent to oil ratio of 4 - 1, and at a temperature of approx. 5 - 6°C below the solvent's critical temperature.

The concentration of non-stoichiometric vanadium sulfide in the batch to the reactor is preferably at least 0.5% by weight, calculated on. base of elemental vanadium. Use of less catalyst results in a very low conversion per review of the asphalt. Excessively high concentrations do not appear to advance the overall results even with extremely contaminated feedstock exhibiting an extremely high asphaltene content. Therefore, the upper limit for vanadium sulphide is approx. 25% by weight.

The colloidal suspension of catalyst and feed material is mixed with at least 350 V/V parts by volume of hydrogen at 15°C at 1 ata. per volume fraction added feed material at 15°C. If you operate there with a smaller hydrogen circulation than this, very poor results are obtained. In the preferred upstream operation of the suspension reactor process, hydrogen is bubbled up through a continuous liquid phase. Due to the presence of a continuous liquid phase, it could be assumed that only the partial pressure of the hydrogen in the reaction zone is important, but this is not the case. Hydrogen transfer to the liquid is to a certain extent diffusion limited. It thus appears that a certain minimum amount of hydrogen that is bubbled through the liquid suspension is necessary. Operated there with less than. the above-mentioned mine imuras hydrogen circulation, very poor results are obtained. It is preferred to work with lOOO - 2000 V/v and approx. 1300 V/V seems to be optimal at the pressure sleepers currently used. The use of more hydrogen than 2000 V/v is not harmful, but does not seem to improve the operation of the process. The compressor costs become very large at hydrogen circulation ratios that exceed 10,000 V/V, so that 10,000 V/V is a practical upper limit for hydrogen circulation.

Reaction zone pressure can be in the range 25 - 500 ata - Higher pressures seem to promote the hydrogenation reactions that occur. Higher pressures also somewhat reduce the capital expenditure and operating costs for the hydrogen recirculation compressors that are often used within the process. Unfortunately, higher pressures will necessitate a heavier construction of the reaction zone.

As the reactions carried out in principle are exothermic, the temperature of the effluent from the reaction zone will be significantly higher than the inlet temperature. In addition, a minimum temperature of 300 - 400°C is necessary for the reaction to take place. Inlet temperatures that exceed 435°C should be avoided because higher temperatures promote dehydrogenation rather than hydrogenation. E.g. by only heating cyclohexane to 350°C, benzene and hydrogen will be produced even if no catalyst is present. Higher temperatures will generally increase the reaction rate, but this is offset by an unfavorable shift in the equilibrium. For the same reasons, the residence time in the reaction zone and the inlet temperature must be such that the outlet temperature is not higher than 500°C. Although suitable results are obtained at a maximum reaction temperature of 500°C, the preferred method of operation limits the maximum reaction temperature to 450°C.

Although the present process can be carried out in an extended reaction zone where the suspension and hydrogen are introduced in the upper part thereof, and the effluent is removed from the lower part, an upstream system is preferred. A significant advantage lies in the fact that the extremely heavy part of the feed material, especially the part that has a normal boiling point above 566°C has a significantly longer residence time inside the reaction zone, which promotes conversion of this heavy fraction of the feed material.

The effluent from the suspension reactor, containing the vanadium sulphide catalyst and unsaturated asphaltenes, is introduced into a suitable separation system from which a hydrogen-rich gas phase is extracted, preferably for recycling as feed material to the suspension reactor. This hydrogen separation system is not considered an essential feature of the present combination process. It may comprise one or more chambers from which hydrogen and other normally gaseous streams can be separated and recovered. The normally liquid portion of the product effluent, again including the vanadium sulphide catalyst and untreated asphaltenes, is preferably introduced into the upper part of a solvent-deasphalting zone, in which it is brought into countercurrent contact with a suitable selective solvent which is introduced into the lower part of the deasphalting zone. The solvent deasphalting zone can also treat the entire feed stream to the process, so that the feed stream to the deasphalting zone can consist of new feed material and the liquid outlet from the suspension reactor.

The solvent deasphalting zone will operate at a temperature in the range 10 - 300°C, preferably in the range 35 - 130°C,

and in a pressure range of 5-75 ata., preferably 15-40 ata. The exact operating conditions will generally depend on the physical properties of the feed material that is fed to the deasphalting unit as well as on the chosen solvent. In general, temperature and pressure are chosen so that desalination takes place in the liquid phase and to ensure that all of the catalyst particles are removed with the solvent-poor heavy phase. Suitable solvents include ethane, methane, propane, butane, isobutane, pentane, isopentane, neopentane, hexane, isohexane, heptane and similar hydrocarbons. Correspondingly, the solution can easily be a normally liquid naphtha fraction containing hydrocarbons with 5-14 carbon atoms per molecule, and preferably a naphtha fraction if it has a final boiling point below 95°C.

The solvent-rich, normally liquid phase is introduced

in a suitable solvent recovery system; the construction and applied technique of such a system is well described within the state of the art. Correspondingly, the solvent-lean, asphaltene-containing phase is introduced into the suspension reactor. This is the case regardless of whether the desalination zone is "opstroms" or "heds troms" Tor the suspension reactor. Black oil conversion appears to be favorably affected when carried out in the presence of hydrogen sulphide. It is therefore within the scope of the present invention to introduce ~2.5 - 25 mol of hydrogen sulphide into the hydrogen which is introduced into the suspension reactor.

An embodiment is shown in the attached drawing by means of a simplified flow diagram showing an embodiment of the present invention where the deasphalting zone is "downstream" in relation to the suspension reactor.

The feed material in pipeline 1 is mixed with recirculating hydrogen from pipeline 2 and supplementary hydrogen from pipeline 3. Non-stoichiometric vanadium sulphide catalyst, mixed, with asphaltenes, is introduced via pipelines 4 and 1 to the lower part of reaction zone 5. The total product effluent, including vanadium sulphide catalyst and unreacted asphaltenes are removed from reactor 5 via pipeline 6 and introduced into a suitable hydrogen separation system 7. The hydrogen separation system 7 can be a multi-chamber separation system in which hydrogen is removed via pipeline 2 for recycling, while other gaseous components can be removed from the process. Normally liquid reactor effluent, e.g. hexane and heavier hydrocarbons, including vanadium sulphide catalyst and unreacted asphaltenes, are extracted from the hydrogen separation zone 7 through pipeline 8 and thereby introduced into the upper part of deasphalting zone 9.

A suitable selective solvent, for example n-butane, is introduced into the lower part of the deasphalting zone 9 via pipe IO. Supplemental solvent can be supplied to pipeline 10 via pipeline 11. The solvent-rich, normally liquid material is extracted from the upper part of deasphalting zone 9 via pipeline 12 and is introduced into the solvent recycling system 13 in which the solvent is recovered and recycled via pipeline IO. The normally liquid product effluent is withdrawn from the process via pipeline 14. The precipitated vanadium sulphide catalyst and unreacted asphaltenes are withdrawn from deasphalting zone 9 and recycled to combine with the feed material and hydrogen. As most black oil feed materials contain significant amounts of metals, especially nickel and vanadium which exist in the form of metallic porphyrins, a tappet drum is preferably drawn off via pipeline 15, and fed to a suitable metal recovery unit. This technique prevents an unnecessary buildup of metals inside the system.

The hydrocarbon feed used in these examples was a vacuum bottoms fraction with a specific gravity of 1.028, an initial boiling point of 286°C, a 10.0 LV% distillation temperature of 514°C and a 24.0 LV% distillation temperature of 566°C. The feed material contained 13.3 wt% asphaltenes, 4.88 wt% sulphur, 0.48 wt% nitrogen, 400 ppm vanadium and 70 ppm nickel, the latter existing in the form of organometallic porphyrins.

Example I

In this example, 200 g/hour of the feed material was mixed with 0.535 m 3 /hour of hydrogen, Tiåolt at 15°C and 1 ata, or approx. 2.680 W/W. 17.0 mol% hydrogen sulphide was present in the recycle gas. The reaction zone pressure was 205 ata, and the reaction zone peak temperature was 443°C. The normally liquid portion of the product effluent was deasphalted with propane at a temperature of 67°C at a pressure sufficient to achieve a liquid phase operation.

About. 3.2 wt% catalyst was used to convert a peaked crude oil with a specific gravity of 1.008 containing 10.53 wt% asphaltenes, 2.80 wt% sulfur and 573 ppm metals. The vanadium sulfide catalyst was treated to remove all benzene soluble materials and asphaltenes, and the asphaltene-free catalyst was reused twice. The results obtained are shown in table I.

Catalyst deactivation is clearly evident. In trial No. 3, the specific gravity had increased to 0.955, residual asphaltenes had increased to 2.40% by weight and 1.81% by weight of sulfur remained in the product.

The feed material was then changed to the vacuum column bottoms fraction as previously described. The vanadium sulfide catalyst, 7.9% by weight, was reused four times after removal of benzene-soluble material and residual asphaltenes. The following table II shows the results of the last three lapsed trials.

The catalyst deactivation is particularly noticeable with regard to the remaining asphaltene concentration, with fresh catalyst more than 99% of the asphaltenes present are converted, while the above figures indicate approx. 87% asphaltene conversion.

Example II

In the next trial series, the catalyst concentration was reduced to 2.8% by weight and 3.5% by weight, calculated as elemental vanadium, and the temperature was lowered to 425 - 430°C. The only other operational change following the method according to the present invention, whereby the catalytic vanadium sulfide was instantly recycled from the deasphalting zone together with unreacted asphaltenes.

The vanadium sulphide catalyst was reused twice, and the catalyst in the initial trial (7) was 3.5% by weight, and in the other two trials (8) and (9) 2.8% by weight. The results are shown in the following table

III.

These results clearly indicate the advantages obtained by utilizing the present combination process, and it is completely unexpected considering that the catalyst concentration has been lowered from 7.9% by weight to 2.8% by weight.

In this example, the process flowchart is modified according to another embodiment of the present invention.

The feed material is first deasphalted, and the solvent-poor, asphaltene-containing fraction is introduced alone to the conversion zone.

The effluent from the conversion zone is fed to a hydrogen separation system. Normally liquid effluent is then recycled to the inlet of the deasphalting zone for treatment of feed material. Thus, both the feed material and the reactor effluent are deasphalted

in the deasphalting zone. Asphaltenes converted in the conversion zone are removed as a product in the solvent-rich phase that is recovered from the deasphalting zone.

Example III

The feed material, a vacuum column fraction, as previously described, is mixed with unreacted asphaltenes and non-stoichiometric vanadium sulphide and subjected to n-butane deasphalting at a temperature of approx. 70°C and at a pressure sufficient to maintain a liquid phase operation. About. 70 UV % of the original feed material is removed as a solvent-rich phase and fed into a suitable solvent recovery system. The remaining heavy oil components, precipitated asphaltenes and non-stoichiometric vanadium sulphide in an amount of 3% by weight, calculated on elemental vanadium, are introduced into the lower part of a reaction zone which is kept at a maximum temperature of approx. 425°C and a pressure of approx. 205 ata. The feed material is mixed with a hydrogen-rich gas in an amount to give 3000 V/V. This gas contains approx. 15 mol% H2S.

After separation of the reaction product effluent to recover hydrogen for recycling to the reaction zone, the liquid is mixed with new hydrocarbon feed material and introduced into the solvent deasphalting zone. After approx. 100 hours of operation draws a drain current of approx. IO LV % of the reactor liquid effluent which, after removal of hydrocarbon-soluble products, is fed to a metal recovery system.

Analysis of the normally liquid product, i.e. the asphaltene-free phase from the deasphalting zone and including hydrocarbon recovered from the tap stream, when possible, indicated a recovery of the initial catalyst activity. At approx. After 50 hours of operation, the remaining asphaltene concentration was approx. 1.25%, at 142 hours it had dropped to approx. 0.50%, at 175 hours the remaining asphaltene content was approx. 0.25%, and after approx. 2QO hours the concentration was -approx. 0.10%.

The foregoing clearly indicates the method by which the present invention is carried out and the advantages obtained from its application. The liquid product obtained by the method according to the present invention is essentially free of asphaltenes, and its metal content is reduced and thus constitutes an ideal feed material for subsequent treatment in fixed bed hydrogen desulphurisation or hydrocracking units.

Claims (8)

1. Catalytic suspension process for converting an asphaltene-containing hydrocarbon feedstock in the presence of a carrier-free non-stoichiometric vanadium sulfide catalyst in a conversion zone and wherein at least a portion of the catalyst is recycled to the conversion zone, characterized in that (a) a recycle stream comprising a solvent lean mixture of non-stoichiometric vanadium sulfide and asphaltenes removed from a solvent extraction zone, and hydrogen is introduced into the conversion zone, (b) the recycle stream is reacted with hydrogen in the conversion zone under such conditions that the asphaltenes are converted to lower-boiling hydrocarbons, (c) a hydrogen containing stream is separated from the conversion zone effluent, (d) the remaining, normally liquid portion of the conversion zone effluent, containing the vanadium sulphide, is introduced into the solvent extraction zone in which said final portion is deasphalted to give a solvent-rich, normally liquid phase which is removed from the process , and a pilot solvent-lean mixture comprising non-stoichiometric vanadium sulfide and asphaltenes as the aforementioned recycle ion stream, and (e) the asphaltene-containing hydrocarbon feedstock is introduced into the recycling system by mixing with at least one of the streams selected from (1) the recycled stream in step (a), and (2) the normally flowing fraction of the effluent from the transformation zone. 2. Method according to claim 1, characterized in that at least part of the hydrogen-containing stream from step (c) is recycled to contribute at least part of the hydrogen required in step (a). 3. Method according to claim 1 or 2, characterized in that the hydrogen hydrocarbon ratio in the conversion zone is kept at 350 - 10,000, preferably 1,000 - 2,000 volume parts of hydrogen at 15°C, 1 ata per volume part hydrocarbon at 15°C, that the pressure in the reaction zone is kept at 25 - 500 ata and that the reaction zone temperature is kept at 300 - 500°C. 4. Method according to claims 1 - 3, characterized in that the non-stoichiometric vanadium sulphide, considered as elemental vanadium, is kept at 0.5-25% by weight of the normally flowing stream which is supplied to the conversion zone. 5. Method according to claims 1-4, characterized in that a light hydrocarbon containing solvent is used. 1 - -7 carbon atoms. 6. Method according to claim 5, characterized in that n-butane is used as solvent. 7. Method according to claims 1-4, characterized in that a naphtha fraction containing hydrocarbons with 5-14 carbon atoms per molecule. 8. Method according to claims 1-7, characterized in that the volume ratio between solvent and hydrocarbon supply is kept at 3:1 to 15:1.