Process for the Conversion of Heavy Hydrocarbon Oils Into Light Hydrocarbon Oils

1. Field of the Invention

This invention relates generally to a process for the conversion of heavy hydrocarbon oils into light hydrocarbon oils and, more particularly, to a two-stage process for the conversion of heavy hydrocarbon oils into light hydrocarbon oils wherein, in the first stage, a heavy hydrocarbon oil is subjected to thermal cracking and, in the second stage, the product of the thermal cracking is subjected to a catalytic hydrotreatment.

2. Description of the Prior Art

The heavy hydrocarbon oils to which this invention is applicable include crude oils, reduced crude oils, vacuum residues, heavy oils extracted from tar sand, liquified coal oils and mixtures thereof. These hydrocarbon oils usually contain a large amount of components with a boiling point of 900.degree. F. or more such as asphaltenes, heavy metals, sulfur compounds, nitrogen compounds or the like. The term "asphaltene" used herein means a substance which is insoluble in normal heptane. Such heavy hydrocarbon oils exist abundantly in nature, and while they are considered as promising hydrocarbon resources, they are presently used merely for producing extremely low grade fuel oils or asphalt for pavement of roads. In view of the present political and economical situation facing energy crisis due to the depletion of high quality petroleum resources in the near future, it is strongly desired to develop effective processes capable of converting heavy hydrocarbon oils into more useful light hydrocarbon oils. The term "light hydrocarbon oils" used herein means oils having a boiling point of not higher than 900.degree. F.

U.S. Pat. No. 4,017,379 issued to Iida et al proposes a two-stage process for the conversion of a heavy hydrocarbon oil into light hydrocarbon oils, in which, in the first stage, the heavy hydrocarbon oil is subjected to thermal cracking at a temperature of 400.degree.-800.degree. C. and a pressure of 1-200 Kg/cm.sup.2 and, in the second stage, the product from the first stage as such is subjected to a catalytic hydrotreatment at a temperature of 370.degree.-480.degree. C. and a total pressure of 50-300 Kg/cm.sup.2. According to this process, a crude oil extracted from tar sand may be converted into light fractions with a yield as high as 98%. This process, however, has been found to involve a problem that the hydrotreated product is not stable so that a sludge is formed during transportation or storage. Therefore, when such a product is subjected to a further refining treatment, for example, the wall surfaces of the apparatus are apt to be fouled. In addition, the product oil has been found to be poor in compatibility. Thus, the oil tends to separate into layers during storage. Poor compatibility also makes it difficult to blend the oil product with other hydrocarbon oils.

It is, therefore, an object of the present invention to provide a process for the conversion of heavy hydrocarbon oils into light hydrocarbon oils, in which the above-mentioned problems involved in the prior art process are overcome.

Another object of the present invention is to provide a two-stage process which includes a thermal cracking stage followed by a catalytic hydrotreating stage and which permits the conversion of heavy hydrocarbon oils into light hydrocarbon oils with a high conversion rate.

It is a further object of the present invention to provide a process of the above-mentioned type by which a hydrotreated oil product having excellent thermal stability and compatibility is obtainable.

In accomplishing the foregoing objects, there is provided in accordance with the present invention a process for the conversion of a heavy hydrocarbon oil into light hydrocarbon oils, comprising subjecting the heavy hydrocarbon oil to thermal cracking conditions, and hydrotreating the product of the thermal cracking in the presence of a catalyst which comprises a porous carrier composed of one or more inorganic oxides of at least one member selected from the group consisting of the elements belonging to Groups II, III and IV of the Periodic Table, and one or more catalytic metal components composited with said carrier, the metal of said catalytic metal components being selected from the group consisting of the metals belonging to Groups VB, VIB, VIII and IB of the Periodic Table, said catalytic metal components being present in an amount of between about 0.1% and about 30% in terms of metal oxide based on the total weight of said catalyst, said catalyst having the following characteristics (a)-(d):

(a) an average pore diameter being between about 200 and about 400 .ANG.

(b) a total volume of pores with a pore diameter of 75 .ANG. or more being between about 0.5 and about 1.5 cc/g, the volume of pores with a diameter of not greater than 100 .ANG. being not more than 0.1 cc/g, the volume of pores with a diameter of between about 200 and about 400 .ANG. being at least about 0.2 cc/g, the volume of pores with a diameter of at least 1500 .ANG. being not greater than about 0.03 cc/g,

(c) a total surface area of pores with a pore diameter of 75 .ANG. or more being at least about 70 m.sup.2 /g,

(d) an average catalyst diameter being between about 0.6 and about 5 mm.

The present inventors have made an extensive research with a view to developing an improved process capable of converting heavy hydrocarbon oils into stable hydrotreated oils, and have found that the toluene insoluble content in a hydrotreated oil is closely related to its stability and that it is essential to maintain the toluene insoluble content below 0.5% by weight in order for the hydrotreated oil to have practically acceptable stability. The term "toluene insolubles" used herein means hydrocarbon components which are insoluble in toluene. The percentages of toluene insolubles are determined by toluene extraction at 80.degree. C. in accordance with UOP Method 614-80. It has also been found that the toluene insoluble content depends upon the catalyst used in the hydrotreatment of the thermally cracked product. The catalyst used in the process of the above-described U.S. patent is composed of one or more metals selected from Groups VI and VIII metals supported on a .gamma.-alumina carrier. The U.S. patent suggests the desirability of the use of a carrier with an average pore diameter of between 120 and 160 .ANG.. So far as such a catalyst is used for hydrotreating virgin heavy hydrocarbon oils, there arises no problem of unstability of the hydrotreated product. However, such a catalyst, when used for hydrotreating a product of thermal cracking of a heavy hydrocarbon oil, is not effective enough to yield a stable, hydrotreated hydrocarbon oil having a sufficiently low content of toluene insoluble matters. As disclosed in the U.S. Pat. No. 4,017,379, thermal cracking of a heavy hydrocarbon oil can increase the amount of asphaltenes, but the asphaltenes in the thermally cracked product differ in quality from those in the oil prior to the thermal cracking; i.e. the asphaltenes after thermal cracking have much lower molecular weight than those before the cracking. From this point of view, it is reasonable to conceive that a catalyst formed of a carrier with an average pore diameter of 120-160 .ANG. may achieve the desired rate of conversion into light hydrocarbon oils. However, it has been unexpectedly found that the use of a catalyst having a large average pore diameter is essential to obtain stable hydrocarbon oils through catalytic hydrotreatment of a product oil from thermal cracking. According to the process of this invention, the product obtained in the thermal cracking stage and containing a large amount of toluene insolubles are hydrotreated with the use of the above-described specific catalyst having an average pore diameter of 200-400 .ANG. so that the toluene insoluble content in the hydrotreated oil product can be reduced to the extent that it can exhibit practically acceptable stability and compatibility.

Other objects, features and advantages of the present invention will become apparent from the detailed description of the invention which follows, when considered in light of the accompanying drawing in which the sole FIGURE is a flow diagram showing one embodiment of the present invention.

Heavy hydrocarbon oils which can be effectively treated by the process of the present invention are highly viscous, high-asphaltene oils having a specific gravity (API) of less than about 20.degree. and containing at least about 10 vol %, especially over about 20 vol % of fractions having a boiling point of at least 900.degree. F. Table 1 shows the properties of typical heavy hydrocarbon oils. In the table, the letters A to C indicate the following oils:

A: Boscan crude oil

B: Khafji vacuum residue

C: Canadian oil sand bitumen

According to the process of this invention, the heavy hydrocarbon feedstock is first introduced into a heat treating zone where it is subjected to thermal cracking conditions. The thermal cracking is performed at a temperature of 400.degree.-530.degree. C., preferably 430.degree.-500.degree. C. and a pressure of 0-200 Kg/cm.sup.2 G, preferably 5-180 Kg/cm.sup.2 G with a residence time of 1-300 min., preferably 10-120 min. The term "residence time" used herein means a value obtained by dividing the inside volume of the reactor for the thermal cracking by the flow rate of the feedstock oil therethrough (calculated on 60.degree. F. basis).

A temperature of the thermal cracking below 400.degree. C. is insufficient to effect the cracking of the heavy hydrocarbon oil to a desired degree within a practically acceptable period of time. Above 530.degree. C., it becomes difficult to control the thermal cracking and, moreover, polycondensation of pitch can proceed very fast, yielding a large amount of coke which cannot be decomposed in the subsequent hydrotreatment.

The pressure at which the thermal cracking is performed is one of the very important factor in minimizing the occurrence of undesirable side reactions such as polycondensation, gasification and coking. The higher the pressure, the lower becomes the yield of gaseous products and, thus, desirable liquid products such as gasoline and kerosene can be obtained with a better yield. Further, the higher the pressure, the greater becomes the amount of the feed oil which is present in the liquid state within the reactor. This is advantageous because heat transfer within the reactor may be facilitated and local superheat can be prevented, whereby the formation of coke may be reduced. From the standpoint of equilibrium, however, an increase in pressure of the thermal cracking is not advantageous. Therefore, the optimum pressure is determined in consideration of the properties of the heavy hydrocarbon oil to be treated, cracking temperature or the like. Generally, the thermal cracking is carried out at a pressure in the range of 0-200 Kg/cm.sup.2 G.

In order to reduce the formation of coke, it is preferred that the thermal cracking be carried out in the presence of a gaseous medium such as steam, a hydrogen gas or a nitrogen gas. A volatile fraction or fractions obtained in the system may also be used as the gaseous medium. The use of hydrogen is particularly preferable because of the additional advantage that there can proceed thermal hydrocracking in which the formation of coke is significantly minimized.

The residence time depends upon the reaction temperature. The higher the temperature, the shorter should be the residence time. With a residence time of more than 5 hours, there is a danger of the coke formation in a considerable amount.

The thermal cracking may be effected with the use of a coil heater, tubular reactor, chamber-type reactor, drum-type reactor or any other suitable reactors.

The thermal cracking may be carried out in a continuous, two-step manner. In the first step the feedstock is subjected to severe cracking conditions to such an extent as to avoid the clogging of the reactor due to coke formation. With a coil heater, for example, the first step is conducted at a temperature of 450.degree.-530.degree. C. and a pressure of 1-200 Kg/cm.sup.2 G with a residence time of 1-20 min. The product from the first step is then fed to the second step employing milder cracking conditions than those in the previous step, for example at a temperature of 400.degree.-500.degree. C. and a pressure of 1-200 Kg/cm.sup.2 G with a residence time of 5-300 min. The second step is preferably performed in the presence of the above-described gaseous medium.

In general, the thermal cracking treatment is conducted so that at least about 15% by volume, preferably at least about 25% by volume of the components of the feedstock oil having a boiling point of at least 900.degree. F. may be decomposed into light hydrocarbon oils having a boiling point of below 900.degree. F.

In the process of the present invention, the product thus obtained in the thermal cracking zone is then introduced into a hydrotreating zone where it is subjected to a hydrotreatment in the presence of a catalyst. If necessary, the product from the thermal cracking zone is rapidly cooled before being introduced into the hydrotreating zone so as to terminate the cracking within the specified range of time. Such rapid cooling may be done either by directly contacting the cracking product with a cooling medium or by means of an indirect-type heat exchanger. The cooling medium may be a hydrogen or nitrogen-rich gas, steam or a gaseous product of the system. The cracking product is generally cooled to a temperature at which the subsequent hydrotreatment is performed. Such a cooling step is unnecessary when the cracking is conducted at a temperature lower than that in the hydrotreating step.

The thus cooled or uncooled product from the cracking zone is introduced into the hydrotreating zone either as such or after the removal of its gaseous components and, if desired, light oil fractions. Whether or not such a removal treatment is to be carried out and what fraction or fractions are to be removed may be determined generally according to the properties of the feedstock oil and intended end product. The separation of the gas and light fractions from the cracking product may be effected by any known ways, for example, by means of a flash drum or by distillation.

The hydrotreatment of the cracking product is carried out in the presence of a specific catalyst. The catalyst includes a porous carrier composed of one or more synthetic inorganic oxides of at least one member selected from the group consisting of the elements belonging to Groups II, III and IV of the Periodic Table. Composited with said carrier is at least one metal component selected from the group consisting of the metals belonging to Groups VB, VIB VIII and IB of the Periodic Table. The catalytic metal component or components are present in an amount of between about 0.1% and about 30% in terms of metal oxide based on the total weight of the catalyst. It is important that the catalyst should have the following properties (a)-(d):

(a) an average pore diameter being between about 200 and 400 .ANG.;

(b) a total volume of pores with a pore diameter of 75 .ANG. or more being between about 0.5 and about 1.5 cc/g, the volume of pores with a diameter of not greater than 100 .ANG. being not more than 0.1 cc/g, the volume of pores with a diameter of between about 200 and about 400 .ANG. being at least about 0.2 cc/g, the volume of pores with a diameter of at least 1500 .ANG. being not greater than about 0.03 cc/g;

(c) a total surface area of pores with a pore diameter of 75 .ANG. or more being at least about 70 m.sup.2 /g;

(d) an average catalyst diameter being between about 0.6 and about 5 mm.

For the purpose of this specification, the value of the average pore diameter, APD, is defined by the following formula, and expressed in .ANG.: ##EQU1## wherein PV and SA represent the total pore volume and total surface area, respectively, of the pores having a diameter of 75 .ANG. or more, per unit catalyst weight, which are expressed in cc/g and m.sup.2 /g, respectively. The total pore volume and the total surface area of the pores having a diameter of at least 75 .ANG., per gram of the catalyst will hereinafter referred to simply as the pore volume and the surface area, respectively, of the catalyst, unless otherwise noted.

The pore diameter, pore volume and surface area of the catalyst were determined by the mercury penetration method [of which details are described in, for example, E. W. Washburn, Proc. Natl. Acad. Sci., 7, page 115 (1921), H. L. Ritter and L. E. Drake, Ind. Eng. Chem. Anal., 17, pages 782 and 787 (1945), L. C. Drake, Ind. Eng. Chem., 41, page 780 (1949), and H. P. Grace, J. Amer. Inst. Chem. Engrs., 2, page 307 (1956)] using a mercury penetration porosimeter, Model 70 (made by Carlo Elba of Milano, Italy). The determination was made with a mercury surface tension of 474 dyne/cm at 25.degree. C., a contact angle of 140.degree. and an absolute mercury pressure which was varied between 1 and 2,000 Kg/cm.sup.2. The diameter of the pores having a diameter of at least 75 .ANG. can be expressed by the following formula: ##EQU2##

The average catalyst diameter, ACD, of the catalyst particles according to this invention is defined by the following formula, and expressed in millimeter: ##EQU3##

In this formula, the average volume and average outer surface area of the catalyst particles represent the volume and outer surface area, respectively, of a spherical equivalent having a diameter which is equal to the average particle diameter of the catalyst which is determined by direct measurement, as specifically set forth in "Particle Size Determination", Powder Engineering Society, Nikkan Kogyo Shinbunsha (1975).

The catalyst according to this invention is composed of one or more specific catalytic metal components composited with a porous carrier formed from inorganic oxides of specific elements, and featured most saliently by its physical properties defined within the aforementioned specific ranges. According to the catalyst of this invention, its ultimate physical properties, particularly its average pore diameter, pore volume, pore distribution, surface area and other features of its pore structure, and the average catalyst diameter of its particles have a significant bearing on the effective hydrotreatment of the thermally cracked product of a heavy hydrocarbon oil.

The catalyst with the above-described physical properties exhibits both a high activity for the hydrogenolytic decomposition of polycondensed high molecular hydrocarbons formed in the thermal cracking stage and an appropriate selectivity for the removal of sulfur and metals from the polycondensed high molecular hydrocarbons in the cracking product. Moreover, the catalyst can maintain its activity for a substantially long period of time with a high degree of stability. Additionally, the catalyst has a sufficiently high mechanical strength.

When a heavy hydrocarbon oil is subjected to thermal cracking for the conversion into light hydrocarbon oils, highly polycondensed materials such as pitch are invariably produced more or less. Sulfur, nitrogen and metal components originally contained in the heavy hydrocarbon oil are accumulated in the highly polycondensed materials in the concentrated state. In addition, the polycondensed materials contain a large amount of solvent-insoluble matters, such as n-heptane insolubles, toluene insolubles and pyridine insolubles.

The toluene insolubles are highly polycondensed, high molecular hydrocarbons formed in the thermal cracking stage by, for example, thermal polycondensation of high molecular hydrocarbons such as asphaltenes. The content of toluene insolubles in a hydrotreated oil is found to serve as an index for evaluating the quality thereof. In order for the hydrotreated oil to have desirable thermal stability and compatibility, it is necessary to control the toluene insoluble content to a level not exceeding 0.5 wt %. Thus, according to the process of this invention, the hydrotreatment is conducted in the presence of the catalyst having the above-described specific properties so that the resulting hydrotreated oil may have a toluene insoluble content of not greater than 0.5 wt %, preferably not greater than 0.15 wt %, more preferably not greater than 0.04 wt %.

The catalyst used in the process of this invention has an average pore diameter of 200-400 .ANG. and is very suited for hydrotreating the thermal cracking product so as to obtain a stable and good quality oil product having a toluene insoluble content of 0.5 wt % or less. An average pore diameter of less than 200 .ANG. is insufficient to decompose the toluene insolubles and to effect desulfurization and demetallization of the highly polycondensed materials to a desired degree. In this connection, the volume of pores with a pore diameter of less than about 100 .ANG. should not be greater than 0.1 cc/g and is preferably not greater than 0.08 cc/g. Above about 400 .ANG. average pore diameter, the surface area of the catalyst pores become so small that the catalyst cannot exhibit desired activity. In addition, such a large average pore diameter causes catalyst poisoning by coke deposition. Thus, the catalyst can be used with a higher degree of efficiency with a greater pore volume defined by pores having a diameter in the range of about 200 and about 400 .ANG.. More specifically, the volume of pores with a diameter of between about 200 and about 400 .ANG. should be at least about 0.2 cc/g, preferably at least about 0.3 cc/g. In other words, a high catalytic activity for the decomposition of toluene insolubles may be achieved when pores having a diameter of about 200-400 .ANG. occupy the major portion of the total pore volume, PV.

The catalyst should have a surface area, SA, of at least about 70 m.sup.2 /g in order to provide a desired catalytic activity.

It is known that the size of the catalyst particles is generally a very important factor in the reactions in which intrapore diffusion prevails (for details, see P. H. Emmett, "CATALYSIS", Vol. II, Reinhold Publishing Corporation, New York (1955), and C. N. Satterfield, "MASS TRANSFER IN HETEROGENEOUS CATALYSIS", M.I. T. Press, Massachusetts (1970)). Too large an average catalyst diameter is disadvantageous to provide a satisfactory level of catalytic activity. If the catalyst particles are too small, on the other hand, it becomes difficult to use them with a fixed, moving or ebullated bed, or other reaction system widely used in the art for the catalytic hydrotreatment of a heavy hydrocarbon oil. For instance, various disadvantages may result from the use of too small catalyst particles with a fixed bed system, including an increased pressure drop in the catalyst bed, pulverization of the catalyst during use, and agglomeration of the particles by deposition of heavy metals and coke. Accordingly, the catalyst should have a particle size of about 0.6 and about 5 mm in terms of average catalyst diameter.

One of the important requirements which an optimum catalyst must satisfy from the standpoint of industrial use is that it should have a satisfactory level of mechanical strength. The mechanical strength of a molded catalyst is generally expressed by way of its crushing strength and abrasion loss. An optimum catalyst having at least a required level of mechanical strength, especially crushing strength, has been found to require a total pore volume, PV, of 0.5-1.5 cc/g. The total pore volume of in such a range is also required in order for the catalyst to maintain its catalytic activity for a sufficiently long period steadily.

A catalyst is imparted with an extremely large abrasion loss, if its pores having a diameter of at least 1,500 .ANG. have a large pore volume in total. In order to reduce the abrasion loss to a desirable level, it is preferable to decrease the pores having a diameter of at least 1,500 .ANG. to the extent that their total volume may be reduced to a level not greater than about 0.03 cc/g. Macropores having a diameter of at least 1,500 .ANG. hardly contribute to providing any catalytic activity for the decomposition of toluene insolubles, but are only concerned with the crushing strength of the catalyst. Therefore, it is desirable to minimize such macropores.

The carrier for the catalyst of this invention comprises one or more inorganic oxides of at least one element selected from among the elements belonging to Groups II, III and IV of the Periodic Table. Examples of the oxide include alumina, silica, titania, boria, zirconia and other oxides of a single element, and silica-alumina, silica-magnesia, alumina-magnesia, alumina-titania, silica-titania, alumina-boria, alumina-zirconia, silica-zirconia and other composite oxides of more than one element. These oxides are used alone, or as a mixture of two or more thereof.

One or more catalytic metal components are composited with the inorganic oxide carrier. The metal of the catalytic metal components is selected from among the metals belonging to Groups VB, VIB, VIII and IB of the Periodic Table, most preferably vanadium, molybdenum, tungsten, chromium, cobalt, nickel or copper. These catalytic metal components can be effectively used, whether in the form of a metal, metal oxide or metal sulfide, or alternatively, they may be partially combined with the carrier material by ion exchange or otherwise. The catalytic metal component should be present in the range of about 0.1% to about 30% by weight in terms of oxide based on the total weight of the catalyst.

The "Periodic Table" as herein referred to is one which appers on page 628 of Webster's 7th New Collegiate Dictionary, G & C Merriam Company, Springfield, Massachusetts, U.S.A. (1965).

These catalytic metal components dictate the activity of the catalyst for various reactions involved in the hydrotreatment of the product of the thermal cracking, such as hydrogenation, hydrogenolytic decomposition, demetallization, desulfurization and denitrification.

The catalyst used in the hydrotreating stage may be prepared by any known ways as long as it can impart the above-described specific physical properties to the catalyst produced thereby. For example, such a catalyst may be prepared by a method including the steps of:

(a) providing a seed hydrosol of at least one member selected from the group consisting of hydroxides of the elements belonging to Groups II, III and IV of the Periodic Table;

(b) mixing a first pH controlling agent with the hydrosol to adjust the pH of the hydrosol to a first region and maintaining the hydrosol at the first pH region, while agitating it, at a temperature and for a period of time sufficient to dissolve fine hydrosol particles;

(c) then mixing a second pH controlling agent with the hydrosol to adjust the pH of the hydrosol to a second region, at least one of the first and second pH controlling agents including at least one hydrosol forming substance containing an element selected from the group consisting of the elements belonging to Groups II, III and IV of the Periodic Table, and maintaining the hydrosol at the second pH region, while agitating it, at a temperature and for a period of time sufficient to cause deposition of the dissolved hydrosol and the hydrosol from the hydrosol forming substance on the undissolved seed hydrosol, steps (b) and (c) being conducted once or repeated in sequence more than once so that the seed hydrosol is caused to grow to an aggregate hydrogel;

(d) molding and drying the hydrogel; and

(e) supporting one or more catalytic metal components on the hydrogel.

The catalytic hydrotreatment of this invention may be carried out by any ordinary flow system, such as a fixed, moving, fluidized or ebullated bed, without causing any catalyst to be carried out of the reaction zone with the reaction product, if the shape of the catalyst, etc. are properly selected. The reactant may be fed into the reaction zone through the top or bottom of the reactor. In other words, the flow of the gas and the liquid in the reactor may be co-current upwardly or downwardly. The catalyst particles may be granular, spherical or cylindrical, but for the hydrotreatment of a cracking product of a heavy hydrocarbon oil containing large quantities of asphaltenes and heavy metals, it is desirable to use extrudate catalyst particles which are hollow cylindrical, non-circular in cross section, e.g., oval, tri-lobed or multi-leafed, or elongated and have a surface provided with at least one groove. When these specially shaped particles are used to form a fixed bed, they increase the voids in the reactor, and it is not only possible to reduce the pressure drop in the catalyst bed, but the blocking of the catalyst bed by deposition of coke and metal among the catalyst particles can also be prevented remarkably.

The conditions for the hydrotreatment according to this invention include a temperature of 350.degree. C. to 450.degree. C., preferably 370.degree. C. to 430.degree. C., a hydrogen pressure of 50 to 250 atm., preferably 80 to 200 atm., and a liquid space velocity of 0.1 to 5 hours.sup.-1, preferably 0.2 to 3 hours.sup.-1.

If the reaction temperature is below 350.degree. C., the catalyst fails to show its full activity, and the reaction and conversion ratio in the hydrotreating process fails to reach any practically acceptable level. If it exceeds 450.degree. C. on the other hand, asphaltenes undergo polycondensation, and tend to increase, rather than decrease. Moreover, coking occurs more actively, thereby causing degradation of the product, reduction in the activity of the catalyst and agglomeration of catalyst particles. If the hydrogen pressure is below 50 atm., coking occurs so actively that it is very difficult to maintain the activity of the catalyst at a normal level. Any hydrogen pressure in excess of 250 atm. brings about too active hydrocracking, and is not practically acceptable from the economical standpoint, since there result an increased hydrogen consumption, a reduced yield of production, and an increased cost of the reactor and other associated equipment. If the liquid space velocity is less than 0.1 hour.sup.-1, a prolonged process time for oil brings about degradation of the product due to the thermal change of its heavy components, while any velocity in excess of 5 hours.sup.-1 brings about a lower reaction and conversion ratio per pass which is practically unacceptable.

The proportion of hydrogen or a gas containing hydrogen to cracking product to be fed into the reaction zone may be 100 to 2,000 volumes of hydrogen at 15.degree. C. per volume of the product at 1 atm. and 15.degree. C. (i.e., 100 to 2,000 Nl/l), preferably 500 to 1,000 Nl/l. If it is less than 100 Nl/l, the reaction zone becomes short of hydrogen, no sufficient hydrogen is fed into the liquid, resulting in coking, which has an adverse effect on the properties of the catalyst and the oil to be produced. Any proportion in excess of 2,000 Nl/l does not provide any additional benefit in the process of this invention, though it does not present any problem in the reaction. The cost of a compressor used for circulating hydrogen depends on the quantity of the hydrogen to be circulated, and is very high for supplying hydrogen in excess of 2,000 Nl/l. Thus, the quantity of 2,000 Nl/l may be a practical upper limit to the circulation of hydrogen. The gas which is rich in hydrogen and circulated through the reaction zone may contain hydrogen sulfide. It does not have any adverse effect, but even tends to promote the reaction if its quantity is appropriate. The catalyst used for the purpose of this invention has some interaction with hydrogen sulfide under the aforementioned conditions for the reaction, and hydrogen sulfide plays a certain role in maintaining the activity of the catalyst. The hydrogen to be fed into the reaction zone can contain up to 10 mol % of hydrogen sulfide within the scope of this invention.

The reaction product obtained by the hydrotreating process under the aforementioned conditions, and not containing any catalyst is delivered to a gas-liquid separation zone, where it is separated into a gas which is rich in hydrogen, and a product consisting substantially solely of a liquid. Any method and apparatus used in an ordinary desulfurization process with a fixed or ebullated bed may be employed for the gas-liquid separation.

The thus separated hydrogen-rich gas may be recycled to the thermal cracking stage or hydrotreating stage. In an alternative, the hydrogen-rich gas may be used for quenching the product from the thermal cracking stage. When the feedstock heavy hydrocarbon oil contains a large amount of asphaltenes and metals, it is advantageous to separate the liquid product obtained by the gas-liquid separation into a substantially asphaltene-free and heavy metal-free light oil fraction and a heavy fraction containing asphaltenes and heavy mctals, the heavy fraction being recycled to the thermal cracking or hydrotreating stage. According to this process, it is possible to convert the feedstock oil containing a large quantity of asphaltenes continuously to a light hydrocarbon oil not containing asphaltenes or heavy metals. With such a recycling treatment, the hydrogen and catalyst consumption may remarkably decrease and the yield of the light hydrocarbon oil may significantly increase. The separation of the liquid product into such light and heavy fractions can be accomplished by any ordinary method, such as distillation and solvent deasphalting. If the separation is conducted by solvent deasphalting, it is possible to use as a solvent at least one low molecular hydrocarbon, such as methane, ethane, propane, butane, isobutane, pentane, isopentane, neopentane, hexane and isohexane. The solvent and the liquid reaction product are brought into counter-current contact with each other. Solvent deasphalting may be carried out at a temperature of 10.degree. to 250.degree. C., preferably 50.degree. to 150.degree. C., and a pressure of 3 to 100 atm., preferably 10 to 50 atm.

According to the hydrotreatment employing the catalyst of this invention, it is possible to obtain a light hydrocarbon oil having improved quality. The hydrocarbon contained in the hydrotreatment product obtained according to this invention has a molecular weight of about 200 to about 2,000. The asphaltenes remaining to a slight extent in the hydrotreatment product have a considerably lower molecular weight than that in the thermal cracking product, and contain sharply reduced quantities of sulfur and heavy metals. A greater proportion of the sulfur and nitrogen in the hydrotreatment product is present in its light fraction, as opposed to their distribution in the cracking product. Moreover, the oil obtained has been found to have unique properties differing from those of the product obtained by any known method, irrespective of the type of the product from the thermal cracking stage. In the process of this invention, even if the thermal cracking stage is conducted under severe conditions so as to obtain as much light hydrocarbon oils as possible, the hydrotreated oil produced in the subsequent catalytic hydrotreating stage are excellent in stability and compatibility. Also, oil products obtained by further processing of the hydrotreated oil have good stability and compatibility. Residues derived from the hydrotreated oil, for example, are very suited as materials for heavy fuel oils because of their good compatibility with light oils of any sort. Such fuel oils are stable and do not give rise to a problem of sludge formation during storage.

The hydrotreating stage in the process of this invention may be carried out in a two-step manner. The two-step hydrotreatment comprises the steps of:

(i) bringing a cracking product from thermal cracking stage into contact with the above-described catalyst at a temperature of 350.degree. to 450.degree. C., a hydrogen pressure of 50 to 250 atm. and a liquid space velocity of 0.1 to 5.0 hours.sup.-1 ; and

(ii) at a temperature of 350.degree. to 450.degree. C., a hydrogen pressure of 50 to 250 atm. and a liquid space velocity of 0.1 to 5.0 hours.sup.-1, bringing at least a portion of the product of step (i) into contact with a catalyst which comprises a porous carrier containing alumina and having composited therewith a first catalytic metal component composed of at least one compound selected from among the oxides and sulfides of the metals belonging to Group VIB of the Periodic Table and a second catalytic metal component composed of at least one compound selected from among the oxides and sulfides of the metals belonging to Group VIII of the Periodic Table, and which catalyst has, with regard to its pores having a diameter of 75 .ANG. or more, the properties: an average pore diameter of about 80 .ANG. to about 200 .ANG., a pore volume of about 0.4 cc/g to about 1.5 cc/g, and a surface area of about 100 m.sup.2 /g to about 400 m.sup.2 /g.

The carrier for the catalyst used in step (ii) supports thereon a combination of at least one compound selected from among the oxides and sulfides of the metals belonging to Group VIB of the Periodic Table, and at least one compound selected from among the oxides and sulfides of the metals belonging to Group VIII of the Periodic Table. This combination is preferably a combination of at least one selected from the oxides and sulfides of chromium, molybdenum and tungsten, and at least one selected from among those of cobalt and nickel. The metal component preferably contains about 2 to 40% by weight of the compound of the metal belonging to Group VIB, and about 0.1 to 10% by weight of the compound of the metal belonging to Group VIII, both in terms of the weight of the oxide, based on the total weight of the catalyst.

Step (ii) is intended for removing sulfur, nitrogen, Conradson carbon residue and residual metal and for minimizing unsaturated components by hydrogenation. The selection and combination of the metal components depend on the reaction on which the utmost importance should be placed on a case to case basis. In order to promote the activity for denitrification and reduction of Conradson carbon residue, too, it is desirable to add as an auxiliary catalyst component at least one of the compounds of titanium, boron, phosphorous and fluorine, or boron phosphate, or boron fluoride. In this case, the catalyst can effectively contain about 1.0 to 30% by weight of any such auxiliary component, in terms of the weight of the oxide, based on the total weight of the catalyst.

The catalyst employed for step (ii) may be prepared by any known method, if there can be obtained a catalyst having a desired porous structure, particularly an average pore diameter of about 80 .ANG. to 200 .ANG., a pore volume of about 0.4 to 1.5 cc/g and a surface area of about 100 to 400 m.sup.2 /g. The method and conditions of the treatment for step (ii), and the particle diameter and shape of the catalyst to be employed therein may be equal to those already described in connection with the hydrotreatment using the catalyst of this invention.

The carrier for the catalyst used for step (ii) is a carrier containing at least 70% by weight of alumina, and preferably composed of alumina alone, or silica-alumina.

A process including the two-step hydrotreating process is a superior process which can be utilized for converting substantially the whole quantity of the heavy feedstock to a high quality desulfurized oil, even if it is required to treat a heavy oil containing still larger quantities of asphaltenes and heavy metals, or even if it is required to conform to very stringent specifications for the product to be obtained. The industrially advantageous two-step treatment of any such bad heavy oil may be carried out by physically separating a heavy fraction containing asphaltenes or heavy metals from the product of the first step, and recycling it to the first step, or alternatively, by physically separating such a heavy fraction from the product of the second step and recycling it to the first and/or second step or the thermal cracking stage. The step of separating the light and heavy fractions does not require any special method, but may be effected by distillation, solvent deasphalting, or any other ordinary method.

As already pointed out, the two-step hydrotreating process of this invention produces a product oil having a high added value. The product oil, which contains reduced quantities of nitrogen and Conradson carbon residue, is an optimum stock for preparing high grade gasoline by catalytic cracking, or kerosene, jet fuel oil and diesel oil by hydrocracking.

One of the preferred embodiments of this invention will be described below with reference to FIG. 1. A heavy hydrocarbon feedstock is supplied through a line 1 and mixed with a hydrogen-rich gas supplied through a line 10. The mixture is introduced through a line 2 into a thermal cracking zone 3 where the mixture undergoes thermal cracking. The product of the thermal cracking is withdrawn from the cracking zone 3 through a line 4 and is cooled by direct contact with a hydrogen gas supplied from a line 5. The mixture of the product and the hydrogen gas is then introduced into a hydrotreating zone 7 through a line 6, where the cracking product is subjected to hydrotreating conditions. The hydrotreatment product is withdrawn from the zone 7 and is fed through a line 8 to a gas-liquid separating zone 9 where the product is separated into a hydrogen-rich gas and a liquid product. The hydrogen-rich gas is recycled through the line 10 to the thermal cracking zone 3. The liquid product is taken through a line 11 for recovery as an end product or for introduction into further treating steps.

The following examples will further illustrate the present invention.

A vacuum residue of Khafji crude oil was employed as a heavy hydrocarbon feedstock oil and was treated in accordance with the process of this invention for continuous conversion thereof into light hydrocarbon oils. The feedstock oil had the properties summarized in Table 2.

The feedstock oil was continuously fed at a flow rate of 500 cc/hr and was mixed with a hydrogen-rich gas of 300 Nl/hr so that the mixture had a hydrogen to oil ratio of 600 Nl/l. The feed of the mixture was preheated to 350.degree. C. in a heater and fed to a thermal cracking zone which included a coil tube reactor placed in a fluid bath heater. The tube had an inner diameter of 7 mm and a length of 2.2 m. The heater was operated so that the temperature within the tube at its outlet was maintained at 485.degree. C. The pressure within the tube was controlled to be 140 Kg/cm.sup.2 G by means of a control valve provided in a line extending from a hydrotreating zone provided downstream of the cracking zone. The residence time within the thermal cracking zone was about 5.7 min.

The reaction mixture containing the cracked product was continuously discharged from the cracking zone and was rapidly cooled, before being introduced into the hydrotreating zone, to 408.degree. C. by direct contact with a pure hydrogen gas supplied at a flow rate of about 200 Nl/hr. Additional hydrogen was also fed directly to the hydrotreating zone so that the total feed rate of hydrogen introduced thereinto was about 1000 Nl/hr. The hydrotreating zone included an isothermal, fixed bed reactor packed with Catalyst I through which reactant streams were flown co-currently downward. Catalyst I had the composition and properties shown in Table 3 and was prepared in the following manner.

Five liters of demineralized water was placed in a stainless steel container provided with an external mantle heater, and heated to 95.degree. C. 1 Kg of aluminum nitrate, Al(NO.sub.3).sub.3.9H.sub.2 O, was dissolved in water to form 2.5 liters of an aqueous solution thereof, and 500 cc of this aqueous solution was added into the container. The solution in the container was held at 95.degree. C., and while it was being stirred, 14% by weight of aqueous ammonia was added to control the solution to pH 8. The solution was aged for one hour under boiling condition to form a seed alumina hydrosol. Then, 500 cc of the aforementioned aqueous aluminum nitrate solution was added into the hydrosol, and the hydrosol was allowed to stand for 5 minutes. The hydrosol had a pH of 4. After it was confirmed that the hydrosol had a temperature of at least 95.degree. C., 14% by weight of aqueous ammonia was added into the hydrosol to control it to pH 9, while it was being stirred, and the hydrosol was allowed to stand for 30 minutes. This pH control operation was repeated nine more times i.e. a total of eighteen pH adjustments. Then, the hydrosol was aged for one hour under boiling condition to form an alumina hydrogel. The washing of this hydrogel was carried out by dispersing it in 20 liters of demineralized water and dehydrating it through a vacuum filter. This washing operation was repeated three times, and the hydrogel was dehydrated through the vacuum filter to yield a cake containing about 25% by weight of alumina in terms of Al.sub.2 O.sub.3. This cake was formed into cylindrical particles having a diameter of 1 mm by an extruder provided with a die having a hole with a diameter of 1.0 mm. The molded product was heated at about 120.degree. C. for two hours for drying, and precalcined at 550.degree. C. in an electric furnace for three hours.

Molybdenum and cobalt were supported on the precalcined product. For this purpose, 400 ml of warm water was added into 151.9 g of ammonium molybdate. An aqueous solution obtained by dissolving 160.5 g of cobalt nitrate in 400 ml of distilled water was added into the ammonium molybdate solution and mixed therewith, and 500 ml of aqueous ammonia having a concentration of 25% by weight was added into the mixture. 35 ml of the solution obtained was diluted with 5 ml of distilled water, and the diluted solution was uniformly sprayed on 50 g of the precalcined product, so that the precalcined product might be impregnated with the solution. The precalcined product was kept in a hermetically sealed condition overnight, and air dried at room temperature. It was, then, dried with hot air at 120.degree. C. for three hours, and calcined at 600.degree. C. for three hours in an air stream, whereby Catalyst I was prepared.

The hydrotreatment was performed at a temperature of 405.degree. C., a pressure of 140 Kg/cm.sup.2 G and a liquid hourly space velocity of 0.33 hr.sup.-1 using a hydrogen to oil ratio of 1000 Nl/l.

The product obtained by the hydrotreatment was successively withdrawn from the hydrotreating zone and was fed to a gas-liquid separating zone for separation thereof into a hydrogen-rich gas and a liquid product. The separation was carried out at a temperature of 150.degree. C. and the same pressure as in the hydrotreatment step. The hydrogen-rich gas was recycled to the thermal cracking zone after the removal of its impurities such as hydrogen sulfide and ammonia by washing. A portion of the recycling gas was diverted out of the system so as to maintain the concentration of a light hydrocarbon gas in the recycling gas below a predetermined level. The liquid product was, after being released of its pressure, introduced into another gas-liquid separating device where it was separated into a cracked gas and a product oil. The properties of the product oil at about 1000 hours process time are shown in Table 4. The stability and compatibility of the product oil were examined in accordance with ASTM D 1661 and 2781. The test procedures are briefly summarized below.

The test sample, circulated by thermosiphon action through a glass apparatus, is in contact for 6 hours with the surface of a steel thimble containing a sheathed heating element. The sample is heated on the hot surface of the steel thimble in a heater chamber, flows upward through a riser tube, then descends in a connecting tube and returns to the heater chamber for recirculation. The temperature between the sheathed heating element and the thimble in the heater chamber is maintained at 350.degree. F. (177.degree. C.). At the end of the test period, the thimble is removed and examined for sediment formation and discoloration on the surface. Thermal stability is rated in accordance with the following ratings:

Equal volumes of the test sample and the distillate fuel oil are blended. A drop of the blend is allowed to spread on chromatographic paper of a specified grade. The spot thus formed is compared with a series of numbered reference spots. The compatibility of the sample is rated on the basis of this comparison and is represented by reference spot number. Larger spot number designates poor compatibility.

Example 1 was repeated using Catalyst II having the composition and properties shown in Table 3 in place of Catalyst I. The properties of the resulting product oil at about 1000 hours process time are shown in Table 4.

Example 1 was repeated without the catalytic hydrotreatment. Thus, the feedstock oil used in Example 1 was subjected to the thermal cracking treatment alone and the cracked oil was directly introduced into the gas-liquid separator without undergoing the catalytic hydrotreatment. The properties of the resulting product oil at about 1000 hours process time were as shown in Table 4.

Example 1 was repeated using Catalyst III having the composition and the properties shown in Table 3 in place of Catalyst I. The properties of the product oil at about 1000 hours process time are shown in Table 4.

Example 1 was repeated using Catalyst IV having the composition and the properties shown in Table 3 in place of Catalyst I. The properties of the product oil at about 1000 hours process time are shown in Table 4.

From the results shown in Table 4, it will be appreciated that the catalysts having an average pore diameter of over 200 .ANG. can give stable oil products with excellent compatibility. On the other hand, the oil products obtained with the use of the catalysts in Comparative Examples 2 and 3 which have an average pore diameter of below 200 .ANG., are unstable and have poor compatibility.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all the changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.