USE OF NAPHTHA IN CARBO-METALLIC OIL CONVERSION

Abstract: A process is disclosed for the production of high octane gasoline and/or other valuable lower molecular weight products from carbo-metallic oils. Examples include crude oil, topped crude, reduced crude, residua, the extract from solvent deasphalting and other heavy hydrocarbon fractions. These carbo-metallic oils contain quantities of coke precur­ sors and heavy metal catalyst poisons substantially in excess of what is normally considered acceptable for Fluid Catalytic Cracking (FCC) and substantial amounts of sulfur, nitrogen and other troublesome components may also be present. Such carbo-metallic oils are converted to the desired pro­ ducts in a catalytic conversion process where the oil feed is mixed with naphtha and brought together with a high-metal content cracking catalyst. Named "RCC" (Reduced Crude Conversion) after a particularly common or useful car­ bometallic feed, the present process is by no means restricted to reduced crude or to oils of petroleum origin, having utility in the processing of oils from coal, shale and other sources.

Description: USE OF NAPHTHA IN CARBO-METALLIC OIL CONVERSION This application is a continuation-in-part application of copending applications Serial Nos. 94,092 and 94,216, each filed on November 14, 1979 and each entitled "Carbo-Metallic Oil Conversion". BACKGROUND OF THE INVENTION In general, gasoline and other liquid hydrocarbon fuels boil in the range of about 1000 to about 6500F. However, the crude oil from which these fuels are made contains a diverse mixture of hydrocarbons and other compounds which vary widely in molecular weight and therefore boil over a wide range. For example, crude oils are known in which 30% to o0% or more of the total volume of oil is composed of compounds boiling at temperatures above 650OF. Among these are crudes in which about 10% to about 30% or more of the total volume consists of compounds so heavy in molecular weight that they boil above 10250F or at least will not boil below 10250F at atmospheric pressure. Because these relatively abundant high boiling components of crude oil are unsuitable for inclusion in gasoline and other liquid hydrocarbon fuels, the petroleum refining industry has developed processes for cracking or breaking the molecules of the high molecular weight, high boiling compounds into smaller molecules which do boil over an appropriate boiling range. The cracking process which is most widely used for this purpose is known as fluid catalytic cracking (FCC). Although the FCC process has reached a highly advanced state, and many modified forms and variations have been developed, their unifying factor is that a vaporized hydrocarbon feedstock is caused to crack at an elevated temperature in contact with a cracking catalyst that is suspended in the feedstock vapors. Upon attainment of the desired degree of molecular weight and boiling point reduction, the catalyst is separated from the desired products. Crude oil in the natural state contains a variety of materials which ter.d to have quite troublesome effects op FCC processes, and only a portion of these troublesome materials can be economically removed from the crude oil. Among these troublesome materials are coke precursors (such as asphaltenes, polynuclear aromatics, etc.), heavy metals (such as nickel, vanadium, iron, copper, etc.), lighter metals (such as sodium, potassium, etc.), sulfur, nitrogen and others. Certain of these, such as the lighter metals, can be economically removed by desalting operations, which are part of the normal procedure for pretreating crude oil for fluid catalytic cracking. Other materials, such as coke precursors, asphaltenes and the like, tend to break down into coke during the cracking operation, which coke deposits on the catalyst, impairing contact between the hydrocarbon feedstock and the catalyst, and generally reducing its potency or activity level. The heavy metals transfer almost quantitatively from the feedstock to the catalyst surface. If the catalyst is reused again and again for processing additional feedstock, which is usually the case, the heavy metals can accumulate on the catalyst to the point that they unfavorably alter the composition of the catalyst and/or the nature of its effect upon the feedstock. For example, vanadium tends to form fluxes with certain components of commonly used FCC catalysts, lowering the melting point of portions of the catalyst particles sufficiently so that they begin to sinter and become ineffective cracking catalysts. Accumulations of vanadium and other heavy metals, especially nickel, also "poison" the catalyst. They tend in varying degrees to promote excessive dehydrogenation and aromatic condensation, resulting in excessive production of carbon and gases with consequent impairment of liquid fuel yield. An oil such as a crude or crude fraction or other oil that is particularly abundant in nickel and/or other metals exhibiting similar behavior, while containing relatively large quantities of coke precursors, is referred to herein as a carbo-metallic oil, and represents a particular challenge to the petroleum refiner In general the coke-forming tendency or coke precursor content of an oil can be ascertained by determining the weight percent of carbon remaining after a sample of that oil has been pyrolyzed. The industry accepts this value as a measure of the extent to which a given oil tends to form non-catalytic coke when employed as feedstock in a catalytic cracker. Two established tests are recognized, the Conradson Carbon and Ramsbottom Carbon tests, the latter being described in ASTSE Test No D524-76. In conventional FCC practice, Ramsbottom carbon values on the order of about 0.1 to about 1.0 are regarded as indicative of acceptable feed. The present invention is concerned with the use of hydrocarbon feedstocks which have higher Ramsbottom carbon values and thus exhibit substantially' greater potential for coke formation than the usual feeds. Since the various heavy metals are not of equal catalyst poisoning activity, it is convenient to express the poisoning activity of an oil containing a given poisoning metal or metals in terms of the amount of a single metal which is estimated to have equivalent poisoning activity. Thus, the heavy metals content of an oil can be expressed by the following formula (patterned after that of W. L. Nelson in Oil and Gas Journal, page 143, October 23, 1961) in which the content of each metal present is expressed in parts per million of such metal, on a weight basis, based on the weight of feed: V Fe Cu Nickel Equivalents = Ni + 4.8 + 7.1 + 1.23 According to conventional FCC practice, the heavy metal content of feedstock for FCC processing is controlled at a relatively low level, e.g. about 0.25 ppm Nickel Equivalents or less. The present invention is concerned with the processing of feedstocks containing metals substantially in excess of this value, and which therefore have a significantly greater potential for accumulating on and poisoning catalyst. The above formula can also be employed as a measure of the accumulation of heavy metals on cracking catalyst, except that the quantity of metal employed in the formula is based on the weight of catalyst (moisture-free basis) instead of the weight of feed. In conventional FCC practice, in which a circulating inventory of catalyst is used again and again in the processing of fresh feed, with periodic or continuing minor addition and withdrawal of fresh and spent catalyst, the metal content of the catalyst is maintained at a level which may, for example, be in the range of about 200 to about 600 ppm Nickel Equivalents. The process of the present invention is concerned with the use of catalyst having a substantially larger metals content, and which therefore has a much greater than normal tendency to promote dehydrogenation, aromatic condensation, gas production or coke formation. Therefore, such higher metals accumulation is normally considered as quite undesirable in FCC processing. There has been a long standing interest in the conversion of carbo-metallic oils into gasoline and other liquid fuels. For example, in the 1950s it was suggested that a variety of carbo-metallic oils could be successfully converted to gasoline and other products in the Houdresid process. Turning from the FCC mode of operation, the Houdresid process employed catalyst particles of Hgranular size" (much larger than conventional FCC catalyst particle size) in a compact gravitating bed, rather than suspending catalyst particles in feed and product vapors in a fluidized bed. The productivity of the process, compared to fluid catalytic cracking with lighter gas oils, was low. But the Houdresid process did offer some advantages. It appeared that the adverse effects previously encountered with heavy metals in the feed were not as great a barrier in the Houdresid process as one might expect in FCC processing. The heavy metals which accumulated on or near the outer surfaces of the catalyst particles apparently could be removed to some extent by an attrition process, which selectively removed an outer layer of metal-contaminated catalyst. The catalysts were very cheap, but also relatively inactive, highly unsuitable by today's standards. While the maximum tolerable limit of heavy metal contamination on catalyst in fluid catalytic cracking was then thought to be about 200 ppm, the Houdresid process did continue to operate satisfactorily even when the total nickel plus vanadium content of the catalyst had reached 870 ppm. Moreover, it was found that the required levels of selectivity could be maintained without withdrawing catalyst from the process, except to the extent that withdrawal was required by normal mechanical losses (e.g., attrition and inadvertent discharge with off gases) and by the attrition used to control metals level. Today such attrition of catalyst to fine particulates would present an expensive environmental problem, thus considerably increasing difficulties involved in practicing the process. Although the Houdresid process obviously represented a step forward in dealing with the effects of metal contamination and coke formation on catalyst performance, its productivity was limited. Because its operation was uneconomical, the first Houdresid unit is no longer operating. Thus, for the 25 years which have passed since the Houdresid process was first introduced commercially, the art has continued its arduous search for suitable modifications or alternatives to the FCC process which would permit commercially successful operation on reduced crude and the like. During this period, a lumber of proposals have been made; some have been used commercially to a certain extent. Several proposals involve treating the heavy oil feed to remove the metal therefrom prior to cracking, such as by hydrotreating, solvent extraction and complexing with Friedel Crafts catalysts, but these techniques have been criticized as unjustified economically. Another proposal employs a combination cracking process having "dirty oil" and "clean oil" units. Still another proposal blends residual oil with gas oil and controls the quantity of residual oil in the mixcure in relation to the equilibrium flash vaporization temperature at the bottom of the riser type cracker unit employed in the process. Still another proposal subjects the feed to a mild preliminary hydrocracking or hydrotreating operation before it is introduced into the cracking unit. It has also been suggested to contact a carbo-metallic oil such as reduced crude with hot taconite pellets to produce gasoline. This is a small sampling of the many proposals which have appeared in the patent literature and technical reports. Notwithstanding the great effort which has been expended and the fact that each of these proposals overcomes some of the difficulties involved, conventional FCC practice today bears mute testimony to the dearth of carbo-metallic oilcracking techniques that are both economical and highly practical in terms of technical feasibility. Some crude oils are relatively free of coke precursors or heavy metals or both, and the troublesome components of crude oil are, for the most part, concentrated in the highest boiling fractions. Accordingly, it has been possible to largely avoid the problems of coke precursors and heavy metals by sacrificing the liquid fuel yield which would be potentially available from the highest boiling fractions. More particularly, conventional FCC practice has employed as feedstock that fraction of crude oil which boils at about 650"F to about 1000 F, such fractions being relatively free of coke precursors and heavy metal contamination. Such feedstock, known as "vacuum gas oil" (VGO), is generally prepared from crude oil by distilling off the fractions boiling below about 650"F at atmospheric pressure and then separating by further vacuum distillation from the heavier fractions a cut boiling between about 650"F and about 900" to 10250F. The vacuum gas oil is used as feedstock for conventional FCC processing. The heavier fractions are normally employed for a variety of other purposes, such as, for instance, production of asphalt, residual fuel oil, &num;6 fuel oil, or marine Bunker C fuel oil, which represents a great waste of the potential value of this portion of the crude oil, especially in light of the great effort and expense which the art has been willing to expend in the attempt to produce generally similar materials from coal and shale oils. The present invention is aimed at the simultaneous cracking of these heavier fractions containing substantial quantities of both coke precursors and heavy metals, and possibly other troublesome components, in conjunction with the lighter oils, thereby increasing the overall yield of gasoline and other hydrocarbon liquid fuels from a given quantity of crude. As indicated above, the present invention by no means constitutes the first attempt to develop such a process, but the long standing recognition of the desirability of cracking carbo-metallic feedstocks, along with the slow progress of the industry toward doing so, show the continuing need for such a process. It is believed that the present process is uniquely advantageous for dealing with the problem of treating such carbo-metallic oils in an economically and technically sound manner. SUMMARY OF THE INVENTION The present invention is notable in providing a simple, relatively straightforward and highly productive approach to the conversion of carbo-metallic feed such as reduced crude or the like to various lighter products such as gasoline. The carbo-metallic feed comprises or is composed of oil which boils above about 6500F. Such oil, or at least the 6500F+ portion thereof, is characterized by a heavy metal content of at least about 4, preferably more than about 5, and most preferably at least about 5.5 ppm of Nickel Equivalents by weight and by a carbon residue on pyrolysis of at least about 1% and more preferably at least about 2% by weight. In accordance with the invention, the carbo-metallic or converter feed, in the form of a pumpable liquid, naphtha, and hot conversion catalyst are brought together in a weight ratio of catalyst to feed mixture in the range of about 2 to about 12 and preferably more than about 4. The carbo-metallic or converter feed is mixed or brought into contact with naphtha, in a weight percentage of the converter feed in an amount ranging from about 1 to about 20%, more preferably from about 1 to about 10% and still more preferably from about 1 to about 5%. The naphtha, and any other additional materials, may be brought into admixture with the converter feed prior to, during or after mixing the feed with the aforementioned catalyst, and either after or, preferably, before, vaporization of the feed. The naphtha may also be mixed with the catalyst prior to being mixed with the converter feed. With this arrangement, the same amount of naphtha is used as when first mixed with the converter feed. This amount being in a weight percentage of the catalyst ranging from 1 to about 9%, more preferably from about 3 to 9% and still more preferably from about 3 to 6%. The carbo-metallic, converter feed of said feed mixture undergoes a conversion step which includes cracking while the combination of feed naphtha and catalyst is flowing through a progressive flow type reactor. The feed, naphtha, catalyst, steam and other materials may be introduced at one or more points. The reactor includes a elongated reaction chamber which is at least partly vertical or inclined and in which the feed mixture, resultant products and catalyst are maintained in contact with one another while flowing as a dilute phase or stream for a predetermined riser residence time in the range of about 0.5 to about 10 seconds. The reaction is conducted at a temperature of about 9000 to about 14000F, measured at the reaction chamber exit, under a total pressure of about 10 to about 50 psia (pounds per square inch absolute) under conditions sufficiently severe to provide a conversion per pass in the range of about 50% or more and to lay down coke on the catalyst in an amount in the range of about 0.3 to about 3% by weight and preferably at least about 0.5%. The overall rate of coke production, based on weight of fresh feed, is in the range of about 4 to about 14% by weight. At the end of the predetermined residence time, the catalyst is projected in a direction established by the elongated reaction chamber or an extension thereof, while the products, having lesser momentum, are caused to make an abrupt change of direction, resulting in an abrupt, substantially instantaneous ballistic separation of products from catalyst. The thus separated catalyst is stripped to remove high boiling components and other entrained or adsorbed hydrocarbons and the stripped catalyst is then regenerated with oxygen-containing combustion-supporting gas under conditions of time, temperature -and atmosphere sufficient to reduce the carbon on the regenerated catalyst to about 0.25% or less and preferably about 0.05% or less by weight. The regenerated catalyst is recycled to the reactor for contact with fresh feed. Depending on how the process of the invention is practiced, one or more of the following advantages may be realized. If desired, and preferably, the process may be operated without added hydrogen in the reaction chamber. If desired, and preferably, the process may be operated without prior hydrotreating of the feed and/or without other process of removal of asphaltenes or metals from the feed, and this is true even where the carbo-metallic oil as a whole contains more than about 4, or more than about 5 or even more than about 5.5 ppm Nickel Equivalents by weight of heavy metal and has a carbon residue on pyrolysis greater than about 1%, greater than about 1.4% or greater than about 2% by weight. Moreover, all of the converter feed, as above described, may be cracked in one and the same conversion chamber. The cracking reaction may be carried out with a catalyst which has previously been used (recycled, except for such replacement as required to compensate for normal losses and deactivation) to crack a carbo-metallic feed under the above described conditions. Heavy hydrocarbons not cracked to gasoline in a first pass may be recycled with or without hydrotreating for further cracking in contact with the same kind of feed in which they were first subjected to cracking conditions, and under the same kind of conditions; but operation in a substantially once-through or single pass mode (e.g. less than about 15% by volume of recycle based on volume of fresh feed) is preferred. The process as above-described may be practiced in conjunction with a number of preferred alternatives, refinements or more commonly encountered conditions, a few of which will be referred to under the heading "Description of Various and Preferred Embodiments" below. BRIEF DESCRIPTION OF THE DRANGNGS Figure 1 is a schematic diagram of a first apparatus for carrying out the invention. Figure 2 is a schematic diagram of a second apparatus for carrying out the invention. Figures 3 through 5 are graphs of data obtained from operating the process of the invention. DESCRIPTION OF VARIOUS A1D PREFERPED EMB0DhNENTS The present invention provides a process for the continuous catalytic conversion of a wide variety of carbometallic oils to lower molecular weight products, while maximizing production of highly valuable liquid products, and making it possible, if desired, to avoid vacuum distillation and other expensive treatments such as hydrotreating. The term "oils", includes not only those predominantly hydrocarbon compositions which are liquid at room temperature (i.e., 68OF), but also those predominantly hydrocarbon compositions which are asphalts or tars at ambient temperature but liquify when heated to temperatures in the range of up to about 800OF. The invention is applicable to carbometallic oils, whether of petroleum origin or not. For example, provided they have a similar boiling range, carbon residue on pyrolysis and heavy metals content, the invention may be applied to the processing of such widely diverse materials as heavy bottoms from crude oil, heavy bitumen crude oil, those crude oils known as "heavy crude" which approximate the properties of reduced crude; shale oil, tar sand extract, products from coal liquifaction and solvated coal, atmospheric and vacuum reduced crude, extracts and/or bottoms (raffinate) from solvent deasphalting, aromatic extract from lube oil refining, tar bottoms, heavy cycle oil, slop oil, other refinery waste streams and mixtures of the foregoing. Such mixtures can for instance be prepared by mixing available hydrocarbon fractions, including oils, tars, pitches and the like. Also, powdered coal may be suspended in the carbo-metallic oil. Persons skilled in the art are aware of techniques for demetalation of carbometallic oils, and demetalated oils may be converted using the invention; but it is an advantage of the invention that it can employ as feedstock carbo-metallic oils that have had no prior demetalation treatment. Likewise, the invention can be applied to-hydrotreated feedstocks; but it is an advantage of the invention that it can successfully convert carbo-metallic oils which have had substantially no prior hydrotreatment. However, the preferred application of the process is to reduced crude, i.e., that fraction of crude oil boiling at and above 6500F, alone or in admixture with virgin gas oils. While the use of material that has been subjected to prior vacuum distillation is not excluded, it is an advantage of the invention that it can satisfactorily process material which has had no prior vacuum distillation, thus saving on capital investment and operating costs as compared to conventional FCC processes that require a vacuum distillation unit. Table I below provides a comparison between a typical vacuum gas oil (VGO) which has been used heretofore in fluid catalytic cracking, with various reduced crudes, constituting a few examples of the many reduced crudes useable in the present invention: TABLE I Volume % of 650+ API Fraction Ramsbottom Wt. ppm Wt. % Weight of Grav. Boiling at Carbon Content Metals(3) S in Nitrogen (ppm) Wt. 650+ (2) 650 650- 650+ Ni 650+ 650+ Fraction ppm Oil or Crude Total 1025 1025+ 1025 Total Ni V Fe Equiv. Total Total Basic Na VGO 28.4 100 0.0 0.38 0.38 0.2 0.1 2.6 0.59 .83 722 260 0.8 Mexican Isthmus 16.9 65.3 34.7 0.49 4.96 2.5+ 33.8 1.9 9.81 2.75 950 450 .9 (21.3) Mexican Kirkuk (1) 17.4 9.30 35.0 99.0 17.0 58.02 2.94 2100 723 1.8 Murban 23.1 78.7 21.3 0.49 3.99 3.0+ 1.5 11.9 4.99 1.64 512 200 7.5 Arabian Light 19.1 64.7 35.3 0.47 6.56 6.4 24.7 3.2 12.00 2.39 940 507 9.2 Arabian Med. 14.5 51.8 48.2 0.46 9.00 19.6 63.0 2.9 33.13 4.43 Ekofisk 22.7 72.8 27.2 0.36 4.42 1.4 3.0 2.4 2.36 0.38 Fosterton 10.9 43.6 56.4 0.42 16.81 48.8 119.0 3.1 74.03 4.22 Iranian Light 17.4 60.8 39.2 0.48 9.01 21.9 60.0 3.1 34.84 2.50 (4) La./Miss Sweet 23.7 80.2 19.8 0.33 4.36 2.7+ --- 8.5 3.90 0.26 Wyoming Sour 12.4 40.7 59.3 0.32 15.1 0.6 70.0 2.0 15.47 3.84 (1) A refinery blend of Mexican and Kirkuk crudes. (2) Throughout the table 650 and 1025 refer to 650 F and 1025 F respectively; 650+ refers to 650F+ material as defined below. (3) Copper level was below 0.5%, except that Mexican Kirkuk contained 0.6%; all metals expressed as metal in ppm, based on the weight of the 650+ fraction. (4) Calculated. In accordance with the invention one provides a carbometallic oil feedstock, at least about 70%, more preferably at least about 85% and still more preferably about 100% (by volume) of which boils at and above about 6500F. All boiling temperatures herein are based on standard atmospheric pressure conditions. In carbo-metallic oil partly or wholly composed of material which boils at and above about 6500F, such material is referred to herein as 6500F+ material; and 6500F+ material which is part of or has been separated from an oil containing components boiling above and below 6500F may be referred to as a 650OF+ fraction. But the terms "boils above" and "6500F+" are not intended to imply that all of the material characterized by said terms will have the capability of boiling. The carbo-metallic oils contemplated by the invention may contain material which may not boil under any conditions; for example, certain asphalts and asphaltenes may crack thermally during distillation, apparently without boiling. Thus, for example, when it is said that the feed comprises at least about 70% by volume of material which boils above about 6500F, it should be understood that the 70% in question may. include some material which will not boil or volatilize at any temperature. These non-boilable materials when present, may frequently or for the most part be concentrated in portions of the feed which do not boil below about 10000F, 10250F or higher. Thus, when it is said that at least about 10%, more preferably about 15% and still more preferably at least about 20% (by volume) of the 6500F+ fraction will not boil below about 10000F or 10250F, it should be understood that all or any part of the material not boiling below about 10000 or 10250F, may or may not be volatile at and above the indicated temperatures. Preferably, the contemplated feeds, or at least the 6500F+ material therein, have a carbon residue on pyrolysis of at least about 2 or greater. For example, the Ramsbottom carbon content may be in the range of about 2 to about 12 and most frequently at least about 4. A particularly common range is about 4 to about 8. Note that the illustrative VGO in Table I has a Ramsbottom carbon residue value of 0.38, and that the 6500 to 10250F fractions of the various reduced crudes have Ramsbottom carbon values between about 0.3 and about 0.5, whereas the various reduced crudes as a whole (650+ Total) vary upwards in Ramsbottom carbon value from about 4 to about 16.8, and still higher values are contemplated. Preferably, the feed has an average composition characterized by an atomic hydrogen to carbon ratio in the range of about 1.2 to about 1.9, and preferably about 1.3 to about 1.8. The carbo-metallic feeds employed in accordance with the invention, or at least the 6500F+ material therein, may contain at least about 4 parts per million of Nickel Equivalents, as defined above, of which at least about 2 parts per million is nickel (as metal, by weight) - Carbo-metallic oils within the above range can be prepared from mixtures of two or more oils, some of which do and some of which do not contain the quantities of Nickel Equivalents and nickel set forth above. It should also be noted that the above values for Nickel Equivalents and nickel represent time-weighted averages for a substantial period of operation of the conversion unit, such as one month, for example. It should also be noted that the heavy metals have in certain circumstances exhibited some lessening of poisoning tendency after repeated oxidations and reductions on the catalyst, and the literature describes criteria for establishing "effective metal" values. For example, see the article by Cimbalo, et al, entitled "Deposited Metals Poison FCC Catalyst", Oil and Gas Journal, May 15, 1972, pp 112-122, the contents of which are incorporated herein by reference. If considered necessary or desirable, the contents of Nickel Equivalents and nickel in the carbo-metallic oils processed according to the invention may be expressed in terms of "effective metal" values. Notwithstanding the gradual reduction in poisoning activity noted by Cimbalo, et al, the regeneration of catalyst under normal FCC regeneration conditions may not, and usually does not, severely impair the dehydrogenation, demethanation and aromatic condensation activity of heavy metals accumulated on cracking catalyst. It is known that about 0.2 to about 5 weight per cent of "sulfur" in the form of elemental sulfur and/or its compounds (but reported as elemental sulfur based on the weight of feed) appears in FCC feeds and that the sulfur and modified forms of sulfur can find their way into the resultant gasoline product and, where lead is added, tends to reduce its susceptibility to octane enhancement. Sulfur in the product gasoline often requires sweetening when processing high sulfur containing crudes. To the extent that sulfur is present in the coke, it also represents a potential air pollutant since the regenerator burns it to SO2 and S03- However, we have found that in our process the sulfur in the feed is on the other hand able to inhibit heavy metal activity by maintaining metals such as Ni, V, Cu and Fe in the sulfide form in the reactor. These sulfides are much less active than the metals themselves in promoting dehydrogenation and coking reactions. Accordingly, it is acceptable to carry out the invention with a carbo-metallic oil having at least about 0.3%, acceptably more than about 0.8% and more acceptably at least about 1.5% by weight of sulfur in the 650OF* fraction. The carbo-metallic oils useful in the invention may and usually do contain significant quantities of compounds containing nitrogen, a substantial portion of which may be basic nitrogen. For example, the total nitrogen content of the carbo-metallic oils may be at least about 0.05% by weight. Since cracking catalysts owe their cracking activity to acid sites on the catalyst surface or in its pores, basic nitrogen-containing compounds may temporarily neutralize these sites, poisoning the catalyst. However, the catalyst is not permanently damaged since the nitrogen can be burned off the catalyst during regeneration, as a result of which the acidity of the active sites is restored. The carbo-metallic oils may also include significant quantities of pentane insolubles, for example at least about 0.5% by weight, and more typically about 2% or more or even about 4% or more. These may include for instance asphaltenes and other materials. Alkali and alkaline earth metals generally do not tend to vaporize in large quantities under the distillation con ditions employed in distilling crude oil to prepare the vacuum gas oils normally used as FCC feedstocks. Rather, these metals remain for the most part in the "bottoms" fraction (the non-vaporized high boiling portion) which may for instance be used in the production of asphalt or other byproducts. However, reduced crude and other carbo-metallic oils are in many cases bottoms products, and therefore may contain significant quantities of alkali and alkaline earth metals such as sodium. These metals deposit upon the catalyst during cracking. Depending on the composition of the catalyst and magnitude of the regeneration temperatures to which it is exposed, these metals may undergo interactions and reactions with the catalyst (including the catalyst support) which are not normally experienced in processing VGO under conventional FCC processing conditions. If the catalyst characteristics and regeneration conditions so require, one will of course take the necessary precautions to limit the amounts of alkali and alkaline earth metal in the feed, which metals may enter the feed not only as brine associated with the crude oil in its natural state, but also as components of water or steam which are supplied to the cracking unit. Thus, careful desalting of the crude used to prepare the carbo-metallic feed may be important when the catalyst is particularly susceptible to alkali and alkaline earth metals. In such circumstances, the content of such metals (hereinafter collectively referred to as "sodium") in the feed can be maintained at about 1 ppm or less, based on the weight of the feedstock. Alternatively, the sodium level of the feed may be keyed to that of the catalyst, so as to maintain the sodium level of the catalyst which is in use substantially the same as or less than that of the replacement catalyst which is charged to the unit. According to a particularly preferred embodiment of the invention, the carbo-metallic oil feedstock constitutes at least about 70% by volume of material which boils above about 6500F, and at least about 10% of the material which boils above about 6500F will not boil below about 1025OF. The average composition of this 6500F+ material may be further characterized by: (a) an atomic hydrogen to carbon ratio in the range of about 1.3 to about 1.8: (b) a Ramsbottom carbon value of at least about 2: (c) at least about four parts per million of Nickel Equivalents, as defined above, of which at least about two parts per million is nickel (as metal, by weight); and (d) at least one of the the following: (i) at least about 0.3% by weight of sulfur, (ii) at least about 0.05% by weight of nitrogen, and (iii) at least about 0.5% by weight of pentane insolubles. Very commonly, the preferred feed will include all of (i), (ii), and (iii), and other components found in oils of petroleum and non-petroleum origin may also be present in varying quantities providing they do not prevent operation of the process. According to the present invention, the naphtha that is mixed with either the carbo-metallic, converter feed or the catalyst, has a higher atomic hydrogen to carbon ratio than that of the converter feed. Although there is no intention of excluding the possibility of using a feedstock which has previously been subjected to some cracking, the present invention has the definite advantage that it can successfully produce large conversions and very substantial yields of liquid hydrocarbon fuels from carbo-metallic oils which have not been subjected to any substantial amount of cracking. Thus, for example, and preferably, at least about 85%, more preferably at least about 90 and most preferably substantially all of the carbo-metallic feed introduced into the present process is oil which has not previously been contacted with cracked7 catalyst under cracking conditions. Moreover, the process of the invention is suitable for operation in a substantially once-through or single pass mode. Thus, the volume of recycle, if any, based on the volume of fresh feed is preferably about 15% or less and more preferably about 10% or less. In general, the weight ratio of catalyst to fresh converter feed (feed which has not previously been exposed to cracking catalyst under cracking conditions) used in the process is in the range of about 3 to about 18. Preferred and more preferred ratios are about 4 to about 12, more preferably about 5 to about 10 and still more preferably about 6 to about 10, a ratio of about 10 presently being considered most nearly optimum. The weight ratio of catalyst to feed mixture (i.e., converter feed and naphtha) ranges from about 2 to about 12 and preferably more than about 4. Within the limitations of product quality requirements, controlling the catalyst to oil or feed mixture ratio at relatively low levels within the aforesaid ranges tends to reduce the coke yield of the process, based on fresh feed. In conventional FCC processing of VGO, the ratio between the number of barrels per day of plant through-put and the total number of tons of catalyst undergoing circulation throughout all phases of the process can vary widely. For purposes of this disclosure, daily plant through-put is defined as the number of barrels of fresh feed boiling above about 6500F which that plant processes per average day of operation to liquid products boiling below about 4300F. For example, in one commercially successful type of FCC-VGO operation, about 8 to about 12 tons of catalyst are under circulation in the process per 1000 barrels per day of plant through-put. In another commercially successful process, this ratio is in the range of about 2 to 3. While the present invention may be practiced in the range of about 2 to about 30 and more typically about 2 to about 12 tons of catalyst inventory per 1000 barrels of daily plant throughput, it is preferred to carry out the process of the present invention with a very small ratio of catalyst weight to daily plant through-put. More specifically, it is preferred to carry out the process of the present invention with an inventory of catalyst that is sufficient to contact the feed for the desired residence time in the above indicated catalyst to oil ratio while minimizing the amount of catalyst inventory, relative to plant through-put, which is undergoing circulation or being held for treatment in other phases of the process such as, for example, stripping, regeneration and the like. Thus, more particularly, it is preferred to carry out the process of the present invention with about 2 to about 5 and more preferably about 2 tons of catalyst inventory or less per thousand barrels of daily plant through-put. In the practice of the invention, catalyst may be added continuously or periodically, such as, for example, to make up for normal losses of catalyst from the system. Moreover, catalyst addition may be conducted in conjunction with withdrawal of catalyst,. such as, for example, to maintain or increase the average activity level of the catalyst in the unit. For example, the rate at which virgin catalyst is added to the unit may be in the range of about 0.1 to about 3, more preferably about 0.15 to about 2, and most preferably to about 0.2 to about 1.5 pounds per barrel of carbometallic oil, converter feed. If on the other hand esuilib- rium catalyst from FCC operation is to be utilized, replacement rates as high as about 5 pounds per barrel can be practiced. Where circumstances are such that the catalyst employed in the unit is below average in resistance to deactivation and/or conditions prevailing in the unit tend to promote more rapid deactivation, one may employ rates of addition greater than those stated above; but in the opposite circumstances, lower rates of addition may be employed. By way of illustration, if a unit were operated with a metal(s) loading of 5000 ppm nickel plus vanadium in parts by weight on equilibrium catalyst one might for example employ a replacement rate of about 2.7 pounds of catalyst introduced for each barrel (42 gallon) of feed process. However, operation at a higher level such as 10,000 ppm nickel plus vanadium on catalyst would enable one to substantially reduce the replacement rate, such as for example to about 1.3 pounds of catalyst per barrel of feed. Thus, the levels of metal(s) on catalyst and catalyst replacement rates may in general be respectively increased and decreased to any value consistent with the catalyst activity which is available and desired for conducting the process. Without wishing to be bound by any theory, it appears that a number of features of the process to be described in greater detail below, such as, for instance, the residence time and optional mixing of steam with the feedstock, tend to restrict the extent to which cracking conditions produce metals in the reduced state on the catalyst from heavy metal sulfide(s), sulfate(s) or oxide(s) deposited on the catalyst particles by prior exposures to carbo-metallic feedstock and regeneration conditions. Thus, the process appears to afford significant control over the poisoning effect of heavy metals on the catalyst, even when the accumulations of such metals are quite substantial. Accordingly, the process may be practiced with catalyst bearing accumulations of heavy metal(s) in the form of elemental metal(s), oxide(s), sulfide(s), or other compounds which heretofore would have been considered quite intolerable in conventional FCC VGO operations. Thus, operation of the process with catalyst bearing heavy metals accumulations in the range of about 3000 or more ppm Nickel Equilvalents, on the average, is contemplated. The concentration of nickel equivalents of metal on catalyst can range up to 50,000 ppm or higher. More specifically, the accumulation may be in the range of about 3000 to about 30,000 ppm, preferably from about 3000 to about 20,000 ppm, and more particularly from about 3000 to about 12,000 ppm. Within these ranges just mentioned, operation at metal(s) levels of about 4,000 or more, or about 5,000 or more or-about 7,000 or more ppm can tend to reduce the rate of catalyst replacement required. The foregoing amounts are based on-parts per million of Nickel Equivalents, in which the metals are expressed as metal, by weight, measured on and based on regenerated equilibrium catalyst. However, in the event that catalyst of adequate activity is available at very low cost, making feasible very high rates of catalyst replacement, the carbo-metallic oil could be converted to lower boiling liquid products with catalyst bearing less than 3,000 ppm Nickel Equivalents of heavy metals. For example, one might employ equi librium catalyst from another unit, for example, an FCC unit which has been used in the cracking of a feed, e.g. vacuum gas oil, having a carbon residue on pyrolysis of less than 1 and containing less than about 4 ppm Nickel Equivalents of heavy metals. In any event, the equilibrium concentration of heavy metals in the circulating inventory of catalyst can be controlled (including maintained or varied as desired or needed) by manipulation of the rate of catalyst addition discussed above. Thus, for example, addition of catalyst may be maintained at a rate which will control the heavy metals acc=ulation on the catalyst in one of the ranges set forth above. In general, it is preferred to employ a catalyst having a relatively high level of cracking activity, providing high levels of conversion and productivity at low residence times. The conversion capabilities of the catalyst may be expressed in terms of the conversion produced during actual operation of the process and/or in terms of conversion produced in standard catalyst activity tests. For example, it is preferred to employ catalyst which, in the course of extended operation under prevailing process conditions, is sufficiently active for sustaining a level of conversion of at least about 505 and more preferably at least about 60,. In this connection, conversion is expressed in liquid volume percent, based on fresh feed. Also, for example, the preferred catalyst may be defined as one which, in its virgin or esuilibrium state, exhibits a specified activity expressed as a percentage in terms of MAT (micro-activity test) conversion. For purposes of the present invention the foregoing percentage is the volume percentage of standard feedstock which a catalyst under evaluation will convert to 4300F end point gasoline, lighter products and coke at 9000F, 16 TESV (weight hourly space velocity, calculated on a moisture free basis, using clean catalyst which has been dried at 11000F, weighed and then conditioned, for a period of at least 8 hours at about 250C and 50% relative humidity, until about one hour or less prior to contacting the feed) and 3C/O (catalyst to oil weight ratio) by ASTM D-32 MAT test D-3907-8C, using an appropriate standard feedstock, e.g. a sweet light primary gas oil, such as that used by Davison, Division of W.R. - Grace, having the following analysis and properties: : API Gravity at 600F, degrees 31.0 Specific Gravity at 600F, g/ce 0.8708 Ramsbottom Carbon, wt. % 0.09 Conradson Carbon, wt. %(est.) 0.04 Carbon, wt. 9 84.92 Hydrogen, wt. % 12.94 Sulfur, wt. % 0.68 Nitrogen. ppm 305 Viscosity at 1000F, centistokes - 10.36 Watson K Factor 11.93 Aniline Point 182 Bromine No. 2.2 Paraffins, Vol. % 31.7 Olefins, Vol. % 1.6 Naphthenes, Vol. % 44.0- Aromatics, Vol. % 22.7 Average Molecular weight 284 Nickel Trace Vanadium Trace Iron Trace Sodium Trace Chlorides Trace BS & Trace Distillation ASTM D-1160 IBP 445 10% 601 30% 664 50% 701 70% 734 90% 787 FBP 834 The gasoline end point and boiling temperature-volume percent relationships of the product produced in the 06AT conversion test may for example be determined by simulated ¯stillation techniques, for example modifications of gas chromate graphic "Sim-D", ASTh D-2887-73. The results of such simulations are in reasonable agreenent with the results obtained by subjecting larger samples of material to standard laboratory distillation techniques. Conversion is calculated by subtracting from 100 the volume percent (based on fresh feed) of those products heavier than gasoline which remain in the recovered product. On page 935-937 of Hougen and Watson, Chemical Process Princinles, John Wiley & Sons, Inc., N.Y. (1947), the concent of "Activity Factors" is discussed. This concept leads to the use of "relative activity" to compare the effectiveness of an operating catalyst against a standard catalyst. Relative activity neasurcmcnt facilitate recognition of how the auantity rectuirements of various catalysts differ from one another. Thus, relative activity is a ratio obtained by dividing the weight of a standard or reference catalyst which is or would be required to produce a given level of conversion, as compared to the weight of an operating catalyst (whether proposed or actually used) which is or would be required to produce the same level of conversion in the same or equivalent feedstock under the same or equivalent conditions. Said ratio of catalyst weights may he expressed as a numerical ratio, but preferably is converted to-a percentage basis. The standard catalyst is preferably chosen from among catalysts useful for conducting the present invention, such as for example zeolite fluid cracking catalysts, and is chosen for its ability to produce a predetermined level of conversion in a standard feed under the conditions of temperature, 7HSV, catalyst to oil ratio and other conditions set forth in the preceding description of the MAT conversion test and in ASTM D-32 MAT test D-3907-80. Conversion is the volume percentage of feedstock that is converted to 4300F endpoint gasoline, lighter products and coke. For standard feed, one may employ the above-mentioned light primary gas oil, or equivalent. For purposes of conducting relative activity determinations, one may prepare a "standard catalyst curve", a chart or graph of conversion (as above defined) vs. reciprocal xfHsV for the standard catalyst and feedstock. A sufficient num- ber of runs is made under ASTM D-3907-80 conditions (as modified above) using standard feedstock at varying levels of T.fHSV to prepare an accurate "curve" of conversion vs. WHSV for the standard feedstock. This curve should traverse all or substantially all of the various levels of conversion including the range of conversion within which it is expected that the operating catalyst will be tested. From this curve, one may establish a standard WHSV for test comparisons and a standard value of reciprocal WHSV correspoinding to that level of conversion which has been chosen to represent 100t relative activity in the standard catalyst. For purposes of the present disclosure the aformentioned reciprocal sESV and level of conversion are, respectively, 0.0625 and 75%. In testing an operating cata lyst of unknown relative activity, one conducts a sufficient number of runs with that catalyst under D-3907-80 conditions (as modified above) to establish the level of conversion which is or would be produced with the operating catalyst at standard reciprocal WHSV. Then, using the above-mentioned standard catalyst curve, one establishes a hypothetical reciprocal WHSV constituting the reciprocal WHSV which would have been required, using the standard catalyst, to obtain the same level of conversion which was or would be exhibited, by the operating catalyst at standard WHSV. The relative activity may then be calculated by dividing the hypothetical reciprocal WISV by the reciprocal standard WHSV, which is 1/16, or .0625. The result is relative activity expressed in terms of a decimal fraction, which may then be multiplied by 100 to convert to percent relative activity. In applying the results of this determination, a relative activity of 0.5, or 50%, means that it would take twice the amount of the operating catalyst to give the same conversion as the standard catalyst, i.e., the production catalyst is 50% as active as the reference catalyst. The catalyst may be introduced into the process in ;ts virgin form or, as previously indicated, in other than virgin form; e.g. one may use equilibrium catalyst withdrawn from another unit, such as catalyst that has been employed in the cracking of a different feed. Whether characterized on the basis of MAT conversion activity or relative activity, the preferred catalysts may be described on the basis of their activity "as introduced" into the process of the present invention, or on the basis of their "as withdrawn" or equilibrium activity in the process of the present invention, or on both of these bases. A preferred activity level of virgin and non-virgin catalyst "as introduced" into the process of the present invention is at least about 60E by "A conversion, and preferably at least about 20%, more preferably at least about 40t and still more preferably at least about 60% in terms of relative activity. However, it will be appreciated that, particularly in the case of non-virgin catalysts supplied at high addition rates, lower activity levels may be acceptable. An acceptable "as withdrawn" or equilibrium activity level of catalyst which has been used in the process of the present invention is at least about 20% or more, but about 40% or more and preferably about 60% or more are preferred values on a relative activity basis, and an activity level of 60% or more on a MAT conversion basis is also contemplated. More preferably, it is desired to employ a catalyst which will, under the conditions of use in the unit, establish an equilibrium activity at or above the indicated level. The catalyst activities are determined with catalyst having less than 0.01 coke, e.g., regenerated catalyst. One may employ any hydrocarbon cracking catalyst having the above indicated conversion capabilities. A particularly preferred class of catalysts includes those which have pore structures into which molecules of feed materials may enter for adsorption and/or for contact with active catalytic sites within or adjacent the pores. Various types of catalysts are available within this classification, including for example the layered silicates, e.g., smectites. Although the most widely available catalysts within this classification are the well-known zeolite-containing catalysts, non-zeolite catalysts are also contemplated. The preferred zeolite-containing catalysts may include any zeolite, whether natural, semi-synthetic or synthetic, alone or in admixture with other materials which do not significantly impair the suitability of the catalyst, provided the resultant catalyst has the activity and pore structure referred to above. For example, if the virgin catalyst is a mixture, it may include the zeolite component associated with or dispersed in a porous refractory inorganic oxide - carrier; in such case the catalyst may for example contain about 1% to about 60%, more preferably about 15 to about 40% and most typically about 20 to about 25% by weight, based on the total weight of catalyst (water-free basis) of the zeolite, the balance of the catalyst being the porous refractory inorganic oxide alone or in combination with any of the known adjuvants for promoting or suppressing various desired and undesired reactions. For a general explanation of the genus of zeolite, molecular sieve catalysts useful in the invention, attention is drawn to the disclosures of the articles entitled "Refinery Catalysts Are a Fluid Business" and "Making Cat Crackers Work on Varied Diet", appearing respectively in the July 26, 1978 and September 13, 1978 issues of Chemical Week magazine. The descriptions of the aforementioned publications are incorporated herein by reference. For the most part, the zeolite components of the zeolitecontaining catalysts will be those which are known to be useful in FCC cracking processes. In general, these are crystalline aluminosilicates, typically made up of tetracoordinated aluminum atoms Associated through oxygen atoms with adjacent silicon atoms in the crystal structure. However, the term "zeolite" as used in this disclosure contemplates not only aluminosilicates, but also substances in which the aluminum has been partly or wholly replaced, such as for instance by gallium and/or other metal atoms, and further includes substances in which all or part of the silicon has been replaced, such as for instance by germanium. Titanium and zirconium substitution may also be practiced. Most zeolites are prepared or occur naturally in the sodium form, so that sodium cations are associated with the electro-negative sites in the crystal structure. The sodium cations tend to make zeolites inactive and much less stable when exposed to hydrocarbon conversion conditions, particularly high temperatures. Accordingly, the zeolite may be ion exchanged, and where the zeolite is a component of a catalyst composition, such ion exchanging may occur before or after incorporation of the zeolite as a component of the composition. Suitable cations for replacement of sodium in the zeolite crystal structure include ammonium (dcornposable to hydrogen), hydrogen, rare earth metals, alkaline earth metals, etc. Various suitable ion exchange procedures and cations which may be exchanged into the zeolite crystal structure are well known to those skilled in the art. Examples of the naturally occurring crystalline alumino- silicate zeolites which may be used as or included in the catalyst for the present invention are faujasite, mordenite, clinoptilote, chabazite, analcite, erionite, as well as levynite, dachiardite, paulingite, noselite, ferriorite, heulandite, scoiccite, stibite, harmotome, phillipsite, brewsterite, flarite, datolite, gmelinite, caumnite, leucite, lazurite, scaplite, mesolite, ptholite, nepheline, matrolite, offretite and sodalite. Examples of the synthetic crystalline aluminosilicate zeolites which are useful as or in the catalyst for carrying out the present invention are Zeolite X, U.S. Patent No. 2,882,244, Zeolite Y, U.S. Patent No. 3,130,007; and Zeolite A, U.S. Patent No. 2,882,243; as well as Zeolite B, U.S. Patent No. 3,008,803; Zeolite D, Canada Patent No. 661,981; Zeolite E, Canada Patent No. 614,495; Zeolite F, U.S. Patent No. 2,996,358; Zeolite H, U.S. Patent No. 3,010,789; Zeolite J, U.S. Patent No. 3,001,869; Zeolite L, Belgian Patent No. 575,117; Zeolite M, U.S Patent No. 2,995,423; Zeolite 0, U.S. Patent No. 3,140,252; Zeolite Q, U.S. Patent No. 2,991,151; Zeolite S, U.S. Patent No. 3,054,657; Zeolite T, U.S. Patent No. 2,950,952; Zeolite W, U.S. Patent No. 3,012,853; Zeolite Z, Canada Patent No. 614,495; and Zeolite Omega, Canada Patent No. 817,915. Also; ZK-4HJ, alpha beta and ZSM-type zeolites are useful. Moreover, the zeolites described in U.S. Patent Nos. 3,140,249, 3,140,253, 3,944,482 and 4,137,151 are also useful, the disclosures of said patents being incorporated herein by reference. The crystalline aluminosilicate zeolites having a faujasite-type crystal structure are particularly preferred for use in the present invention. This includes particularly natural faujasite and Zeolite X and Zeolite Y. The crystalline aluninosilicate zeolites, such as synthetic faujasite, will under normal conditions crystallize as regularly shaped, discrete particles of about one to about ten microns in size, and, accordingly, this is the size range frequently found in commercial catalysts which can be used in the invention. Preferably, the particle size of the zeolites is from about 0.5 to about 10 microns and more preferably is from about 0.1 to about 2 microns orless. For example, zeolites prepared in situ from calcined kaolin may be characterized by even smaller crystallites. Crystalline zeolites exhibit both an interior and an exterior surface area, which we have defined as "portal" surface area, with the largest portion of the total surface are being internal. By portal surface area, we refer to the outer surface of the zeolite crystal through which reactants are considered to pass in order to convert to lower boiling products. Blockage of the internal channels by, for example, coke formation, blockage of entrance to the-internal channels by deposition of coke in the portal surface area, and contamination by metals poisoning, will greatly reduce the total zeolite surface area. Therefore, to minimize the effect of contamination and pore blockage, crystals larger than the normal size cited above are preferably not used in the catalysts of this invention. Commercial zeolite-containing catalysts are available with carriers containing a variety of metal oxides and combination thereof, including for example silica, alumina magnesia, and mixtures thereof and mixtures of such oxides with clays as e.g., described in U.S. Patent No. 3,034,948. One may for example select any of the zeolite-containing molecular sieve fluid cracking catalysts which are suitable for production of gasoline from vacuum gas oils. However, certain advantages may be attained by judicious selection of catalysts having marked resistance to metals. A metal resistant zeolite catalyst is, for instance, described in U.S. Patent No. 3,944,482, in which the catalyst contains 140 weight percent of a rare earth-exchanged zeolite, the balance being a refractory metal oxide having specified pore volume and size distribution. Other catalysts described as "metals-tolerant" are described in the above-mentioned Cimbalo, et al article. In general, it is preferred to employ catalysts having an overall particle size in the range of about 5 to about 160, more preferably about 40 to about 120, and most preferably about 40 to about 80 microns. For example, a useful catalyst may have a skeletal density of about 150 pounds per cubic foot, and an average particle size of about 60-70 microns, with less than 10% of the particles having a size less than about 40 microns and less than 80% having a size less than about 50-60 microns. Although a wide variety of other catalysts, including both zeolite-containing and non-zeolite-containing may be employed in the practice of the invention the following are examples of commercially available catalysts which have been employed in practicing the invention: TABLE II Specific Weight Percent Surface Zeolite Catalvst mg/g Content A1XO3 SiO2 Nav0 Fe9O Tri02 AGZ-290 300 11.0 29.5 59.0 0.40 0.11 0.59 GRz-l 162 14.0 23.4 69.0 0.10 0.4 0.9 CCZ-220 129 11.0 34.6 60.0 0.60 0.57 1.9 Super DX 155 13.0 31.0 65.0 0.80 0.57 1.6 F-87 240 10.0 44.0 50.0 0.80 0.70 1.6 FOC-90 240 8.0 44.0 52.0 0.65 0.65 1.1 HFZ 20 310 20.0 59.0 40.0 0.47 0.54 2.75 HEZ-55 210 19.0 59.0 35.2 0.60 0.60 2.5 The AGZ-290, GRZ-1, CCZ-220 and Super DX catalysts referred to above are products of W. R. Grace and Co. F-87 and FOC90 are products of Filrrol, while HFZ-20 and HEZ-55 are products of Engelhard/Houdry. The above are properties of virgin catalyst and, except in the case of zeolite content, are adjusted to a water free basis. i.e., based on material ignited at 1750 F. The zeolite content is derived by comparison of the X-ray intensities of a catalyst sample and of a standard material composed of high purity sodium Y zeolite in accordance with draft &num;6, dated January 9, 1978, of proposed ASTM Standard Method entitled "Determination of the Faujasite Content of a Catalyst." It is considered an advantage that the process of the present invention can be conducted in the substantial absence of tin and/or antimony or at least in the presence of -a catalyst which is subs tan- tially free of either or both of these metals. Among the above-mentioned commercially available catalysts, the Super D family and especilly a catalyst designated GRZ-1 are particularly preferred. For example, the Super DX has given particularly goods results with Arabian light crude. The GRZ-1 although substantially more expensive than the Super DX at present, apperas somewhat more metals tolerant. Although not yet commercially available, it is believed that the best catalysts for carrying out the present invention will be those which, according to proposals advanced by Dr. William P. Hettinger, Jr., and Dr. James E. Lewis, are characterized by matrices with feeder pores having large minimum diameters and large mouths to facilitate diffusion of high molecular weight molecules through the matrix to the portal surface area of molecular sieve particles within the matrix. Such matrices preferably also have a relatively large pore volume in order to soak up unvaporized portions of the carbo-metallic oil feed. Thus significant numbers of liquid hydrocarbon molecules can diffuse to active catalytic sites both in the matrix and in sieve particles on the surface of the matrix. In general it is preferred to employ catalysts with matrices wherein the feeder pores have diameters in the range of about 400 to about 7000 angstrom units, and preferably about 1000 to about 6000 angstrom units. The process of the present invention may be operated with the above described carbo-metallic converter feed oil and catalyst, and with naphtha as substantially the only additional material charged to the reaction zone. But the charging of other additional materials is not excluded. The charging of recycled oil to the reaction zone has already been mentioned. As described in greater detail below, still other additional materials fulfilling a variety of functions may also be charged. In such case the carbo-metallic oil and catalyst usually represent the major proportion by weight of the total of all materials charged to the reaction zone. The naphtha that is added to or admixed with the carbometallic, converter feed and/or catalyst, may be sulfolane extracted and/or may be unprocessed raw naphtha and contain less than 5% olefins. It may also contain at least 10% naphthenes which can be cyclopentyl and cyclohexal rings. The naphtha may be virgin naphtha or it may be derived from a cracking process as for example an RCC process or a thermal cracking process. The naptha has a boiling point ranging from about 1600 to about 4000 F, and more specifically, from about 2000 to about 3500F. With respect to the hydrogen to carbon ratio, the naphtha has a greater ratio than does the converter feed. In general, the naphtha performs functions which offer significant advantages over the process as performed with only the carbo-metallic converter feed oil and catalyst. Among these functions are: inducing a lower pressure of hi-boiling hudrocarbons of converter feed; enhancing selectivity of and increasing yield of products; converting carbasetallic oil product gasoline to a higher octane gasoline; providing a source of hydrogen (H+) thus inducing transfer of hydrogen to potential coke formers and quenching coking reactions by hydrogen neutralization of the free radical carbon (C) precursor; donating hydrogen to hydrogen-poor carbo-metallic oil feedstock; enhancing dispersion of "hi-boiling" oil feedstock onto catalyst; preventing agglomeration of catalyst and converter oil feed in reactor; reducing unfavorable dehydrogenation and coking of carbo-metallic oil feed; assisting in volatilization and disengagement of heavier metals in reactor stripper; controlling the effects of heavy metals and other catalyst contaminants by conditioning the catalyst with a material which partially deactivates poisonous metals such as nickel and vanadium by forming selective carbon reactant products with nickel and vanadium; assisting in fluidization of the catalyst in the lower section of the riser where fluidization of the catalyst with unreacted high boiling oil is impaired; enhancing catalyst activity; improving yield and selectivity (Volume % yield of gasoline divided by Volume % conversion of feed); absorbing excess heat in the catalyst as received from the regenerator; converting pollutants to a form or forms in which they may be more readily separated from products and/or disposed of; controlling catalyst temperature; adjusting feed/catalyst contact time; and possibly enhancing distillation of products. Certain of the metals in the heavy metals accumulation on the catalyst are more active in promoting undesired reactions when they are in the form of elemental metal than they are when in the oxidized form produced by contact with oxygen in the catalyst regenerator. However, the time of contact between catalyst and vapors of feed and product in past conventional catalytic cracking was sufficient so that hydrogen released in the cracking reaction was able to reconvert a significant portion of the less harmful oxides back to the more harmful elemental heavy metals. One can take advantage of this situation through the introduction of additional materials which are in gaseous (including vaporous) form in the reaction zone in admixture with the catalyst and vapors of feed and products. The increased volume of material in the reaction zone resulting from the presence of such additional materials tends to increase the velocity of flow through the reaction zone with a corresponding decrease in the residence time of the catalyst and oxidized heavy metals borne thereby. Because of this reduced residence time, there is less opportunity for reduction of the oxidized heavy metals to elemental or reduced valency form and therefore less of the harmful elemental metals are available for contacting the feed and products. For all these reasons, the present invention provides a method of operating with specific naphthas in conjunction with carbo-metallic oil to attain superior results. Other materials in addition to naphtha may be introduced into the process in any suitable fashion, some examples of which follow. For instance, they may be admixed with the carbo-metallic oil feedstock prior to contact of the latter with the catalyst. Alternatively, the added materials may, if desired, be admixed with the catalyst prior to contact of the latter with the feedstock. Separate portions of the added materials may be separately admixed with both catalyst and carbo-metallic oil. Moreover, the feedstock, catalyst and additional materials may, if desired, be brought together substantially simultaneously. A portion of the added materials may be mixed with catalyst and/or carbo-metallic oil in any of the above-described ways, while additional portions are subsequently brought into admixture. For example, a portion of the added materials may be added to the carbo-metallic oil and/or to the catalyst before they reach the reaction zone, while another portion of the added materials is introduced directly into the reaction zone. The added materials may be introduced at a plurality of spaced locations in the reaction zone or along the length thereof, if elongated. The amount of additional materials which may be present in the feed, catalyst or reaction zone for carrying out the above functions, and others, may be varied as desired; but said amount will preferably be sufficient to substantially heat balance the process. These materials, including the naptha, may for example be introduced into the reaction zone in a weight ratio relative to feed of up to about 0.4, preferably in the range of about 0.02 to about 0.4, more preferably about 0.03 to about 0.3 and most preferably about 0.05 to about 0.25. For example, many or all of the above desirable functions may be attained by mixing naphtha with the converter feed and/or by also introducing H2O to the converter feed in the reaction zone in combination with naphtha. The naphtha is added in a weight percentage of the converter feed in an amount ranging from about 1 to about 20%, and still more preferably from about 1 to about 5%. The H2O is added to the reaction zone with naphtha, the carbo-metallic oil feed and catalyst in an amount ranging from about 0.04 to about 0.25 parts per part by weight of the converter feed, preferably from about 0.04 to about 0.20 parts and more preferably from about 0.05 to about 0.15 parts per part of converter feed. According to the present invention, the H20 is preferably added in a combination, i.e., in the form of both steam and liquid water. As suggested above, the addition or use of naphtha tends to assist in providing a greater yield of gasoline product from the converter feed oil and and a higher octane gasoline. Moreover, the naphtha being a hydrogen donor may donate the hydrogen from the hydrogen-rich naphtha to hydrogenpoor carbo-metallic, converter feed. In adding H2O (i.e., in the form of steam or a liquid or a combination thereof) to the carbo-metallic feedstock and catalyst in the reaction zone, it appears that the use of H20 tends to further inhibit reduction of catalyst-borne oxides, sulfites and sulfides to the free metallic form which is believed to promote condensation-dehydrogenation with consequent promotion of coke and hydrogen yield and accompanying loss of product. Moreover, H2O may also, to some extent. reduce deposition of metals onto the active catalyst surface. There may also be some tendency to desorb nitrogen-containing and other heavy contaminant-containing molecules from the surface of the catalyst particles. or at least some tendency to inhibit their absorption by the catalyst. It is also believed that added H20 tends to increase the acidity of the catalyst by Bronsted acid formation which in turn enhances the activity of the catalyst. In the reaction zone, the presence of H20 can dilute the carbo-metallic oil vapors, thus reducing their partial pressure and tending to increase the yield of the desired products. It has been reported that H20 is useful in combination with other materials in generating hydrogen during cracking; thus it may be able to act as a hydrogen donor for hydrogen-deficient carbo-metallic oil feedstocks. The H20 may also serve certain purely mechanical functions such as: assisting in the atomizing or dispersion of the feed; competing with high molecular weight molecules for absorption on the surface of the catalyst. thus interrupting coke formation; steam distillation of vaporizable product from unvaporized feed material; and disengagement of high boiling products from the catalyst by steam distillation upon conclusion of the cracking reaction. It is particularly preferred to bring together H2O, naphtha, catalyst and carbometallic oil substantially simultaneously. For example, one may admix H20 and feedstock in a spray nozzle and immediately direct the resultant spray into contact with the catalyst at the downstream end of the reaction zone. The addition of steam to the reaction zone is frequently mentioned in the literature of fluid catalytic cracking. Addition of liquid water to the feed is discussed relatively infrequently, compared to the introduction of steam directly into the reaction zone. However, in accordance with the present invention it is particularly preferred that H2O in the form of steam and liquid in combination be brought into intimate admixture with the carbo-metallic oil in a weight ratio of about 0.04 to about 0.25 at or prior to the time of introduction of the oil into the reaction zone, whereby the H2 0 enters the reaction zone as part of the flow of feedstock which enters such zone. Other materials may be added to the reaction zone to perform one or more of the above described functions. For example, the dehydrogenation-condensation activity of heavy metals may be inhibited by introducing hydrogen sulfide gas, or H2S dissolved into the reaction zone. As discussed above, hydrogen may be made available for hydrogen deficient carbo-metallic oil feedstocks by the naphtha, or by introducing into the reaction zone relatively low molecular weight carbonhydrogen fragment contributors, including for example: light paraffins; low molecular weight alcohols and other compounds that chemically combine to generate hydrogen in the reaction zone such as by reaction of carbon monoxide with water, or with alcohols, or with olefins, or with other materials or mixtures of the foregoing. All of the above mentioned additional materials, including H2O, alone or in conjunction with each other or in conjunction with other materials, such as nitrogen or other inert gases, light hydrocarbons, and others, may perform the above-described functions for which they are suitable, including without limitation, acting as diluents to reduce feed partial pressure and/or as heat sinks to absorb excess heat present in the catalyst as received from the regeneration step. However, with the addition of naphtha, we are able to obtain a preferred result. The foregoing is a discussion of some of the functions - which can be performed by materials other than catalyst and carbo-metallic oil feedstock introduced into the reaction zone, and it should be understood that other materials may be added or other functions performed without departing from the spirit of the invention. The invention may be practiced in a wide variety of apparatus. However, the preferred apparatus includes means for rapidly vaporizing as much feed as possible and efficiently admixing feed and catalyst (although not necessarily in that order), for causing the resultant mixture to flow as a dilute suspension in a progressive flow mode, and for separating the catalyst from cracked products and any uncracked or only partially cracked feed at the end of a predetermined residence time or times, it being preferred that all or at least a substantial portion of the product should be abruptly separated from at least a portion of the catalyst. For example, the apparatus may include, along its elongated reaction chamber, one or more points for introduction of carbo-metallic feed, one or more points for introduction of catalyst, one or more points for introduction of additional material, one or more points for withdrawal of products and one or more points for withdrawal of catalyst. The means for introducing feed, catalyst and other material may range from open pipes to sophisticated jets or spray nozzles, it being preferred to use means capable of breaking up the liquid feed into fine droplets. Preferably, the catalyst, liquid water (when used) naphta and fresh feed are brought together in an apparatus similar to that disclosed in U.S. Patent Application Serial No. 969,601 of George D. Myers, et al, filed December 14, 1978, the entire disclosure of which is hereby incorporated herein by reference. According to a particularly preferred embodiment based on a suggestion which is understood to have emanated from Mr. Steven M. Kovach, the liquid water and carbo-metallic oil, prior to their introduction into the riser, are caused to pass through a propeller, apertured disc, or any appropriate high shear agitating means for forming a "homogenized mixture" containing finely divided droplets of oil and/or water with oil and/or water present as a continuous phase. It is preferred that the reaction chamber, or at least the major portion thereof, be more nearly vertical than horizontal and have a length to diameter ratio of at least about 10, more preferably about 20 or 25 or more. Use of a vertical riser type reactor is preferred. If tubular, the reactor can be of uniform diameter throughout or may be provided with a continuous or step-wise increase in diameter along the reaction path to maintain or vary the velocity along the flow path. In general, the charging means (for catalyst and feed) and the reactor configuration are such as to provide a relatively high velocity of flow and dilute suspension of catalyst. For example, the vapor or catalyst velocity in the riser will be usually at least about 25 and more typically at least about 35 feed per second. This velocity may range up to about 55 or about 75 feet or about 100 feet per second or higher. The vapor velocity at the top of the reactor may be higher than that at the bottom and may for example be about 80 feet per second at the top and about 40 feet per second at the bottom. The velocity capabilities of the reactor will in general be sufficient to prevent substantial buildup of a catalyst bed in the bottom or other portions of the riser, whereby the catalyst loading in the riser can be maintained below about 4 or 5 pounds, as for example about 1.5 pounds, and below about 2 pounds, as for example about 0.8 pounds per cubic foot, respectively, at the upstream (e.g. bottom) and downstream (e.g. top) ends of the riser. The progressive flow mode involves, for example, flowing of catalyst, feed mixture and products as a stream in a positively controlled and maintained direction established by the elongated nature of the reaction zone. This is not to suggest however that there must be strictly linear flow. As is well known, turbulent flow and "slippage" of catalyst may occur to some extent especially in certain ranges of vapor velocity and some catalyst loadings, although it has been reported advisable to employ sufficiently low catalyst loadings to restrict slippage and back-mixing. Most preferably the reactor is one which abruptly separates a substantial portion or all of the vaporized cracked products from the catalyst at one or more pdints along the riser, and preferably separates substantially all of the vaporized cracked products from the catalyst at the downstream end of the riser. The process of the present invention uses ballistic separation of catalyst and products; that is, catalyst is projected in a direction established by the riser tube, and is caused to continue its motion in the general direction so established, while the products, having lesser momentum, are caused to make an abrupt change of direction, resulting in an abrupt, substantially instantaneous separation of product from catalyst In a preferred embodiment referred to as a vented riser, the riser tube is provided with a substantially unobstructed discharge opening at its downstream end for discharge of catalyst. An exit port in the side of the tube adjacent the downstream end receives the products. The discharge opening communicates with a catalyst flow path which extends to the usual stripper and regenerator, while the exit port communicates with a product flow path which is substantially or entirely separated from the catalyst flow path and leads to separation means for separating the products from the relatively small portion of catalyst, if any, which manages to gain entry to the product exit port. Examples of a ballistic separation apparatus and technique as above described, are found in U.S. Patents 4;066,533 and 4,070,159 to Myers, et al, the disclosure of which patents are hereby incorporated herein by reference in their entireties. According to a particularly preferred embodiment, based on a suggestion understood to have emanated from Paul W. Walters, Roger M. Benslay and Dwight F. Barger, the ballistic separation step includes at least a partial reversal of direction by the product vapors upon discharge from the riser tube; that is, the product vapors make a turn or change of direction which exceeds 900 at the riser tube outlet. This may be accomplished for example by providing a cup-like member surrounding the riser tube at its upper end, the ratio of cross-sectional area of the riser tube outlet being low, i.e., less than 1 and preferably less than about 0.6. Preferably the lip of the cup is slightly downstream of, or above the downstream end or top of the riser tube, and the cup is preferably concentric with the riser tube. By means of a product vapor line communicating with the interior of the cup but not the interior of the riser tube, having its inlet positioned within the cup interior in a direction upstream of the riser tube outlet, product vapors emanating from the riser tube and entering the cup by reversal of direction are transported away from the cup to catalys. and product separation equipment. Such an arrangement can produce a high degree of completion of the separation of catalyst from product vapors at the riser tube outlet, so that the required amount of auxiliary catalyst separation equipment such as cyclones is greatly reduced, with consequent large savings in capital investment and operating cost. Preferred conditions for operation of the process are described below. Among these are feed, Catalyst and reaction temperatures, reaction and feed pressures, residence time and levels of conversion, coke production and coke laydown on catalyst. In conventional FCC operations with VGO, the feedstock is customarily preheated, often to temperatures significantly higher than are required to make the feed sufficiently fluid for pumping and for introduction into the reactor. For example, preheat temperatures as high as about 7000 or 8000F have been reported. But in our process as presently practiced it is preferred to restrict preheating of the feed, so that the feed is capable of absorbing a larger amount of heat from the catalyst while the catalyst raises the feed to conversion temperature, at the same time minimizing utilization of external fuels to heat the feedstock, thereby, also conserving energy. Thus, where the nature of the feedstock permits, it may be fed at ambient temperature. Heavier stocks may be fed at preheat temperatures of up to about 6000F, typically about 2000F to about 5000F, but higher preheat temperatures are not necessarily excluded. The catalyst fed to the reactor may vary widely in temperature, for example from about 11000 to about 16000F, nore preferably about 12000 to about 15000F and most preferably about 13000 to about 14000F, with about 1325 to about 13750F being considered optimum at present. As indicated previously, the conversion of the carbonetallic oil to lower molecular weight products may be conducted at a temperature of about 9000 to about 14000F, measured at the reaction chamber outlet. The reaction temperature as measured at said outlet is more preferably maintained in the range of about 9650 to about 13000F, still more preferably about 9750 to about 12000F, and most preferably about 9800 to about 11500F. Depending upon the temperature selected and the properties of the feed, all of the feed may or may not vaporize in the riser. Although the pressure in the reactor may, as indicated above, range from about 10 to about 50 psia, preferred and more preferred pressure ranges are about 15 to about 35 and about 20 to about 35. In general, the partial (or total) pressure of the feed may be in the range of about 3 to about 30, more preferably about 7 to about 25 and most preferably about 10 to about 17 psia. The feed partial pressure may be controlled or suppressed by the introduction of gaseous (including vaporous) materials into the reactor, such as for instance the naphtha, steam, water, and other additional materials described above. The process has for example been operated with the ratio of feed partial pressure relative to total pressure in the riser in the range of about 0.2 to about 0.8, more typically about 0.3 to about 0.7 and still more typically about 0.4 to about 0.6, with the ratio of added gaseous forming material (which includes naphtha as previously described, and may include recycled gases and/or steam resulting from introduction of H2 0 to the riser in the form of steam and/or liquid water) relative to total pressure in the riser correspondingly ranging from about 0.8 to about 0.2, more typically about 0.7 to about 0.3 and still more typically about 0.6 to about 0.4. In the illustrative operations just described, the ratio of the partial pressure of the added gaseous forming material relative to the partial pressure of the feed has been in the range of about 0.25 to about 4.0, more typically about 0.4 to about 2.3 and still more typically about 0.7 to about 1.7. Although the residence time of feed and product vapors in the riser may be in the range of about 0.5 to about 10 seconds, as described above, preferred and more preferred values are about 0.5 to about 6 and about 1 to about 4 seconds, with about 1.5 to about 3.0 seconds currently being considered about optimum. For example, the process has been operated with a riser vapor residence time of about 2.5 seconds or less by introduction of copious amounts of gaseous or gaseous forming materials into the riser, such amounts being sufficient to provide for example a partial pressure ratio of added gaseous or gaseous forming materials relative to hydrocarbon feed of about 0.8 or more. By way of further illustration, the process has been operated with said residence time being about two seconds or less, with the aforesaid ratio being in the range of about 1 to about 2. The combination of low feed partial pressure, very low residence time and ballistic separation of products from catalyst are considered especially beneficial for the conversion of carbo-metallic oils. Additional benefits may be obtained in the foregoing combination when there is a substantial partial pressure of added gaseous forming material, especially naphtha, as described above. Depending upon whether there is slippage between the catalyst and hydrocarbon vapors in the riser, the catalyst riser residence time may or may not be the same as that of the vapors. Thus, the ratio of average catalyst reactor residence time versus vapor reactor residence time, i.e. slippage, may be in the range of about 1 to about 5, more preferably about 1 to about 4 and most preferably about 1 to about 3, with about 1 to about 2 currently being considered optimum. In practice there will usually be a small amount of slippage, e.g., about 1.05 or 1.2. In an operating unit there may be, for example, a slippage of about 1.1 at he bottom of the riser and about 1.05 at the top. In certain types of known FCC units, there is a riser which discharges catalyst and product vapors together into an enlarged chamber, usually considered to be part of the reactor, in which the catalyst is disengaged from product and collected. Continued contact of catalyst, uncracked feed (if any) and cracked products in such enlarged chamber results in an overall catalyst feed contact time appreciably exceeding the riser tube residence times of the vapors and catalysts with attendant over-cracking of desired liquid gasoline, the preferred product. When practicing the process of the present invention with ballistic separation of catalyst and vapors at the downstream (e.g. upper) extremity of the riser, such as is taught in the above mentioned Myers, et al, patents, the riser residence time and the catalyst contact time are substantially the same for a major portion of the feed and product vapors. It is considered advantageous if the vapor riser residence time and vapor catalyst contact time are substantially the same for at least about 80%, more preferably at least about 90% and most preferably at least about 95% by volume of the total feed and product vapors passing through the riser. By denying such vapors continued contact with catalyst in a catalyst disengagement and collection chamber one may avoid a tendency toward re-cracking and diminished selectivity. In general, the combination of catalyst to oil ratio, temperatures, pressures and residence times should be such as to effect a substantial conversion of the carbo-metallic oil feedstock. It is an advantage of the process that very high levels of conversion can be obtained in a single pass; for example the conversion may be in excess of 50% and may range to about 90% or higher. Preferably, the aforementioned conditions are maintained at levels sufficient to maintain conversion levels in the range of about 60 to about 90% and more preferably about 70 to about 85%. The foregoing conversion levels are calculated by subtracting from 100% the percentage obtained by dividing the liquid volume of fresh feed into 100 times the volume of liquid product boiling at and above 4300F (tbp, standard atmospheric pressure). These substantial levels of conversion of carbometallic oil, may and usually do result in relatively large yields of coke, such as for example about 4 to about 14% by weight based on fresh feed, more commonly about 6 to about 13% and most frequently about 10 to about 13%. The coke yield can more or less quantitatively- deposit upon the catalyst. At contemplated catalyst to oil ratios, the resultant coke laydown may be in excess of about 0.3, more commonly in excess of about 0.5 and very frequently in excess of about 1% of coke by weight, based on the weight of moisture-free regenerated catalyst. Such coke laydown may range as high as about 2%, or about 3%, or even higher. In common with conventional FCC operations on VGO, the present process includes stripping of spent catalyst after disengagement of the catalyst from product vapors. Persons skilled in the art are acquainted with appropriate stripping agents and conditions for stripping spent catalyst, but in some cases the present process may require somewhat more severe conditions than are commonly employed. This may result, for example, from the use of a carbo-metallic oil having constituents which do not volatilize under the conditions prevailing in the reactor, which constituents deposit themselves at least in part on the catalyst. Such adsorbed, unvaporized material can be troublesome from at least two standpoints. First, if the gases (including vapors) used to strip the catalyst can gain admission to a catalyst disengagement or collection chamber connected to the downstream end of the riser, and if there is an accumulation of catalyst in such chamber, vaporization of these unvaporized hydrocarbons in the stripper can be followed by adsorption on the bed of catalyst in the chamber. More particularly, as the catalyst in the stripper is stripped of adsorbed feed material, the resultant feed material vapors pass through the bed of catalyst accumulated in the catalyst collection and/or disengagement chamber and may deposit coke and/or condensed material on the catalyst in said bed. As the catalyst bearing such deposits moves from the bed and into the stripper and from thence to the regenerator, the condensed products can create a demand for more stripping capacity, while the coke can tend to increase regeneration temperatures and/or demand greater regeneration capacity. For the foregoing reasons, it is preferred to prevent or restrict contact between stripping vapors and catalyst accumulations in the catalyst disengagement or collection chamber. This may be done for example by preventing such accumulations from forming, e.g. with the exception of a quantity of catalyst which essentially drops out of circulation and may remain at the bottom of the disengagement and/or collection chamber, the catalyst that is in circulation may be removed from said chamber promptly upon settling to the bottom of the chamber. Also, to minimize regeneration temperatures and demand for regeneration capacity, it may be desirable to employ conditions of time, temperature and atmosphere in the stripper which are sufficient to reduce potentially volatile hydrocarbon material borne by the stripped catalyst to about 10% or less by weight of the total carbon loading on the catalyst. Such stripping may for example include reheating of the catalyst, extensive stripping with steam, the use of gases having a temperature considered higher than normal for FCC/VGO operations, such as for instance flue gas from the regenerator, as well as other refinery stream gases such as hydrotreater off-gas (H25 containing), hydrogen and others. For example, the stripper may be operated at a temperature of about 3500F rising steam at a pressure of about 150 psig and a weight ratio of steam to catalyst of about 0.002 to about 0.003. On the other hand, the stripper may be operated at a temperature of about 10250F or higher. Substantial conversion of carbo-metallic oils to lighter products in accordance with the invention tends to produce sufficiently large coke yields and coke laydown on catalyst to require some care in catalyst regeneration. In order to maintain adequate activity in zeolite and non-zeolite catalysts, it is desirable to regenerate the catalyst under conditions of time, temperature and atmosphere sufficient to reduce the percent by weight of carbon remaining on the catalyst to about 0.25% or less, whether the catalyst bears a large heavy metals accumulation or not. Preferably this weight percentage is about 0.1% or less and more preferably about 0.05% or less, especially with zeolite catalysts. The amounts of coke which must therefore be burned off of the catalysts when processing carbo-metallic oils are usually substantially greater than would be the case when cracking VGO. The term coke when used to describe the present invention, should be understood to include any residual unvapotized feed or cracking product, if any such material is present on the catalyst after stripping. Regeneration of catalyst, burning away of coke deposited on the catalyst during the conversion of the feed, may be performed at any suitable temperature in the range of about 11000 to about 1600OF, measured at the regenerator catalyst outlet. This temperature is preferably in the range of about 12000 to about lSOO0F, more preferably about 12750 to about 14250F and optimally about 13250 to about 13750F. The process has been operated, for example, with a fluidized regenerator with the temperature of the catalyst dense phase in the range of about 13000 to about 14000F. When regenerating catalyst to very low levels of carbon on regenerated catalyst, e.g. about 0.1% or less or about 0.05% or less, based on the weight of regenerated catalyst, it is acceptable to burn off at least about the last 10% or at least about the last 5% by weight of coke (based on the total weight of coke on the catalyst immediately prior to regeneration) in contact with combustion producing gases containing excess oxygen. In this connection it is contemplated that some selected portion of the coke, ranging from all of the coke down to about the last 5 or 10% by weight, can be burned with excess oxygen. By excess oxygen is meant an amount in excess of the stoichiometric requirement for burning all of the hydrogen, all of the carbon and all of the other combustible components, if any, which are present in the above-mentioned selected portion of the coke immediately prior to regeneration. The gaseous products of combustion conducted in the presence of excess oxygen will normally include an appreciable amount of free oxygen. Such free oxygen, unless removed from the by-product gases or converted to some other form by a means or process other than regeneration, will normally manifest itself as free oxygen in the flue gas from the regenerator unit. In prior commercial processing, in order to provide sufficient driving force to complete the combustion of coke to low levels, when burning all or a major portion of the coke with excess oxygen, the amount of free oxygen will normally be not merely appreciable but substantial, i.e. there will be a concentration of at least about 2 mole percent of free oxygen in the total regeneration flue gas recovered from the entire, completed regeneration operation. While such technique is effective in attaining the desired low levels of carbon on regenerated catalyst, it has its limitations and difficulties as will become apparent from the discussion below As conventionally practiced, the burning of coke during regeneration produces some H2O because of the small amount of hydrogen normally found in coke; but carbon monoxide and carbon dioxide are generally regarded as the principal products. The conversion of the carbon content of coke to carbon monoxide and carbon dioxide are highly exothermic reactions. For instance, the reaction of oxygen with coke to product carbon dioxide produces 14,108 BTUs per pound of coke, while the reaction of oxygen with coke or carbon to form carbon monoxide produces approximately 3967 BTUs per pound of coke. The larger the amount of coke which must be burned from a given weight of catalyst, the greater the amount of heat released during combustion in the regenerator. Heat released by combustion of coke in the regenerator is absorbed by the catalyst and can be readily retained thereby until the regenerated catalyst is brought into contact with fresh feed. When processing carbo-metallic oils to the relatively high levels of conversion involved in the present invention, the amount of regenerator heat which is transmitted to fresh feed by way of recycling regenerated catalyst can substantially exceed the level of heat input which is appropriate in the riser for heating and vaporizing the feed and other materials, for supplying the endothermic heat of reaction for cracking, for making up the heat losses of the unit and so forth. Thus, in accordance with the invention, the amount of regenerator heat transmitted to fresh feed may be controlled, or restricted where necessary, within certain approximate ranges. The amount of heat so transmitted may for example be in the range of about 500 to about 1200, more particularly about 600 to about 900, and more particularly about 550 to about 850 BTUs per pound of fresh feed. The aforesaid ranges refer to the combined heat, in BTUs per pound of fresh feed, which is transmitted by the catalyst to the feed and reaction products (between the contacting of feed with catalyst and the separation of product from catalyst) for supplying the heat of reaction (e.g. for cracking) and the difference in enthalpy between the products and the fresh feed. Not included in'the foregoing are the heat made available in the reactor by the adsorption of coke on the catalyst, or the heat consumed by heating, vaporizing or reacting recycle streams and such added materials as water, steam, naphtha and other hydrogen donors, flue gases and inert gases, or by radiation and other losses. One or a combination of techniques may be utilized in this invention for controlling or restricting the amount of regeneration heat transmitted via catalyst to fresh feed. For example, one may add a combustion modifier to the cracking catalyst in order to reduce the temperature of combustion of coke to carbon dioxide and/or carbon monoxide in the regenerator. Moreover, one may remove heat from the catalyst through heat exchange means, including for example heat exchangers (e.g. steam coils) built into the regenerator itself, whereby one may extract heat from the catalyst during regeneration. Heat exchangers can be built into catalyst transfer lines, such as for instance the catalyst return line from the regenerator to the reactor, whereby heat may be removed from the catalyst after it is regenerated. The amount of heat imparted to the catalyst in the regenerator may be restricted by reducing the amount of insulation on the regenerator to permit some heat loss to the surrounding atmosphere, especially if feeds of exceedingly high coking potential are planned for processing; in general, such loss of heat to the atmosphere is considered economically less desirable than certain of the other alternatives set forth herein. One may also inject cooling fluids into the regenerator, for example water and/or steam, whereby the amount of inert gas available in the regenerator for heat absorption and removal is increased. Another suitable and preferred technique for controlling or restricting the heat transmitted to fresh feed via recycled regenerated catalyst involves maintaining a specified ratio between the carbon dioxide and carbon monoxide formed in the regenerator while such gases are in heat exchange contact or relationship with catalyst undergoing regeneration. In general, all or a major portion by weight of the coke present on the catalyst immediately prior to regeneration is removed in at least one combustion zone in which the aforesaid ratio is controlled as described below. More particularly, at least the major portion more preferably at least about 65% and more particularly at least about 80% by weight of the coke on the catalyst is removed in a combustion zone in which the molar ratio of CO2 to CO is maintained at a level substantially below 5, e.g. about 4 or less. Looking at the CO2/Co relationship from the inverse standpoint, it is preferred that the CO/CO2 molar ratio should be at least about 0.25 and preferably at least about 0.3 and still more preferably about 1 or more or even 1.5 or more. While persons skilled in the art are aware of techniques for inhibiting the burning of CO to CO2, it has been suggested that the mole ratio of CO:C02 should be kept less than 0.2 when regenerating catalyst with large heavy metal accumulations resulting from the processing of carbo-metallic oils; in this connection see for example U.S. Patent No. 4,162,213 to Zrinscak, Sr., et al. In this invention however, maximizing CO production while regenerating catalyst to about 0.1% carbon or less, and preferably about 0.05% carbon or less, is a particularly preferred embodiment of this invention. Moreover, according to a preferred method of carrying out the invention the subprocess of regeneration, as a whole, may be carried out to the above-mentioned low levels of carbon on regenerated catalyst with a deficiency of oxygen; more specifically, the total oxygen supplied to the one or more stages of regeneration can be and preferably is less than the stoichiometric amount which would be required to burn all hydrogen in the coke to H2O and to burn all carbon in the coke to CO2. If the coke includes other combustibles, the aforementioned stoichiometric amount can be adjusted to include the amount of oxygen required to burn them. Still another particularly preferred technique for controlling or restricting the regeneration heat imparted to fresh feed via recycled catalyst involves the diversion of a portion of the heat borne by recycled catalyst to naphtha and other materials which may be introduced into the reactor, such as the water, steam, hydrogen donors other than naptha, flue gases, inert gases, and other gaseous or vaporizable materials. The larger the amount of coke which must be burned from a given weight of catalyst. the greater the potential for exposing the catalyst to excessive temperatures. Many otherwise desirable and useful crac1x ng catalysts are parti- cularly susceptible to deactivation at high temperatures, and among these are quite a few of the costly molecular sieve or zeolite types of catalyst. The crystal structures of zeolites and the pore structures of the catalyst carriers generally are somewhat susceptible to thermal and/or hydro thermal degradation. The use of such catalysts in catalytic conversion processes for carbo-metallic feeds creates a need for regeneration techniques which will not destroy the catalyst by exposure to highly severe temperatures and steaming. Such need can be met by a multi-stage regenera tion process which includes conveying spent catalyst into a first regeneration zone and introducing oxidizing gas thereto. The amount of oxidizing gas that enters said first zone and the concentration of oxygen or oxygen--bearing gas therein are sufficient for only partially effecting the desired conversion of coke on the catalyst to carbon oxide gases. The partially regenerated catalyst is then removed from the first regeneration zone and is conveyed to a second regenera tion zone. Oxidizing gas is introduced into the second regeneration zone to provide a higher concentration of oxygen or oxygen- containing gas than in the first zone, to complete the removal of carbon to the desired level. The regenerated catalyst may then be removed from the second zone and recycled to the reactor for contact with fresh feed. An example of such multi-stage regeneration process is described in U.S. Patent Application Serial No. 969,602 of George D. Myers, et al, filed December 14, 1978, the entire disclosure of which is hereby incorporated herein by reference. Another example may be found in U.S. Patent No. 2,938,739. Multi-stage regeneration offers the possibility of combining oxygen deficient regeneration with the control of the CO:CO2 molar ratio. Thus, about 50% or more, more preferably about 65% to about 95%, and more preferably about 80% to about 95% by weight of the coke on the catalyst immediately prior to regeneration may be removed in one or more stages of regeneration in which the molar ratio of CO:CO2 is controlled in the manner described above. In combination with the foregoing, the last 5% or more, or 10% or more by weight of the coke originally present, up to the entire amount of coke remaining after the preceding stage or stages, can be removed in a subsequent stage of regeneration in which more oxygen is present. Such process is susceptible of operation in such a manner that the total flue gas recovered from the entire, completed regeneration operation contains little or no excess oxygen, i.e. on the order of about 0.2 mole percent or less, or as low as about 0.1 mole percent or less, which is substantially less than the 2 mole percent which has been suggested elsewhere. Thus, multi-stage regeneration is particularly beneficial in that it provides another convenient technique for restricting regeneration heat transmitted to fresh feed via regenerated catalyst and/or reducing the potential for thermal deactivation, while simul taneously affording an opportunity to reduce the carbon level on regenerated catalyst to those very low percentages (e.g. about 0.18 or less) which particularly enhance catalyst activity. For example, a two-state regeneration process may be carried out with the first stage burning about 80% of the coke at a bed temperature of about 1300 CF to produce CO and CO2 in a molar ratio of CO/CO2 of about 1 and the second stage burning about 20% of the coke at a bed temperature of about 13500F to produce substantially all CO2 mixed with free oxygen. Use of the gases from the second stage as combustion supporting gases for the first stage, along with additional air introduced into the first stage bed, results in an overall CO to CO2 ratio of about 0.6, with a catalyst residence time of about 5 to 15 minutes total in the two zones. Moreover, where the regeneration conditions, e.g., temperature or atmosphere, are substantially less severe in the second zone than in the first zone (e.g. by at least about 10 and preferably at least about 200F), that part of the regeneration sequence which involves the most severe conditions is performed while there is still an appreciable amount of coke on the catalyst. Such operation may provide some protection of the catalyst from the more severe conditions. A particularly preferred embodiment of the invention is two-stage fluidized regeneration at a maximum temperature of about lSOO0F with a reduced temperature of at least about 10 or about 200F and preferably at least about 500F in the dense phase of the second stage as compared to the dense phase of the first stage, and with reduction of carbon on catalyst to about 0.05% or less or even about 0.025% or less by weight in the second zone. In fact, catalyst can readily be regenerated to carbon levels as low as 0.01% by this technique, even though the carbon on catalyst prior to regeneration is as much as about 1%. In most circumstances, it will be important to insure that no adsorbed oxygen containing gases are carried into the riser by recycled catalyst. Thus, whenever such action is considered necessary, the catalyst discharged from the regenerator may be stripped with appropriate stripping gases to remove oxygen containing gases. Such stripping may for instance be conducted at relatively high temperatures, for example about 13500 to about 13700F, using steam, nitrogen or other inert gases. The use of nitrogen and other inert gases is beneficial from the standpoint of avoiding a tendency toward hydro-thermal catalyst deactivation which may result from the use of steam. The following comments and discussion relating to metals mangement, carbon management and heat management may be of assistance in obtaining best results when operating the invention. Since these remarks are for the most part directed to what is considered the best mode of operation, it should be apparent that the invention is not limited to the particular modes of operation discussed below. Moreoever, since certain of these comments are necessarily based on theoretical considerations, there is no intention to be bound by any such theory, whether expressed herein or implicit in the operating suggestions set forth hereinafter. Although discussed separately below, it is readily apparent that metals management, carbon management and heat management are inter-related and interdependent subjects both in theory and practice. While coke yield and coke laydown on catalyst are primarily the result of the relatively large quantities of coke precursors found in carbo-metallic oils, the production of coke is exacerbated by high metals accumulations, which can also significantly affect catalyst performance. Moreover, the degree of success experienced in metals management and carbon management will have a direct influence on the extent to which heat management is necessary. Moreover, some of the steps taken in support of metals management have proved very helpful in respect to carbon and heat management. As noted previously the presence of a large heavy metals accumulation on the catalyst tends to aggravate the problem of dehydrogenation and aromatic condensation, resulting in increased production of gases and coke for a feedstock of a given Ramsbottom carbon value. The introduction of substantial quantities of naphtha and H20 into the reactor, either in the form of steam or liquid water, appears highly beneficial from the standpoint of keeping the heavy metals in a less harmful form, i.e. the oxide rather than metallic form. This is of assistance in maintaining the desired selectivity. Also, a unit design in which system components and residence times are selected to reduce the ratio of catalyst reactor residence time relative to catalyst regenerator residence time will tend to reduce the ratio of the times during which the catalyst is respectively under reduction and oxidation conditions. The management of metals content of the catalyst mav be s- served by monitoring the total hydrogen plus methane or the ratio of hydrogen to methane thus produced. In general, it is considered that the hydrogen to methane mole ratio should be less than about 1 and preferably about 0.6 or less, with about 0.4 or less being considered about optimum. In actual practice the hydrogen to methane ratio may range from about 0.5 to about 1.5 and average about 0.8 to about 1. Careful carbon management can improve both selectivity, (the ability to maximize production of valuable products) and heat productivity. In general, the techniques of metals control described above are also of assistance in carbon management. The usefulness of naphtha and water addition in respect to carbon management has already been spelled out in considerable detail in that part of the specification which relates to added materials for introduction into the reaction zone. In general, those techniques which improve dispersion of the feed in the reaction zone should also prove helpful; 'these include for instance the use of fogging or misting devices to assist in dispersing the feed. Catalyst to oil ratio is also a factor in heat management. In common with prior FCC practice on VGO, the reactor temperature may be controlled in the practice of the present invention by respectiely increasing or decreasing the flow af;hot regenerated catalyst to the reactor in response to decreases and increases in reactor temperature, typically the outlet temperature n the case of a riser tyDe reactor. Where the automatic controller for catalyst introduction is set to maintain an excessive catalyst to oil ratio, one can expect unnecessarily large rates of carbon production and heat release, relative to the weight of fresh feed charged to the reaction zone. Relatively high reactor temperatures are also beneficial from the standpoint of carbon management. Such higher temperatures foster more complete vaporization of feed and diengagen.ent of product from catalyst. Carbon management can also be facilitated by suitable restriction of the total pressure in the reactor and the partial pressure of the feed. In general, at a given level of conversion, relatively small decreases in the aforementioned pressures can substamtially reduce coke production. This may be due to the fact that restricting total pressure tends to enhance vaporization of high boiling components of the feed, encourage cracking and facilitate disengagement of both unconverted feed and higher boiling cracked products from the catalyst. It may be. of assistance in this regard to restrict the pressure drop of equipment downstream of and in communication with the reactor. But if it is desired or necessary to operate the system at higher total pressure, such as for instance because of operas nag limitations (e.g. pressure drop in downstream equipment) the above described benefits may be obtained by restricting the feed partial pressure. Suitable ranges for total reactor pressure and feed partial pressure have been set forth above, and in general it is desirable to attempt to minimize the. pressures within these ranges. The abrupt separation of catalyst from product vapors and unconverted feed (if any) is also of great assistance. It is for this reason that the so-called vented riser apparatus and technique disclosed in U.S. Patent Nos. 4,070,159 and 4,066,533 to George D. Myers, et al, is the preferred type of apparatus for conducting this process. For similar reasons, it is beneficial to reduce insofar as possible the elapsed time between separation of catalyst from product vapors and the commencement of stripping. The vented riser and prompt stripping tend to reduce the opportunity for coking of unconverted feed and higher boiling cracked products adsorbed on the catalyst. In the method of this invention of using naphtha, a hydrogen donor, in the vented riser, a standpoint of carbon management, is to operate the process while maintaining the feed partial pressure and total reactor pressure as low as possible, and incorporating relatively large amounts of the naphtha, and additionally water and steam, and, if desired, other diluents which provide the numerous benefits discussed in greater detail above. The feeding of liquid water, steam, hydrogen donors, hydrogen and other gaseous or vaporizable materials to the reaction zone, provides an opportunity for exercising additional control over catalyst to oil ratio. Thus, for example, the practice of increasing or decreasing the catalyst to oil ratio for a given amount of decrease or increase in reactor temperature may be reduced or eliminated by substituting either appropriate reduction or increase in the charging ratios of the water, steam and other gaseous or vaporizable material, or an appropriate reduction or increase in the ratio of water to steam and/or other gaseous materials introduced into the reaction zone. Heat management includes measures taken to control the amount of heat released in various parts of the process and/or for dealing successfully with such heat as may be released. Unlike conventional FCC practice using VGO, wherein it is usually a problem to generate sufficient heat during regeneration to heat balance the reactor, the processing of carbo-metallic oils generally produces so much heat as to require careful management thereof. Heat management can be facilitated by various techniques associated with the materials introduced into the reactor. Thus, heat absorption by feed can be maximized by minimum preheating of feed, it being necessary only that the feed temperature be high enough so that it is sufficiently fluid for successful pumping and dispersion in the reactor. The addition of naphtha to the feed prior to introducing the feed into the reactor increases the fluidity of the feed, thus permitting lower feed temperatures than would be feasible without the naphtha addition. When the catalyst is maintained in a highly active state with the suppression of coking (metals control), so as to achieve higher conversion, the resultant higher conversion and greater selectivity can increase the heat absorption of the reaction. In general, higher reactor temperatures promote catalyst conversion activity in the face of more refractory and higher boiling constituents with high coking potentials. While the rate of catalyst deactivation may thus be increased, the higher temperature of operation tends to offset this loss in activity. Higher temperatures in the reactor also contribute to enhancement of octane number, thus off-setting the octane depressant effect of high carbon laydown. Other techniques for absorbing heat have also been discussed above in connection with the introduction of water, steam, and other gaseous or vaporizable materials into the reactor. The severe stripping and various regeneration techniques discussed above are useful in controlling heat release in the regenerator. While removal of heat from catalyst in or downstream of the regenerator by means of heat exchangers (including steam coils) has been suggested as a means for controlling heat release, the above described techniques of multi-stage regeneration and control over the CO/CO2 ratio (in either single or multi-stage regeneration) are considered more advantageous. The use of steam coils is considered to be partly self-defeating, in that a steam coil or heat exchanger in the regenerator or catalyst return line will generally cause an increase in the catalyst to oil ratio with a resultant increase in the rates of carbon production in the reactor and heat release in the regenerator. As noted above, the invention can be practiced in the above described modes and many others. Two illustrative, non-limiting examples are described by the accompanying schematic diagrams in Figures 1 and 2 and by the descriptions of those figures which follow. Figure 1 is a schematic diagram of an apparatus for carrying out the process of the present invention. The carbo-metallic oil feed (which may have been heated in a feed preheater not shown), naphtha and water (when used) supplied through delivery pipe 9, are fed by feed supply pipe 10 having a control valve 11 to a wye 12 in which they mix with a flow of catalyst delivered through supply pipe 13 and controlled by valve 14. Of course, a variety of mixing arrangements may be employed, and provisions may be made for introducing the other added materials discussed above. The mixture of catalyst, feed and naphtha, with or without such additional materials, is then introduced into riser 18. Although riser 18 appears vertical in the drawing, persons skilled in the art will recognize that the riser need not be vertical, as riser type reactors are known in which an appreciable portion of the riser pipe is nonvertical. Thus, riser pipes having an upward component of direction are contemplated, and usually the upward component of their upwardly flowing inclined portions is substantial, i.e. at least about 30". It is also known to provide risers which have downwardly flowing inclined or vertical portions, as well as horizontal portions. Folded risers are also known, in which there are both upwardly extending and downwardly extending segments. Moreover, it is entirely feasible to practice the process of the invention in an inclined and/or vertical pipe in which the feed and catalyst are introduced at an upper elevation and in which the feed and catalyst moves under the influence of gravity and the down flow of the feed to a lower elevation Thus, in general, the invention also contemplates the use of reaction chambers having a long L/D ratio and having a significant deviation from horizontal. At the upper end of the riser 18 is a chamber 19 which receives the catalyst from the riser. Means are provided for causing product vapors to undergo a sufficient change of direction relative to the direction traveled by the catalyst particles, whereby the vapors are suddenly and effectively separated from the catalyst. Thus, there is "ballistic" separation of catalyst particles and product vapors as described above. In the present schematic diagram, the disengagement chamber 19 includes an upward extension 20 of riser pipe 18 having an open top 21 through which the catalyst particles are discharged. This embodiment makes use of the so-called vented riser described in the above-mentioned Myers, et al patents. Because of the refractory nature of carbo-metallic fractions, relatively high severity is required, but the rapid disengagement of catalyst from lighter cracked products in the vented riser prevents over cracking of desirable liquid products such as gasoline to gaseous products. The product vapors are caused to undergo a sudden change of direction into lateral port 22 in the side of riser extension 20, the catalyst particles being, for the most part, unable to follow the product vapors into port 22. The vapors and those few particles which do manage to follow them into port 22 are transferred by cross pipe 23 to a cyclone separator 24. It is an advantage of the vented riser system shown that it can function satisfactorily with a single stage cyclone separator. However, in the present embodiment the cyclone separator 24 is employed as a first stage cyclone separator -which is connected via transfer pipe 17 with optional secondary cyclone separator 25. The cyclone separator means, whether of the single- or multi-stage type, separates from the product vapors those small amounts of catalyst which do enter the lateral port 22. Product vapors are discharged from disengagement chamber 19 through product discharge pipe 26. The catalyst particles which discharge from open top 21 of riser pipe extension 20, and those catalyst particles which are discharged from the discharge legs 27 and 28 of primary and secondary cyclones 24 and 25 drop to the bottom of disengagement chamber 19. The inventory and residence time of catalyst in chamber 19 are preferably minimized. During startup those catalyst particles which are present may be kept in suspension by fluffing jets 30 supplied with steam through steam supply pipe 29. Spent catalyst spilling over from the bottom of disengagement chamber 19 passes via drop leg 31 to a stripper chamber 32 equipped with baffles 33 and steam jet 34. Any of the other stripping gases referred to above may be employed with or in place of the steam. Carbon is burned from the surface of the catalyst in the combustor 38 which receives stripped catalyst via downcomer pipe 39 and control valve 40. Blowers 41 and 42, in association with a valve and piping arrangement generally indicated by 44, supply air to combustion air jets 48 at the bottom of the combustor and to fluffing jets 49 at an elevated position. Air preheater 43, although usually unused when processing heavy hydrocarbons in accordance with the invention, may be employed when starting up the unit on VGO; then, when the unit is switched over to the carbo-metallic feed, preheater operation may be discontinued (or at least reduced). Supplemental fuel means may be provided to supply fuel through the combustion air jets 48; but such is usually unnecessary since the carbon laydown on the catalyst supplies more than enough fuel to maintain the requisite temperatures in the regeneration section. Regenerated catalyst, with most of the carbon burned off, departs the combustor through an upper outlet 50 and cross pipe 51 to a secondary chamber 52, where it is deflected into the lower portion of the chamber by a baffle 53. Although the use of two stage regeneration is contemplated, and preferred, in this particular embodiment the secondary chamber 52 was operated primarily as a separator chamber, although it can be used to remove additional carbon down to about 0.01% or less in the final stages of regeneration. Catalyst moves in up to three different directions from the secondary chamber 52. A portion of the catalyst may be circulated back to combustor 38 via catalyst recirculation loop 55 and control valve 56 for heat control in the combustor. Some of the catalyst is entrained in the product gases, such as CO and/or CO2 produced by burning the carbon on the catalyst in the combustor, and the entrained catalyst fines pass upwardly in chamber 52 to two sets of primary and secondary cyclones generally indicated by 57 and 58 which separate these catalyst fines from the combustion gases. Catalyst collected in the cyclones 57, 58 and discharged through their drop legs is directed to the bottom of chamber 52 where catalyst is kept in suspension by inert gas and/or steam jets 59 and by a baffle arrangement 54, the latter facilitating discharge of regenerated catalyst through outlet 69 to catalyst supply pipe 13 through which it is recirculated for contact with fresh feed at wye 12, as previously described. Combustion product gases produced by regeneration of the catalyst and separated from entrained catalyst fines by the sets 57, 58 of primary and secondary cyclones in chamber 52, discharge through effluent pipes 61, 62 and heat exchangers 60, 63. If such gases contain significant amounts of CO, they may be sent via gas supply pipe 64 to an optional furnace 65 in which the CO is burned to heat heating cqil 66 connected with steam boiler 67. Additional heat may be supplied to the contents of the boilers through conduit loop 68, which circulates fluid from the boiler 67 to heat exchangers 60, 63 and back to the boiler. This is of course only one example of many possible regeneration arrangements which may be employed. The amount of heat passed from the regenerator back to the riser via regenerated catalyst may be controlled in any of the other ways described above; however, it is preferred to control the relative proportions of carbon monoxide and carbon dioxide produced while the catalyst is in heat exchange relationship with the combustion gases resulting from regeneration. An example of this technique is disclosed in the particularly preferred embodiment described in Figure 2. In Figure 2 reference numeral 80 identifies a feed control valve in feedstock supply pipe 82. Supply pipe 83 (when used) introduces naphtha into the feed. Heat exchanger 81 in supply pipe 82 acts as a feed preheater, whereby preheated feed material may be delivered to the catalyst being regulated by a control valve 87 and suitable automatic control equipment (not shown) with which persons skilled in the art of designing and operating riser type crackers are familiar. The riser 91 may optionally include provision for injection of water, steam and, if desired, other gaseous and/or vaporizable material for the purpose described above. The reactor is equipped with a disengagement chamber 92 similar to the disengagement chamber 19 of the reactor shorn in Figure 1, and the Figure 2 embodiment thus includes means for causing product vapors to undergo a change of direction for sudden and effective separation from the catalyst as in the previous embodiment. Catalyst departs disengagement chamber 92 through stripper 94 which operates in a manner similar to stripper 32 of Figure 1. Spent catalyst passes from stripper 94 to regenerator 101 via spent catalyst transfer pipe 97 having a slide valve 98 for controlling the flow. Regenerator 101 is divided into upper chamber 102 and lower chamber 103 by a divider panel 104 intermediate the upper and lower ends of the regenerator. The spent catalyst from transfer pipe 97 enters upper chamber 102 in which the catalyst is partially regenerated. A funnel-like collector 106 having a bias-cut upper edge receives partially regenerated catalyst from the upper surface of the dense phase of catalyst in upper chamber 102 and delivers it via drop leg 107 having an outlet 110 beneath the upper surface of the dense phase of catalyst in lower regeneration chamber 103. Instead of the internal catalyst drop leg 107, one may use an external drop leg. Valve means in such external drop leg can control the catalyst residence time and flow rate in and between the upper and lower chambers. Air is supplied to the regenerated through an air supply pipe 113. A portion of the air travels through a branch supply pipe 114 to bayonet 115 extending upwardly in the interior of plenum 111 along its central axis. Catalyst in chamber 103 has access to the space within plenum 111 between its walls and the bayonet 115. A small bayonet (not shown) in the aforementioned space fluffs the catalyst and urges it upwardly toward a horizontally arranged ring distributor (not shown) where the open top of plenum 111 opens into chamber 103. The remainder of the air passing through air supply pipe 113 may be heated in air heater 117 (at least during start-up with VGO) and is then introduced into the inlet 118 of the aforementioned ring distributor, which may be provided with holes, nozzles or other apertures which produce an upward flow of gas to fluidize the partially regenerated catalyst in chamber 103. The air introduced in the manner described above com- pletes in chamber 103 the regeneration of the partially regenerated catalyst received via drop leg 107. The amount of air that is supplied is sufficient so that the air and/or the resultant combustion gases are still able to support combustion upon reaching the top of chamber 103. The aforementioned drop leg 107 extends through an enlarged aperture in panel 104, to which is secured a gas distributor 120 which is concentric with and surrounds the drop leg. Via gas distributor 120, combustion supporting gases, which have now been partially depleted of combustion supporting gas, are introduced into the upper regenerator chamber 102 where they contact for purposes of partial oxidation the incoming spent catalyst from spent catalyst transfer pipe 97. Apertured probes 121 or other suitable means in gas distrLbutor 120 assist in achieving a uniform distribution of the partially depleted combustion supporting gas in upper chamber 102. Supplemental air or other fluids may be introduced into upper chamber 102, if desired through supply pipe 122, which discharges into or through gas distributor 120. Fully regenerated catalyst with less than about 0.25% carbon, preferably less than about 0.1% and more preferably less than about 0.05%, is discharged from lower regenerator chamber 103 through a regenerated catalyst stripper 128, whose outlet feeds into the catalyst standpipe 86 mentioned above. Thus, regenerated catalyst is returned to riser 91 for contact with additional fresh feed from feed supply pipe 82. Whatever heat is introduced into the recycled catalyst in the regenerator 101 is available for heat transfer with the fresh feed in the riser. The division of the regenerator into upper and lower regeneration chambers 102 and 103 not only smooths out variations in catalyst regenerator residence time but is also uniquely of assistance in restricting the quantity of regeneration heat which is imparted to the fresh feed while yielding a regenerated catalyst with low levels of residual carbon for return to the reactor. Because of the arrangement of the regenerator, the spent catalyst from transfer line 97, with its high loading of carbon, contacts in chamber 102 combustion supporting gases which have already been at least partially depleted of oxygen by the burning of carbon from partially regenerated catalyst in lower regenerator chamber 102. Because of this, it is possible to control both the combustion and the quantity of carbon dioxide produced in upper regenerator chamber 102. Although the air or other regenerating gas introduced through air supply pipe 113 and branch conduit 114 may contain a relatively large quantity of oxygen, the partially regenerated catalyst which they contact in lower regenerator chamber 103 has already had part of its carbon removed. The high concentration of oxygen and the temperature of the partially regenerated catalyst combine to rapidly remove the remaining carbon in the catalyst, thereby achieving a clean regenerated catalyst with a minimum of heat release. Thus, here again, the combustion temperature and the CO:CO2 ratio in the lower regeneration chamber are therefore readily controlled. The regeneration off gases are discharged from upper regenerator chamber 102 via off gas pipe 123, regulator valve 124, catalyst fines trap 125 and outlet 126. The vapor products from disengagement chamber 92 may be processed in any convenient manner such as for example, by discharge through vapor discharge line 131 to the inlet of fractionator 132. Said fractionator includes a bottoms outlet 133, side outlet 134, flush oil stripper 135, and stripper bottom outlet and discharge line 136 connected to pump 137 for discharging flush oil. The overhead product from stripper 135 is routed via stripper overhead return line 138 to the fractionator 132. The main overhead discharge line 139 of the fractionator is connected to overhead receiver 142 having a bottoms discharge line 143 feeding into pump 144 for discharging gasoline product. If desired, a portion of this product may be sent via recirculation line 145, the flow being controlled by recirculation valve 146, back to the fractionator 132; The overhead receiver also includes a water receiver 147 and a water discharge line 148. The gas outlet 150 of the overhead receiver discharges a stream which is mainly below C5, but containing some C5, C6 and C7 material. If desired, the C5 and above material in this gas stream may be separated by compression, coiling and fractionation and recycled to the overhead receiver with a compressor, cooler and fractionator (not shown). Also, according to the present invention, naphtha can be added to the feed line and passed through the gas stream and it may be recycled therethrough. However, a virgin naphtha is preferred to be used with the feed. EXAMPLES The following examples are given only by way of illustration and not for limiting the invention. The properties of the feedstocks and catalysts employed in Examples 1-10 are set forth in Tables I and II above. The properties of the feedstock and naphtha of Example 11 are provided below in Tables IV and V. EXAMPLES 1-3 A unit constructed in accordance with Figure 1 was operated in the manner and with the results set forth in Table III and in accordance with the teachings of Figure 1 and the associated description thereof. EXAMPLES 4-6 These examples were conducted as indicated in Table III with a unit constructed in accordance with the teachings of Figure 2. Regeneration was carried out in two stages. EXAMPLES 7-10 These examples were conducted in a pilot scale unit using feedstock, catalyst and operating conditions as shown in Table III. Figures 3-5 show the yield in volume percent of (3) C3 olefins, (4) C4 saturates and olefins and (5) gasoline, respectively versus volume percent conversion, obtained through use of the feeds and catalysts of examples 7-10 repeated at various levels. TABLE III E X A M P L E S 1 2 3 4 5 6 7 8 9 10 Feedstock Murb.- ABL.R.C.5 ABL.R.C Murb.WGO Murb.R.C. Mex. R.C. ABL.R.C. Mex. Feed &num;/hr.1 75,215 74,478 72,209 2741.72 2556 2268 4.16 4.98 4.52 2.27 Steam &num;/hr. 4,050 3,840 3,910 249.2 188.4 193.0 Water &num;/hr. 8,210 9,230 9,380 N/A N/A 96.0 Catalyst GRZ-1 GRZ-1 GRZ-1 Eq.SDX4 Eq.SDX Eq.SDX Eq.SDX Cat/Oil Ratio 6.0 5.14 5.8 8.1 6.2 9.6 10.32 8.46 12.35 12.5 Riser Vapor Residence Time 1.5 1.6 2.6 Rx Temperature 1,010 1,012 1,010 1025 1002 980 1056 1051 1100 1055 Pressure (psia) 35 36.3 34.6 34.1 33.6 35.1 Products Dry Gas Wt. % 6.1 4.9 5.6 5.5 5.5 4.8 C3 Sat. Vol. % 3.5 3.2 2.2 2.7 3.4 2.7 C3 Olefins Vol. % 8.9 5.4 5.9 9.6 11.1 9.2 C4 Sat. Vol. % 3.9 6.8 3.5 5.9 7.7 7.0 C4 Olefins Vol. % 7.1 7.3 8.3 10.5 11.4 9.7 C5-430 Vol. % 46.1 33.4 45.2 52.2 48.0 51.7 53.50 53.44 46.55 45.85 Coke Wt. % 10.0 8.9 10.0 8.4 10.5 11.6 7.22 8.1 9.53 16.47 Material Balance Wt. % 105.8 97.4 102.7 99.0 98.2 95.1 98.81 96.7 99.4 96.6 Selectivity .72 .58 .70 .68 .63 .63 .64 .73 .63 .57 TABLE III (Continued) E X A M P L E S 1 2 3 4 5 6 7 8 9 10 Conversion, Vol. % 63.6 59.8 64.3 76.7 76.6 82.1 83.2 72.95 74.1 79.96 Volume Yield 123.6 111.3 118.1 118.8 118.1 111.0 Gasoline API Gravity 54.2 53.0 52.7 47.7 60.5 47.8 42.5 36.9 37.8 @@.3 Distillation IBP 94 94 104 152 95 168 50% 238 254 255 258 212 264 95% 434 438 435 398 398 420 RONC 91.6 89.4 89.2 93.8 93.6 92.3 92.0 93.2 94 95.4 MONC 80.6 78.7 78.9 80.8 82.2 82.7 70.7 78.1 80.1 Regeneration Temp. 1350 1336 1330 N/A N/A N/A 1283 1360 1360 1295 Upper Regen. Temp. N/A N/A N/A 1249 1407 1303 N/A N/A N/A N/A Lower Regen. Temp. N/A N/A 1 N/A 1355 1422 1325 N/A N/A N/A N/A Spent Catalyst Wt. %C 1.67 1.72 1.72 1.14 1.63 1.17 .87 1.49 .94 2.54 Regenerated Cataltlyst %C .43 .53 .53 .01 .01 .01 .17 .53 .17 .73 Circulating Catalyst %C .49 .42 .34 O2 Effluent 0 0 0 0 0 0 2.51 4.9 2.52 2.32 CO2/CO lb./lb. 2.67 2.43 3.19 5.8 4.7 3.81 4.43 18.2 20 3.84 1Fresh feed only, except that Example 3 includes recycle of 9.477 &num;/hr. of HCO (heavy cycle oil), having an API gravity of 18.3 and boiling at about 550 to 775, injected into the reduced crude line to the riser. 2Blend of 67% Mexican and 37% Murban (by volume) reduced crudes. 3The VGO described in Table I. 4Equilibrium Davison Super DX 5Arabian Light Reduced Crude EXAMPLE 11 The following is a discussion of Experiments A and B carried out to illustrate some of the advantages of the use of a naphtha in processing carbo-metallic feeds over that where there is no use of naphtha. In the experiments, a fluid catalytic cracking unit with a capacity for processing 6000 BPD of feedstock was operated on a feed containing 2000 BPD of virgin gas oil and 3600-3700 BPD of a reduced crude, the properties of which feed are as follows: TABLE IV Property Reduced Crude Virgin Gas Oil API Gravity 23.S 26.1 Distillation, Volume % 4560F 5% 55% 6670F 30% 71.9% 795 CF 50% 77.9% 9800F 80% 87.1% 1000 F 95% 100% UOPK 11.64 11.86 Vis 210 SUS 55.4 43.8 Wt. % S 0.36 0.42 RSM Btm. %C 3.94 0.30 Eeptane Insol 0.77 0.03 Ni ppm 4.8 0.5 V ppm 4.2 0.5 In both Experiments A and B, the same feedstock was used. To this feedstock as used in Example B, 860 BPD of a naphtha was added with the following properties: NPI 58.6 IBP 1250F 5% 1750F 10% 1960F 50% 2640F 90% 3300F EP 380"F UOPK 11.98 RON 40.8 Paraffins 54.6% Olefins 0.4 Naphthenes 39.4 Aromatics 5.6 The results of the experiments are shown below. The feedstock used in both Experiments A and B was derived from a crude slate consisting of 4200 to 7600 BPD of Louisiana, 18,000 to 23,000 BPD of Illinois Basin, and 60 to 580 3PD of Murbon during this period of operation. RESULTS EYSERIMENTS A B Gas Oil Charge, BPD 2060 z 1940 Reduced Crude Charge BPD 3620 3730 HSR Naphtha, BPD 0 860 Total H20 to Riser, Lb/Hr 7930 6960 Reactor Temp. TI-33 1005 1003 Conversion, LV% 64.4 67.5 Ni on Catalyst, ppm 4200 4000 V on catalyst, ppm - 2300 2500 H2/C Ratio .654 .774 Mat'i Balance Error, Wt. % -2.0 +3.3 C2 and Lighter, Wt. % 6.9 7.2 Total C3, Vol. % 9.7 12.7 Total C4, Vol. % 9.5 11.7 Total C - 430 EP Gaso, Vol % 39.8 45.8 430 - 630 LCO, Vol. 7. 14.7 17.7 630 IBP Slurry, Vol. % -20.8 14.8 Coke, Wt. % 7.8 7.9 From these results, it can be seen that when adding naphtha to the feedstock (Exp. B) the conversion was increased from 64.4 ; 67.5 Vol. %, and the yield of high octane gasoline was increased by 6.0 Vol. % and the research octane number (RON) of the product gasoline was 89.6. On the basis of these results, it would be expected that if the straight run naphtha was blended with a catalytically cracked gasoline in a 25-75% proportion, a RON of 87.4 would result. Similarly, based on 85% of the straight run naphtha charge being recovered in the debutanizer bottoms, an increase of approximately 34-40 RON's of the naphtha was seen, suggesting that reforming as well as other hydrogen donor reactions were occurring. The H2/C1 ratio also increased from 0.654 to 0.774 confirming that increased hydrogen was released. These results also confirm the several other advantages claimed in processing carbo-metallic oils over high metals containing catalysts. Although conversion increased, apparently, coke waste received remained essentially constant, C3 and C4 yields increased, light cycle oil (LCO) increased 3 Vol. % while 6300F product, indicating a tendency to form coke or to be deficient in hydrogen, dropped 6 Vol. %. The catalyst utilized in this series of runs contained 4000 ppm Nickel and 2500 ppm Vanadium in Experiment B, and 4200 ppm of Nickel and 2300 ppm Vanadium in Experiment A. For all practical purposes, these concentrations are essentially equivalent, being within the limits of analytical error. The above results also show that best control was maintained (reactor temperature) by the use of naphtha, in that 997 lb/hr of water to the reactor was removed and satisfactorily replaced by 860 BPD of naphtha. The reactor actually operated at 10030F while on naphtha and was about 10050F prior to introducing naphtha. Regenerator temperatures also dropped significantly, dropping from 13900F while on water, to 13690F when on naphtha. Absorbed H2, another indicator of hydrogen transfer, rose from 20.7 Vol. % to 25.7 Vol. %, showing that additional hydrogen available for the aforementioned reactions, was being generated. This run demonstrates the many advantages of utilizing naphtha when processing a heavy, hydrogen deficient, high metals content, and coke forming feedstock, especially over a high nickel equivalent metals containing catalyst. Replacing water with naphtha also resulted in a favorable reduction in that the catalyst to oil ratio dropped from 8.9 to 8.5 in Experiment B. In conjunction with the runs above, when 750 BPD of straight run naphtha plus 300 BPD of C4's were added to 2250 BPD of debutanizer bottoms in Experiment A, an octane number of 89.0 was obtained. -In Experiment B, in which naphtha was added to the feed to the catalytic cracking unit, plus enough C4 to produce an equivalent 8.0 RVP, an increase to 91.4 RONC was obtained, resulting in an increase of 2.4 RON's. MONC also increased from 77.4 to 78.7.

Claims: CLAIMS: 1. A process for converting carbo-metallic oils to lighter products, comprising: (a) providing a converter feed containing 6500F+ material, said 6500 material being characterized by a carbon residue on pyrolysis of at least about 1 and by containing at least about 4 ppm of Nickel Equivalents of heavy metals(S); (b) Bringing said converter feed and naphtha together with a high-metal content cracking catalyst; ; (c) forming a stream containing a mixture of said converter feed, said naphtha and said catalyst to form a stream comprising a suspension of said catalyst in said feed and naphtha and causing the resultant stream to flow through a progressive flow type reactor having an elongated reaction chamber which is at least in part vertical or inclined for a predetermined vaporizer residence time in the range of about 0.5 to about 10 seconds at a temperature of about 9000 to about 14000F and under a pressure of about 10 to about 50 pounds per square inch absolute sufficient for causing a conversion per pass in the range of about 50% to about 90% while producing coke in amounts in the range of about 4 to about 14% by weight based on fresh feed, and laying down coke on the catalyst in amounts in the range of about 0.3 to about 3% by weight; ; (d) separating said catalyst from the resultant cracking products; (e) stripping adsorbed hydrocarbons from said separated catalyst; (f) regenerating said catalyst with oxygencontaining combustion-supporting gas under conditions of time, temperature and atmosphere sufficient to reduce the carbon on the catalyst to about 0.25% by weight or less, while forming a gaseous combustion product comprising CO and/or CO2 ; and (g) recycling the regenerated catalyst to the reactor for contact with fresh converter feed and napbtha. 2. A process according to Claim 1, wherein said naphtha contains at least 10% naphthenes. 3. A process according to Claim 2, wherein said naphthenes are cyclopentyl and cyclohexal rings. 4. A process according to Claim 1, wherein said naphtha boils within the range of about 1600 to about 4000F. 5. A process according to Claim 1, wherein said naphtha comprises from about 1 to about 10% by weight of said converter feed. 6. A process according to Claim 1, wherein said naphtha contains a greater hydrogen to carbon ratio than does said converter feed and wherein said reaction takes place at a temperature ranging from about 9650F to about 13000F. 7. A process according to Claim 1, wherein said catalyst has a metal content greater than 5000 ppm of Nickel equivalents. 8. A process according to Claim 1, wherein water and steam are added to said converter feed in an amount ranging from about 0.04 to about 0.25 parts pcr part by weight of said converter feed. 9. A process according to Claim 1, wherein water and steam are added to said converter feed in an amount ranging from about 0.05 to about 0.15 parts per part by weight of said converter feed. 10. A process for converting carbo-metallic oils to lighter products comprising: (a) bringing a naphtha feed together with a high metal content cracking catalyst; (b) providing a converter feed containing 6500F+ material, being characterized by a carbon residue on pyrolysis of at least about 1 and by containing at least about 4 ppm of Nickel equivalent of heavy metals; ; (c) bringing said converter feed together with said naphtha and cracking catalyst to form a stream comprising a suspension of said catalyst and naphtha in said feed and causing the resultant steam to flow through a progressive flow-type reactor having an elongated reaction chamber which is at least in part vertical or inclined for a predetermined vaporizer residence time in the range of about 0.5 to about 10 seconds at a temperature ranging from about 9000 to about 14000F and under a pressure of about 10 to 50 pounds per square inch absolute sufficient for causing a conversion per pass in the range of about 50% to about 90% while producing coke in amounts in the range of about 4 to about 14% by weight based on fresh feed, laying down coke on the catalyst in amounts in the range of about 0.3 to about 3% by weight; (d) separating said catalyst from the resultant cracking products; ; (e) stripping adsorbed hydrocarbon from said separated catalyst; (f) regenerating said catalyst with oxygencontaining combustion-supporting gas under conditions of time, temperature and atmosphere sufficient to reduce the carbon on the catalyst to about 0.25% by weight of less, while forming gaseous combustion product gases comprising CO and/or CO2 ; and (g) recycling the regenerated catalyst to the reactor for contact with fresh naphtha and converter feed.

Inventors: MYERS, GEORGE D.
Applicants: ASHLAND OIL, INC.
Application year: 1982
Main classification: C10G11
IPC codes: C10G11