Abstract:
In the preparation of a heavy oil with a low Ramsbottom Carbon Test (RCT) from a long residue by a two-stage process comprising catalytic hydrotreatment followed by solvent deasphalting and recycle of the asphalt to the first stage the catalytic hydrotreatment for RCT reduction in the first stage is carried out at such a severity that the C 4   -   gas production per percent RCT reduction is kept between defined limits.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a process for the preparation of a hydrocarbon mixture having a Ramsbottom Carbon Test value (RCT) of (a) %w and an initial boiling point of T 1  ° C. 
     The RCT is an important parameter in the assessment of the suitability of heavy hydrocarbon mixtures as feedstocks for catalytic conversion processes, such as catalytic cracking, carried out in the presence or absence of hydrogen, for the preparation of light hydrocarbon distillates, such as gasoline and kerosine. According as the feed has a higher RCT, the catalyst will be deactivated more rapidly in these processes. 
     Residual hydrocarbon mixtures, such as residues obtained in the distillation of a crude mineral oil and asphaltic bitumen separated in the solvent deasphalting of the said distillation residue or of residues obtained in the distillation of a hydrotreated residual fraction of a crude mineral oil generally have too high an RCT to be suitable without previous treatment for use as feeds for the above-mentioned catalytic conversion processes. Since the RCT of residual hydrocarbon mixtures is mainly determined by the percentage of asphaltenes present in the mixtures, a reduction of the RCT of these mixtures can be obtained by reducing the asphaltenes content. Basically, this may be achieved in two ways. Part of the asphaltenes may be separated from the mixture by solvent deasphalting, or part of the asphaltenes may be converted by subjecting the mixture to a catalytic hydrotreatment. For the reduction of the RCT of distillation residues the latter method is preferred, in the first place, because its yield of heavy product with a low RCT is higher and further because, in contrast to the former method, where asphaltic bitumen is obtained as a by-product, it yields a valuable C 5   +   atmospheric distillate as a by-product. In view of the fact that when the former method is applied to asphaltic bitumen, yields are low, only the latter method is eligible for the preparation of heavy product with a low RCT from asphaltic bitumen or from mixtures of asphaltic bitumen and distillation residue. A drawback to the latter method, however, is that it gives rise to the formation of an undesirable C 4   -  fraction which, moreover, contributes considerably to the hydrogen consumption of the process. 
     Applicants have carried out an investigation into the reduction of the RCT through catalytic hydrotreatment of mixtures of an atmospheric residue obtained in the distillation of a crude mineral oil (for the sake of brevity hereinafter referred to as &#34;atmospheric residue I&#34;) and an asphaltic bitumen separated in the solvent deasphalting of a residue obtained in the distillation of a hydrotreated residual fraction of a crude mineral oil (for the sake of brevity hereinafter referred to as &#34;asphaltic bitumen I&#34;), which mixtures comprise less than 50 pbw of asphaltic bitumen I per 100 pbw of atmospheric residue I. The investigation has shown that, according as the catalytic hydrotreatment is carried out under more severe conditions in order to attain a greater RCT reduction, the parameter &#34;C 4   -  production per % RCT reduction&#34; (for the sake of brevity hereinafter referred to as &#34;G&#34;) at first remains virtually constant (G c ) and subsequently shows a fairly sharp increase. In view of the hydrogen consumption of the process it is important to take care that the RCT reduction is not carried beyond the value corresponding with G=2×G c . This means that in practice there will be a number of cases in which it is undesirable, starting from a mixture of an atmospheric residue I and an asphaltic bitumen I, which mixture comprises less than 50 pbw of asphaltic bitumen I per 100 pbw of atmospheric residue I, to employ nothing but a catalytic hydrotreatment for the preparation of a product from which, after separation of an atmospheric distillate, an oil can be obtained having an initial boiling point of T 1  ° C. and an RCT of (a) %w. In those cases there is nevertheless an attractive manner of preparing from a vacuum residue I an oil having the afore-mentioned initial boiling point and RCT from a mixture of an atmospheric residue I and an asphaltic bitumen I which mixture comprises less than 50 pbw of asphaltic bitumen I per 100 pbw of atmospheric residue I (for the sake of brevity hereinafter referred to as &#34;residual feed mixture&#34;). To this end the product obtained in the catalytic hydrotreatment is separated by distillation into an atmospheric distillate and an atmospheric residue having an initial boiling point of T 1  ° C. (for the sake of brevity hereinafter referred to as &#34;atmospheric residue II&#34;). The process may be continued in two ways. First, from the atmospheric residue II so much asphaltic bitumen may be separated by solvent deasphalting that a deasphalted atmospheric residue having the desired RCT of (a) %w is obtained. Secondly, atmospheric residue II may be separated by distillation into a vacuum distillate and a vacuum residue and from the vacuum residue so much asphaltic bitumen may be separated by solvent deasphalting that a deasphalted vacuum residue is obtained having an RCT which is such that when this deasphalted vacuum residue is mixed with the previously separated vacuum distillate, an oil is obtained which has the desired RCT of (a) %w. The most attractive balance between yields of: C 4   -  fraction, C 5   +  atmospheric distillate, asphaltic bitumen and oil having an initial boiling point of T 1  ° C. and an RCT of (a) %w is obtained when the catalytic hydrotreatment is carried out under such conditions that G lies between 1.5×G c  and 2.0×G c . When the catalytic hydrotreatment is carried out under such conditions that G&lt;1.5×G c , a low C 4   -  production is still obtained, but the yield of oil having an initial boiling point of T 1  ° C. and an RCT of (a) %w in the combination process is unsatisfactory. When the catalytic hydrotreatment is carried out under such conditions that G&gt;2.0×G c , a high yield of oil having an initial boiling point of T 1  ° C. and an RCT of (a) %w in the combination process is still obtained, but is attended with unacceptably high C 4   -  production. 
     Applicants have found that the RCT reductions in the catalytic hydrotreatment of a residual feed mixture, in which for G values are reached which correspond with 1.5×G c  and 2.0×G c , are dependent on 
     (1) the desired initial boiling point of the oil having an RCT of (a) %w to be prepared (T 1  ° C.), 
     (2) the RCT of atmospheric residue I (b %w), 
     (3) the percentage by weight of atmospheric residue I boiling below 520° C. (f %w), 
     (4) the RCT of asphaltic bitumen I (c %w), and 
     (5) the asphaltic bitumen I/atmospheric residue I mixing ratio in the residual feed mixture, expressed in pbw of asphaltic bitumen I per 100 pbw of atmospheric residue I (r pbw), 
     and are given by a numerical relation. 
     SUMMARY OF THE INVENTION 
     A process is disclosed for the preparation of a hydrocarbon mixture having an RCT of (a) %w and an initial boiling point of T 1  ° C., wherein a mixture of an atmospheric residue I obtained in the distillation of a crude mineral oil, which atmospheric residue has an RCT of (b) %w and boils below 520° C. to an extent of (f) %w, and an asphaltic bitumen I separated in the solvent deasphalting of a residue obtained in the distillation of a hydrotreated residual fraction of a crude mineral oil, which asphaltic bitumen has an RCT of (c) %w, which mixture comprises less than 50 pbw of asphaltic bitumen I per 100 pbw of atmospheric residue I, is subjected to a catalytic hydrotreatment in order to reduce the RCT; the product obtained is separated by distillation into an atmospheric distillate and an atmospheric residue II having an initial boiling point of T 1  ° C.; either so much asphaltic bitumen is separated from the atmospheric residue II by solvent deasphalting that a deasphalted atmospheric residue having the desired RCT of (a) %w is obtained, or atmospheric residue II is separated by distillation into a vacuum distillate and a vacuum residue, from which vacuum residue so much asphaltic bitumen is separated by solvent deasphalting that a deasphalted vacuum residue is obtained which has such an RCT that, when it is mixed with the vacuum distillate, a mixture having the desired RCT of (a) %w is obtained; and the catalytic hydrotreatment is carried out under such conditions as to satisfy the relation: ##EQU1## d=the RCT of the mixture of atmospheric residue I and asphaltic bitumen I, e=the RCT of atmospheric residue II, and 
     r=the number of pbw of asphaltic bitumen I per 100 pbw of atmospheric residue I present in the feed mixture. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The relation found by the Applicants in the first place offers an opportunity of determining whether, in view of the maximum acceptable value of G (corresponding with 2.0×G c ), it is possible by catalytic hydrotreatment alone, starting from a residual feed mixture having a given mixing ratio r, in which atmospheric residue I has an RCT of (b) %w, (f) %w boils below 520° C. and asphaltic bitumen I has an RCT of (c) %w, to prepare a product from which, by distillation, an atmospheric residue II can be obtained which has a given RCT of (a) %w. If, according to the relation, this proves impossible and, therefore, the combination route has to be applied, the relation further indicates the limits between which, in the catalytic hydrotreatment of the combination route, the RCT reduction should be chosen in order to ensure optimum efficiency of the combination route. 
     The present patent application therefore relates to a process for the preparation of a hydrocarbon mixture with an RCT of (a) %w and an initial boiling point of T 1  ° C., in which a residual feed mixture is subjected to a catalytic hydrotreatment, in which the product obtained is separated by distillation into an atmospheric distillate and an atmospheric residue II, in which either so much asphaltic bitumen is separated from the atmospheric residue II by solvent deasphalting that a deasphalted atmospheric residue having the desired RCT of (a) %w is obtained, or the atmospheric residue II is separated by distillation into a vacuum distillate and a vacuum residue, from which vacuum residue so much asphaltic bitumen is separated by solvent deasphalting that a deasphalted vacuum residue is obtained which has such an RCT that, when it is mixed with the vacuum distillate, a mixture having the desired RCT of (a) %w is obtained, and in which the catalytic hydrotreatment is carried out under such conditions that the afore-mentioned relation is satisfied. 
     In the process according to the invention the RCT (b) of the atmospheric residue (I) used as feed component, the RCT (c) of asphaltic bitumen I used as feed component, the RCT (a) of the hydrocarbon mixture to be prepared, and the RCT (e) of atmospheric residue II should be known. When the hydrocarbon mixture to be prepared is a mixture of a vacuum distillate and a deasphalted vacuum residue, the RCT&#39;s of the two components of the mixture and the RCT of the vacuum residue II that was deasphalted, should be known as well. As regards the way in which the RCT&#39;s of the various hydrocarbon mixtures are determined, the following three cases may be distinguished. 
     (a) The viscosity of the hydrocarbon mixture to be investigated is so high that it is impossible to determine the RCT by ASTM method D 524. In this case, the CCT (Conradson Carbon Test value) of the mixture is determined by ASTM method D 189, and the RCT is computed from the CCT according to the formula: 
     
         RCT=0.649×(CCT).sup.1.144 
    
     (b) The viscosity of the hydrocarbon mixture to be investigated is such that the RCT can still be determined according to the ASTM D 524 method, but this method gives an RCT value which lies above 20.0% w. In this case, as in the case mentioned under (a), the CCT of the mixture is determined by ASTM method D 189 and the RCT is computed from the CCT according to the formula mentioned under (a). 
     (c) The viscosity of the hydrocarbon mixture to be investigated is such that the RCT can be determined by ASTM method D 524 and this method gives an RCT value not higher than 20.0% w. In this case the value thus found is taken to be the RCT of the mixture concerned. 
     In practice, for the determination of the RCT&#39;s of vacuum distillates, atmospheric residues, deasphalted distillation residues and mixtures of vacuum distillates and deasphalted distillation residues the direct method described under (c) will in many cases be sufficient. In the determination of the RCT of vacuum residues both the direct method described under (c) and the indirect method described under (b) are used. In the determination of the RCT of asphaltic bitumens the indirect method described under (a) is usually the only one eligible. 
     The process according to the invention is a two-step process in which reduction of the RCT is attained through reduction of the asphaltenes content. In the first step of the process the asphaltenes content is reduced by converting part of the asphaltenes by means of a catalytic hydrotreatment. In the second step of the process the asphaltenes content is reduced by separating part of the asphaltenes by means of solvent deasphalting. 
     Residual feed mixtures usually contain an appreciable percentage of metals, especially vanadium and nickel. When such residual feed mixtures are subjected to a catalytic treatment, e.g., a catalytic hydrotreatment for RCT reduction, as in the process according to the invention, these metals will be deposited on the RCT-reduction catalyst, thus shortening its life. In view of this, residual feed mixtures having a vanadium+nickel content of more than 50 ppmw should preferably be subjected to demetallization before being contacted with the RCT-reduction catalyst. This demetallization may very suitably be carried out by contacting the residual feed mixture, in the presence of hydrogen, with a catalyst consisting more than 80% w of silica. Both catalysts consisting entirely of silica and catalysts containing one or more metals having hydrogenating activity, in particular a combination of nickel and vanadium, on a carrier substantially consisting of silica, are eligible for the purpose. Very suitable demetallization catalysts are those which meet certain given requirements as regards their porosity and particle size and which are described in Netherlands patent application No. 7,309,387. When in the process according to the invention a catalytic demetallization in the presence of hydrogen is applied to the residual feed mixture, this demetallization may be carried out in a separate reactor. Since the catalytic demetallization and the catalytic RCT reduction can be carried out under the same conditions, both processes may very suitably be carried out in the same reactor containing, successively, a bed of demetallization catalyst and a bed of RCT-reduction catalyst. 
     It should be noted that in the catalytic demetallization the reduction of the metal content is accompanied by some reduction of the RCT. The same applies to the catalytic RCT reduction in which the RCT reduction is accompanied by some reduction of the metal content. For application of the relation upon which the present invention is based, RCT reduction should be taken to be the total RCT reduction occurring in the catalytic hydrotreatment (i.e., including that occurring in a possible catalytic demetallization process). 
     Suitable catalysts for carrying out the catalytic RCT reduction are those which contain at least one metal chosen from the group formed by nickel and cobalt and, in addition, at least one metal chosen from the group formed by molybdenum and tungsten on a carrier, which carrier consists more than 40% w of alumina. Very suitable RCT-reduction catalysts are those which comprise the metal combination nickel/molybdenum or cobalt/molybdenum on alumina as the carrier. 
     The catalytic RCT reduction is preferably carried out at a temperature of 300-500° C., a pressure of 50-300 bar, a space velocity of 0.02-10 g.g -1 .h -1  and a H 2  /feed ratio of 100-5000 Nl/kg. Particular preference is given to carrying out the catalytic RCT reduction at a temperature of 350-450° C., a pressure of 75-200 bar, a space velocity of 0.1-2 g.g -1 .h -1  and a H 2  /feed ratio of 500-2000 Nl/kg. As regards the conditions to be used in a catalytic demetallization process in the presence of hydrogen, to be carried out if necessary, the same preference applies as that stated hereinbefore for the catalytic RCT reduction. 
     The desired RCT reduction in the first step of the process according to the invention may, for instance, be achieved by application of the space velocity (or temperature) pertaining to that RCT reduction, which can be read from a graph composed on the basis of a number of catalytic hydrotreatment scouting experiments with the residual feed mixture carried out at different space velocities (or temperatures) and in which the RCT reductions achieved have been plotted against the space velocities (or temperatures) used. Apart from the space velocity or temperature, which is variable, the other conditions in the scouting experiments are kept constant and chosen equal to those which will be used when the process according to the invention is applied in practice. 
     The second step of the process according to the invention is a solvent deasphalting step applied to a residue from the distillation of the hydrotreated product of the first step. The distillation residue to which the solvent deasphalting step is applied may be an atmospheric residue or a vacuum residue from the hydrotreated product. Preferably, a vacuum residue from the hydrotreated product is used for the purpose. Suitable solvents for carrying out the solvent deasphalting are paraffinic hydrocarbons having 3-6 carbon atoms per molecule, such as n-butane and mixtures thereof, such as mixtures of propane with n-butane and mixtures of n-butane with n-pentane. Suitable solvent/oil weight ratios lie between 7:1 and 1:1 and in particular between 4:1 and 2:1. The solvent deasphalting is preferably carried out at a pressure between 20 and 100 bar. When n-butane is used as the solvent, the deasphalting is preferably carried out at a pressure of 35-45 bar and a temperature of 100-150° C. 
     When the RCT reduction in the second step of the process according to the invention takes place by solvent deasphalting of an atmospheric residue, the desired RCT of the deasphalted atmospheric residue may be attained, for instance, by using the deasphalting temperature pertaining to that RCT, which can be read from a graph composed on the basis of a number of deasphalting scouting experiments with the atmospheric residue carried out at different temperatures, in which the RCT&#39;s of the deasphalted atmospheric residues obtained have been plotted against the temperatures applied. Apart from the temperature, which is variable, the other conditions in the scouting experiments are kept constant and chosen equal to those which will be used when the process according to the invention is applied in practice. 
     When the RCT reduction in the second step of the process according to the invention takes place by solvent deasphalting of a vacuum residue, after which the deasphalted vacuum residue is mixed with the vacuum distillate separated earlier, the RCT and the quantity of the deasphalted vacuum residue should be adjusted to the quantity and the RCT of the vacuum distillate as follows. When a given quantity of vacuum distillate (VD) of A pbw having a given RCT VD  is available, then, in order to obtain a mixture M having a given RCT M  by mixing the vacuum distillate with deasphalted vacuum residue (DVR), B pbw of deasphalted vacuum residue will have to be prepared, its RCT DVR  being such that it obeys the relation: ##EQU2## or, expressed otherwise, 
     
         A(RCT.sub.M -RCT.sub.VD)=B(RCT.sub.DVR -RCT.sub.M). 
    
     In the equation mentioned hereinabove the left-hand member is known. In addition, in the right-hand member RCT M  is known. On the basis of a number of deasphalting scouting experiments carried out with vacuum residue at, for instance, different temperatures, a graph can be composed in which the term B(RCT DVR  -RCT M ) has been plotted against the temperature used. The temperature to be applied in the deasphalting in the second step of the process according to the invention may be read from this graph, this being the temperature at which the term B(RCT DVR  -RCT M ) has the given value A(RCT M  -RCT VD ). Apart from the temperature, which is variable, the other conditions in the scouting experiments on deasphalting are kept constant and chosen equal to those which will be applied when the process according to the invention is used in practice. 
     Besides the RCT, the metal content is also an important parameter in assessing the suitability of heavy hydrocarbon oils as feeds for catalytic conversion processes, in the presence or absence of hydrogen, for the preparation of light hydrocarbon distillates, such as gasoline and kerosine. According as the feed has a higher metal content, the catalyst will be deactivated more rapidly in these processes. As a rule, residual feed mixtures obtained in the distillation of a crude mineral oil have not only too high an RCT, but also too high a metal content to be suitable, without treatment, as feeds for the afore-mentioned catalytic conversion processes. The product obtained in the process according to the invention is a deasphalted atmospheric residue or a mixture of a vacuum distillate and a deasphalted vacuum residue, which product, in addition to a low RCT, has a very low metal content. This is due to a considerable extent to the fact that the metal-containing distillation residue which is subjected to solvent deasphalting has been catalytically hydrotreated. For, the solvent deasphalting of such metal-containing residues shows a very high metal-removing selectivity. 
     The asphaltic bitumen I used in the process according to the invention as a component of the feed for the first step should be separated in the solvent deasphalting of a residue obtained in the distillation of a hydrotreated residual fraction of a crude mineral oil. Examples of the said residual fractions are atmospheric residues and vacuum residues obtained in the distillation of a crude mineral oil and asphaltic bitumen separated in the solvent deasphalting of these residues. A very attractive embodiment of the process according to the invention is that in which the asphaltic bitumen I used as a component of the feed for the first step is the asphaltic bitumen obtained in the solvent deasphalting in the second step. The conditions for attaining the desired RCT reduction in the first step of the process, with recirculation of asphaltic bitumen, may be determined as follows. The relations found is used to determine the RCT reduction to be employed in the catalytic hydrotreatment in order to ensure optimum efficiency in the combination process, when atmospheric residue I is the only feed used. The space velocity to be used for the purpose is determined on the basis of a number of catalytic hydrotreatment scouting experiments using atmospheric residue I as the feed. Using this space velocity and starting from atmospheric residue I as the feed, an oil is prepared in the combination process, which has the desired RCT of (a) %w and the desired initial boiling point of T 1  ° C., and an asphaltic bitumen (asphaltic bitumen A) is obtained as a by-product. Subsequently, the relation found is used to determine the RCT reduction to be employed in the catalytic hydrotreatment in order to ensure optimum efficiency in the combination process when a mixture of atmospheric residue I and asphaltic bitumen A having the desired ratio r is used as the feed. The space velocity to be used for the purpose is determined on the basis of a number of catalytic hydrotreatment scouting experiments using the mixture of vacuum residue I and asphaltic bitumen A as the feed. Using this space velocity and starting from the mixture of atmospheric residue I and asphaltic bitumen A, an oil is prepared in the combination process which has the desired RCT of (a) %w and the desired initial boiling point of T 1  ° C., and an asphaltic bitumen (asphaltic bitumen B) is obtained as a by-product. Optionally these experiments are repeated once or several times, in each case using the asphaltic bitumen from a preceding series of experiments as the mixing component for atmospheric residue I (at constant values of r) in a following series of experiments, until the moment has come when two successive series of experiments yield asphaltic bitumens having virtually equal RCT&#39;s, thus is determined the space velocity which is required for the application in actual practice of the process according to the invention with recirculation of asphaltic bitumen. Generally, three series of experiments are sufficient to produce the stationary state. 
    
    
     The invention is now illustrated with the aid of the following example, which is intended to be a complete specific embodiment of the invention and is not intended to be regarded as a limitation thereof. 
     EXAMPLE 
     In the first part of the investigation a residual feed mixture AB was used which had been obtained by mixing 100 pbw of an atmospheric residue A and 15 pbw of an asphaltic bitumen B. Atmospheric residue A had been obtained through the distillation of a crude mineral oil. Atmospheric residue A had an RCT of 9.8% w (determined by ASTM method D 524), and a vanadium+nickel content of 95 ppmw and boiled below 520° C. to an extent of 50% w. Asphaltic bitumen B had been separated in the solvent deasphalting with butane of a vacuum residue obtained in the distillation of a hydrotreated vacuum residue from a crude mineral oil. Asphaltic bitumen B had an RCT of 35% w (calculated from the CCT determined by ASTM method D 189) and a vanadium+nickel content of 110 ppmw. 
     As regards the question whether it is possible, in view of the maximum permissible value of G, starting from residual feed mixture AB, to prepare by nothing but catalytic hydrotreatment a product from which, by distillation, an atmospheric residue can be obtained which has an initial boiling point of 370° C. and an RCT lower than that of residual feed mixture AB, application of the relation found, in the form: ##EQU3## (where F max  is the maximum value of the right-hand member of the relation), with substitution of b=9.8, c=35, r=15, T 1  =370 and f=50, shows that this is quite feasible provided that the atmospheric residue with an initial boiling point of 370° C. to be prepared has an RCT (c) higher than 7.1% w. This means, for instance, that, starting from the residual feed mixture AB, for the preparation of an atmospheric residue having an initial boiling point of 370° C. and an RCT (e) of 8.5% w a catalytic hydrotreatment alone will be sufficient. 
     If, however, from residual feed mixture AB an oil is to be prepared having an initial boiling point of 370° C. and an RCT of 1.5% w, a catalytic hydrotreatment alone is not sufficient in view of the maximum permissible value of G. Then, in addition to the catalytic hydrotreatment, a solvent deasphalting treatment should be applied. Application of the relation found, in the form: 
     maximum RCT reduction=F max , and 
     minimum RCT reduction=F min   
     (where F max  and F min  are the maximum and the minimum value, respectively, of the right-hand member of the relation), with substitution of b=9.8, c=35, r=15, T 1  =370 and f=50, shows that for optimum utilization of the combination process care should be taken that the RCT reduction in the catalytic hydrotreatment is between 34.6 and 46.2%. 
     With the object of preparing atmospheric residues having an initial boiling point of 370° C. and different RCT&#39;s (e), residual feed mixture AB was subjected to catalytic hydrotreatment in eleven experiments. The experiments were carried out in a 1000 ml reactor containing two fixed catalyst beds of a total volume of 600 ml. The first catalyst bed consisted of a Ni/V/SiO 2  catalyst comprising 0.5 pbw of nickel and 2.0 pbw of vanadium per 100 pbw of silica. The second catalyst bed consisted of a Co/Mo/Al 2  O 3  catalyst comprising 4 pbw of cobalt and 12 pbw of molybdenum per 100 pbw of alumina. The weight ratio between the Ni/V/SiO 2  and Co/Mo/Al 2  O 3  catalysts was 1:2.5. All the experiments were carried out at a temperature of 385° C., a pressure of 150 bar and a H 2  /oil ratio of 1000 Nl/kg. Various space velocities were used in the experiments. The results of Experiments 1-10 are listed in Table A. The values given relate to observations carried out at run hour 425. 
     For each experiment the table gives the space velocity used, the RCT reduction ##EQU4## achieved and the corresponding C 4   -  production (calculated as %w on feed). Experiments 1-10 were carried out in pairs, the difference in space velocity between the two experiments of each pair being such as to achieve a difference in RCT reduction of about 1.0%. The table further gives the C 4   -  production per % RCT reduction (G) for each pair of experiments. 
     
                       TABLE A______________________________________               RCT        C.sub.4.sup.-Experiment   Space velocity               reduction, production,                                  G,Number  g.g..sup.-1.h.sup.-1               %          % w     % w______________________________________1       5.31        10.1       0.217   0.0272       4.84        11.0       0.2363       2.09        22.3       0.475   0.0224       2.02        23.1       0.4935       1.19        34.2       0.838   0.0326       1.13        35.4       0.8767       0.68        46.1       1.280   0.0428       0.65        47.0       1.3189       0.41        60.3       2.057   0.08610      0.40        61.8       2.168______________________________________ 
    
     Experiment 11 was carried out at a space velocity of 0.86 g.g -1 .h -1 . The RCT reduction was 40.5% and the C 4   -  production 1.03% w. 
     Of Experiments 1-11 only Experiments 6, 7 and 11 are experiments according to the invention. The other experiments fall outside the scope of the invention. They have been included in the patent application for comparison. As can be seen in Table A, in Experiments 1-2 and 3-4, in which RCT reductions were achieved of about 10 and 24%, respectively, G remains virtually constant (G c ). In Experiments 5-6 and 7-8, in which RCT reductions were achieved of about 35 and 47%, respectively, G was about 1.5×G c  and 2.0×G c , respectively. In Experiments 9-10, in which RCT reductions were achieved of about 61%, G was larger than 3×G c . 
     Comparison of Experiments 6 and 9 shows that reduction of the space velocity from 1.13 to 0.41 g.g -1 .h.sup. -1 at a constant temperature of 385° C., results in an increase in RCT reduction from 35 to 60% and an increase in C 4   -  production from 0.88 to 2.06% w. For comparison with Experiment 6, Experiment 12 was carried out, in which an increase in RCT reduction from 35 to 60% was realized by an increase in temperature from 385° to 420° C. at a constant space velocity of 1.13 g.g -1 .h -1 . In Experiment 12 the C 4   -  production was 3.42% w (instead of 2.06% w, as in Experiment 9). 
     In three experiments (Experiments 13, 14 and 15, respectively) the products obtained in the catalytic hydrotreatment carried out according to Experiments 3, 9 and 11 were separated by successive atmospheric distillation and vacuum distillation into a C 4   -  fraction, a H 2  S+NH 3  fraction, a C 5  -370° C. atmospheric distillate, a 370°-520° C. vacuum distillate and a 520° C. +  vacuum residue. The vacuum residues were deasphalted with n-butane at a pressure of 40 bar and a solvent/oil weight ratio of 3:1, and the deasphalted vacuum residues obtained were mixed with the corresponding vacuum distillates. The results of these experiments (of which only Experiment 15 is an experiment according to the invention) are listed in Table B. 
     
                       TABLE B______________________________________Experiment Number       13     14     15______________________________________H.sub.2 -treated product from Experiment Number                   3      9      11DistillationYield of products calculated on100 pbw residual feed mixture AB, pbwC.sub.4.sup.-           0.5    2.1    1.0H.sub.2 S + NH.sub.3    1.6    3.1    2.8C.sub.5 - 370° C.                   9.5    14.0   12.3370-520° C. (vacuum distillate)                   43.7   50.1   48.8520° C..sup.+ (vacuum residue)                   45.2   32.0   36.1RCT of the vacuum distillate, % w                   0.4    0.3    0.3RCT of the vacuum residue, % w                   19.7   12.9   17.9DeasphaltingTemperature, °C. 135    132    133Yield of deasphaltized vacuum residue, pbw                   22.0   22.4   21.8Yield of asphaltic bitumen, pbw                   23.2   9.6    14.3RCT of the deasphaltized vacuum residue, % w                   3.7    4.2    4.2MixingYield of mixture of vacuum distillate and                   65.7   72.5   70.6deasphaltized vacuum residue, pbwInitial boiling point of the mixture, °C.                   370    370    370RCT of the mixture, % w 1.5    1.5    1.5______________________________________ 
    
     In the second part of the investigation three experiments (Experiments 16-18) were carried out with the object of preparing an oil having an initial boiling point of 370° C. and an RCT of 1.5% w. In the experiments three different residual feedstocks were subjected to a catalytic hydrotreatment. The experiments were carried out in a 1000 ml reactor containing two fixed catalyst beds of a total volume 600 ml. The catalyst beds consisted of the same Ni/V/SiO 2  and Co/Mo/Al 2  O 3  catalysts as were used in Experiments 1-12. The weight ratio between the Ni/V/SO 2  and Co/Mo/Al 2  O 3  catalysts was 1:3. The experiments were carried out at a temperature of 390° C., a pressure of 125 bar and a H 2  /oil ratio of 1000 Nl/kg. The products from the catalytic hydrotreatment were separated by successive atmospheric distillation and vacuum distillation into a C 4   -  fraction, a H 2  S+NH 3  fraction, a C 5  -370° C. atmospheric distillate, a 370°-520° C. vacuum distillate and a 520° C. +  vacuum residue. The vacuum residues were deasphalted with n-butane at a pressure of 40 bar and a solvent/oil weight ratio of 3:1, and the deasphalted vacuum residues obtained were mixed with the corresponding vacuum distillates. 
     EXPERIMENT 16 
     The feed used in this experiment was atmospheric residue C. Atmospheric residue C had been obtained in the distillation of a crude mineral oil. Atmospheric residue C had an RCT of 10% w (determined by ASTM method D 524) and a vanadium+nickel content of 70 ppmw, and boiled below 520° C. to an extent of 50% w. Application of the relation found, in the form: 
     maximum RCT reduction=F max , and 
     minimum RCT reduction=F min , 
     with substitution of b=10, c=0, r=0, T 1  =370 and f=50, shows that for optimum utilization of the combination process care should be taken that the RCT reduction in the catalytic hydrotreatment is between 54.1 and 64.1%. In the catalytic hydrotreatment of Experiment 16 the space velocity used was 0.50 g.g -1 .h -1  and the RCT reduction achieved was 59%. In the solvent deasphalting of Experiment 16 an asphaltic bitumen D was separated which had an RCT of 41% w. 
     EXPERIMENT 17 
     The feed used in this experiment was a residual feed mixture CD obtained by mixing 100 pbw of atmospheric residue C with 12 pbw of asphaltic bitumen D separated in Experiment 16. Application of the relation found, in the form: 
     maximum RCT reduction=F max , and 
     miminum RCT reduction=F min , 
     with substitution of b=10, c=41, r=12, T 1  =370 and f=50, shows that for optimum utilization of the combination process care should be taken that the RCT reduction in the catalytic hydrotreatment is between 36.5 and 47.7%. In the catalytic hydrotreatment of Experiment 17 the space velocity used was 0.43 g.g -1 .h -1 , and the RCT reduction achieved was 42.1%. In the solvent deasphalting an asphaltic bitumen E was separated which had an RCT of 39% w. 
     EXPERIMENT 18 
     The feed used in this experiment was a residual feed mixture CE obtained by mixing 100 pbw of atmospheric residue C with 12 pbw of asphaltic bitumen E separated in Experiment 17. Application of the relation found, in the form: 
     maximum RCT reduction=F max , and 
     minimum RCT reduction=F min , 
     with substitution of b=10, c=39, r=12, T 1  =370 and f=50, shows that for optimum utilization of the combination process care should be taken that the RCT reduction in the catalytic hydrotreatment is between 37.1 and 48.3%. In the catalytic hydrotreatment of Experiment 18 the space velocity used was 0.43 g.g -1 .h -1 , and the RCT reduction achieved was 42.7%. In the solvent deasphalting an asphaltic bitumen F was separated which had an RCT of 39% w. Since the RCT of asphaltic bitumen F is equal to that of asphaltic bitumen E, this is the moment when the recycling process has reached its stationary state. The results of Experiments 16-18 are listed in Table C. 
     
                       TABLE C______________________________________Experiment No.          16     17     18______________________________________DistillationYield of productscalculated on 100 pbw feed,C.sub.4.sup.-           1.03   0.92   0.93H.sub.2 S + NH.sub.3    3.2    3.0    3.0C.sub.5 - 370° C.                   10.5   9.8    9.9370-520° C. (vacuum distillate)                   58.1   51.9   52.0520° C..sup.+ (vacuum residue)                   28.9   36.1   35.9RCT of the vacuum distillate, % w                   0.3    0.3    0.3RCT of the vacuum residue, % w                   13.6   18.3   17.9DeasphaltingTemperature, °C. 133    135    136Yield of deasphaltized vacuum residue, pbw                   21.8   21.6   21.8Yield of asphaltic bitumen, pbw                   7.1    14.5   14.1RCT of the deasphalted vacuum residue, % w                   4.8    4.4    4.3RCT of the asphaltic bitumen, % w                   41     39     39MixingYield of mixture of vacuum distillate and                   79.9   73.5   73.8deasphalted vacuum residue, pbwInitial boiling point of the mixture, °C.                   370    370    370RCT of the mixture, % w 1.5    1.5    1.5______________________________________