US2756186A

US2756186A - Method for thermal recycle cracking

Info

Publication number: US2756186A
Application number: US401631A
Authority: US
Inventors: Owen Charley Hillman; James W Slover
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1953-12-31
Filing date: 1953-12-31
Publication date: 1956-07-24
Anticipated expiration: 1973-07-24

Description

July 24 1956 c. H. OWEN ET Ax.

METHOD FOR THERMAL RECYCLE CRACKING 2 Sheets-Sheet l Filed Dec. 3l, 1953 July 24, 1956 c. H. OWEN ET AI. 2,756,186

METHOD FOR THERMAL REcYcLE CRACKING Filed Dec. 3l, 1953 2 Sheets-Sheet 2 FITCH FROM UNIT A PITCH FROM UNIT E IIo TEMPERATURE, F

F/G. Z

o o r N o SGNODES Q 6'SVIJ OOI NOILVHLSNBd INVENTORS `I.w. sLovER C.H. OWEN BY A TTORNEKS United States Patent O 2,756,186 METHOD FOR THERMAL RECYCLE CRACKING Charley Hillman Owen and James W. Slover, Phillips,

Tex., assignors to Phillips Petroleum Company, a corporation of Delaware Application December 31, 1953, Serial No. 401,631 12 Claims. (Cl. 196-49) Many cracking methods are known and a wide range of cracking conditions have been disclosed in the prior art. In recent years catalytic cracking processes have been in the limelight and have received much attention from petroleum technologists. However, despite the re markable advances which have been made there still remain several types of hydrocarbon oils which cannot be economically cracked in catalytic cracking processes. Such oils must still be cracked by thermal cracking methods.

Among the hydrocarbon oils which must be cracked by thermal methods are the residual oils and the refractory distillate oils. Of the residual oils those most diicult to crack, without excessive coke formation, are those which have been subjected to one cracking operation. These once cracked oils, which it is desirable to reduce still l.

further, are usually of such viscosity and have such a high carbon to hydrogen ratio, that excessive coke formation occurs. Large quantities of these cracked residual oils are produced daily in the petroleum industry and their disposal is a major problem in the industry.

When an oil is passed through a pipe or cracking still furnace tube there exists next to the inner wall of the tube a film of oil which travels at a lower velocity than the main body of oil in the center of the tube. Since the transfer of heat through the metal tube and oil film is a conduction phenomena a difference in temperature must exist betwee the main body of oil and the tube wall. The greater the thickness of the oil film the greater the temperature diiference between the tube wall and the main body of the oil within the tube. Therefore, the thicker the oil film the greater the volume' of oil (in the lm) that is exposed to the high temperature of the tube wall. Coking conditions exist when the oil lm is of such thickness that excessive temperatures must be employed on the outside of the tube in order to obtain the desired temperature in the main body of oil in the center of the tube. As the relatively slow moving film of oil proceeds through the tube it cracks with the formation of tarry high molecular weight materials. These tarry materials have a greater viscosity than the original oil, consequently move at a lower velocity, and are in turn further cracked. This cracking continues until eventually solid coke is deposited on the inside of the tube wall. Thus, the coking conditions lead to carbon forming conditions, i. e., the laydown of solid petroleum coke which, although it contains some hydrogen, is substantially pure carbon.

One method commonly employed to mitigate the coking condition is to increase the velocity of the oil through the tube, and thereby increase the velocity of the oil film. However, the oil velocities which can be employed are limited by heat transfer rates for obvious reasons.

This is particularly true in the case of heavy viscous oils such as cracked residues.

We have found that when cracking a cracked residual oil, coking conditions within a cracking still furnace tube can be eliminated or mitigated by employing an aromatic gas oil as a diluent for the cracked residual oil. We have further found that when the said aromatic gas oil contains components of high molecular weight an improved petroleum pitch can be recovered.

Thus, according to our invention there is provided a process for cracking a cracked residual oil, such as a cracked topped crude, which comprises adding an aromatic gas oil to said residual oil and severely cracking said residual oil under conditions which, absent said aromatic oil, would be coking conditions.

Further, according to our inventionkthere is provided an improved petroleum pitch.

The added aromatic oil of our invention serves at least two purposes; (1) to decrease the thickness of the oil lm by reducing the viscosity of the oil and (2) to act as a solvent for the tarry materials which form in the oil lm, as described above. Prior to our invention it was customary, when cracking a black oil such as a cracked topped crude, to remove the tarry material formed in the first cracking operation before attempting to crack the oil further. Otherwise, the tarry materials went into theoil film immediately to comprise the greater part of said film. Excessive coke formation then occurred as described above. When employing our invention it is not necessary to remove the said tarry materials from a cracked residual oil prior to further cracking of said oil. indeed, in a preferred embodiment of our invention we, in effect, increase the concentration of the said tarry materials by employing a heavy aromatic gas oil of high molecular weight as a portion of the added aromatic oil of the invention. A preferred added aromatic oil of our invention is a gas oil having a broad boiling range, i. e., one having a broad range of molecular weights. We have found that when suchan oil is employed an improved petroleum pitch is obtained. Although the exact nature of the reactions taking place during the process of our invention is not understood at this time it is presently be lieved that the following occurs. Normally an aromatic gas oil is highly refractory and passes through a cracking zone substantially unchanged. However, when such oils have an extremely high molecular weight cracking occurs due to the size of the oil molecules, whereas an oil of equal aromaticity but lower molecular weight would not crack under the same conditions. Therefore, it is presently believed that, when the added aromatic oil is an oil containing low molecular weight components and high molecular weight components, the low molecular weight components pass through the cracking zone substantially unchanged, whereas a substantial portion of the high molecular weight components are cracked. The low molecular weight components of the added aromatic oil thus serve as a diluent and/ or solvent for the tarry materials originally present in the cracked residual oil as well as those formed by cracking of the high molecular weight components of the added aromatic oil and further cracking of the cracked residual oil. It is presently believed that the improved petroleum pitch of our invention results from the cracking of the cracked residual oil in the presence of, and along with, aromatics; the added aromatic oil of the invention furnishing lower molecular weight aromatics to serve as a diluent and/ or solvent in the presence of which the cracking occurs, and higher molecular weight aromatics to be cracked. Another operation customarily employed prior to our invention was to coke the cracked residual oils in regular coking stills. Thus, when employing our invention it is possible to crack a cracked residual oil at higher temperatures under non-Coking conditions and thereby reduce the oil further, than has heretofore been possible, and further, to recover a valuable petroleum" pitch having more uses than petroleum coke.

Figure 1 illustrates diagrammatically the several embodiments of our invention. n

Figure 2 is a plot of the pitch penetration data given 1n Table II below.

Referring now to Figure `1: in a first embodiment of the invention crude `oil is introduced through line into crude oil distillation zone 11 which comprises a modern crude oil distillation unit having a vacuum reducl tion tower. Fractions comprising gas oil and gasoline are withdrawn through lines 12 and 1,3, respectively. Topped crude is withdrawn through line` 14 and passed to mild cracking zone 15 from where `fractions comprising gas oil and gasoline are withdrawn through lines 16 and 17 respectively. Cracked topped crude isV withdrawn through line 18 and passed tosevere cracking zone 22.

` Prior to entering severe cracking zone 2,2 an aromatic gas oil is added to the oil in line 21 as a diluent. Said diluent canbe added `from an outside source through line 23 or from a source described below through line 35. If de sired, depending upon the nature of the oil in line 21 and the nature of the diluent oil,` all of said diluent can come from line 23 or all from line 35, or, the amounts introduced through each line with respect to the other can be varied through any desired ratio. lf desired water can also be injected through line 61. The effluent from severe cracking zone 22 is` passed through line 24 into hot oil separator 25 wherein a separation between the higher boiling components comprising fuel oil, and the lower boiling components comprising gas oil and lighter, is effected. Said fuel oil is withdrawn from hot oil separator 2 5 through line 26 and passed intolsurge tank 27 from where it is withdrawn through line 28 and passed through heater 29 into vacuum tower 30. A vacuum is maintained on vacuum tower 30 through line 31 connected to vacuum jets not shown. An improved petroleum pitch is withdrawn to storage through line 32. Gas oil is withdrawn from said vacuum tower through line 33 and ret turned through

lines

34 and 35, to line 21 as a portion or all of said diluentfor the oil in line 21. If desired a portion of the gas oil withdrawn from vacuum tower 30 through'line 33 may be passed to storage through line 36. The overhead from hot oil separator 25 is passed through line 37 to fractionation zone 38. Gasoline is removed overhead from said fractionation `zone by means of line 39 through condenser 40 into run tank 41 and to storage through line 42. A portion of the gasoline is returned to said fractionation zone via line 43 as reux in a conventional manner. Gas oil is withdrawn from fractionation zone 38 through line 44 and a portion thereof passed `through line 45 and pump 46 into line 3S as a portion or all of the diluent added to the cracked topped crude being passed to severe cracking zone 2,2. If desired, a portion of the gas oil withdrawn through

lines

44, 45 and pump 46 can be passed through line`47, cooler 48, line 49 and into line 24 as quench for the euent from severe cracking zone 2 2. Similarly, if desired, a portion of the gas oil passed through line 47 and cooler 48 may be passed through line 50 into fractionation zone 38 as a cold spray to aid in the` operation thereof. If desired a portion of the gas oil inline 47 can be passed through line 52 for use or treatment elsewhere.

In a second embodiment of the invention the cracked topped crude withdrawn from mild cracking zone `15 can be passed through line 18 into vacuum distillation zone 19 from which an overhead t gas oil stream is withdrawn through line 20 for treatment elsewhere. Vacuum reduced cracked topped crude is withdrawn through line 21 and passed into severe cracking zone 22 for further treatment as described in connection with the first ernbodiment.

In a third embodiment of the invention a portion of the gas oil withdrawn from vacuum distillation zone 19 through line 20 can be returned through line 51 into mild cracking zone 15.` However, in many instances it will be more desirable to withdraw all of the gas oil from vacuum distillation zone 19 through line 20 for further treatment in other operations such as catalytic cracking operations.

In a fourth embodiment of the invention the gas oil withdrawn from fractionation zone 38 through line 44 can be passed to separation zone 53 where a separation is effected between the more aromatic portion and the less aromatic portion of said gas oil. The said less aromatic portion from separation zone 53 is withdrawn through line 54 for other treatment or use elsewhere. The said more aromatic portion from separation zone 53 is withdrawn through line 55 and passed through

lines

34 and 35 and added to the cracked vacuum reduced topped crude in line 21 as a portion or all of the aromatic diluent for the oil therein.

In a ifth embodiment of the invention the gas oil withdrawn from vacuum tower 30 via

lines

33 and 34 can be passed via line 56 into separation zone 57. In separation zone 57, depending upon the method of separation employed, a separation is effected between the less aromatic portions and the more aromatic portions of said gas oil, or, a separation is effected between the lower molecular weight portions and the higher molecular weight portions in said gas oil. If the separation method employediu separation zone 57 is of the type to separate the gas oil into less aromatic and more aromatic portions, the more aromatic portion is withdrawn through line 58 into lino 34 and passed by means of line 35 into line 21 as a portion of the aromatic diluent therein. The less aromatic por tion from separation zone 57 is withdrawn through line 59 for treatment or use elsewhere. If the separation method employed in separation zone 57 is of a type to separate the gas oil charged thereto into portions of lower molecular weight and portions of higher molecular weight, the higher molecular weight material is withdrawn through line 58 and passed through

lines

34 and 35 into line 21 and the lower molecular weight material is withdrawn through line 59 for treatment or use elsewhere. The separation in separation zone 57 according to molecular weights is advantageous in those instances where it is desirable to increase the concentration of high molecular weight aromatic components in the added aromatic gas oil.

In a sixth embodiment of the invention a gas oil can be charged through line 62 to cracking zone 15 to prepare a cracked residue which is then withdrawn through either

lines

18 or 60 for further treatment in accordance with one of the previously described embodiments.

It should be understood that while the various embodiments of the invention have been describedseparately they can be practiced in various combinations with each other. For example, in the rst embodiment the gas oil withdrawn from fractionator 38 through line 44 can be passed through separation zone 53. Or, the gas oil in line 34 can be passed through separation zone 5,7. In other words, it should be understood, for example, that

separation zones

53 and 57 can be employed either singly or simultaneously with the embodiment of the invention described as the rst embodiment.

Separation zones

53 and 57 can comprise any known method for separating an oil into fractions having different properties, e. g., solvent extraction, propane de asphalting, vacuum distillation, etc.

In general, operating conditions in the various operating zones will depend upon the nature of the oil being processed and the particular combination of the various embodiments of the invention employed as will be understood by those skilled in the art.

The operating conditions in mildcracking zone 15 are in general those employed in` what is known in the art as visbreaking cotidianas, i. e... high velocity o f ou through the greets@ coil, high heat transfer rates, etc. When charging a topped crude from a crude oil comprising a mixture of West Texas and Texas Panhandle crude oils, we can operate under the following conditions: temperature of furnace coil outlet, within the range from 850 to 1000 F., preferably from 900 to 1000 F., and more preferably from 900 to 950 C.; and pressures within the range from 50 p. s. i. g to 150 p. s. i. g., preferably from 50 p. s. i. g. to 100 p. s. i. g., and more preferably as low as condenser conditions will permit.

ln severe cracking zone 22, when charging a cracked topped crude from the said mixture of crude oils the coil outlet pressures employed can be within the range of 50 p. s. i. g. to 400 p. s. i. g The temperatures employed should be at least 900 F. The upper temperature is limited only by consideration of such factors as nature of the oil charge, desired properties of products produced, particular equipment limitations, etc. Operating conditions which comprise severe cracking conditions will vary with the nature of the oil being cracked. The longer an oil is exposed to high temperature conditions, at a given pressure, the more it will be cracked. Thus, temperature, residence time in the coil, and pressure are the most important factors to consider. Those skilled in the art will be able, in the light of this disclosure, to select the upper temperature limit for a given oil after a consideration of the above factors.

Operating conditions in vacuum tower 30 will depend upon the degree of reduction desired to be effected as will be understood by those skilled in the art. When charging a fuel oil from severe cracking zone 2.2, prepared as described above from the said mixture of crude oils, to produce the improved pitch of their invention, temperatures up to 850 F., preferably up to 800 F., and more preferably up to 750 F. can be employed.' The pressure can be from 20 to 0.1 mm. Hg abs., and preferably from 2 to 0.1 mm. Hg. abs.

The volumetric ratio of total diluent to cracked oil charge will vary in accordance with the nature of the cracked oil to be cracked further. We have found that when cracking a vacuum reduced topped crude of the type described above that said ratio should be at least 6: 1.

It should be noted that the severe cracking zone of our invention is a producer of aromatic gas oil suitable for use as the aromatic oil admixed with the charge thereto. As mentioned aromatic gas oil' from an outside source can be employed. Likewise, the added aromatic gas oil can be the gas oil recovered from the bottom of fractionator 38, or the aromatic overhead gas oil from vacuum tower 30, or a mixture from all three said sources. At present a preferred embodiment is to employ a mixtue of the gas oil from fractionator 38 and vacuum tower 30. The inclusion of the heavier gas oil containing high molecular weight components results in a petroleum pitch having a iiatter penetration curve with a sharper break at a lower softening point as shown in Figure 2 discussed below. The relative ratios of the different gas oils comprising the added aromatic oil, and the amount of high molecular weight components included in said added aromatic oil, will vary in accord with the nature of the cracked oil being cracked and the properties desired in the linal products.

Example l below illustrates a now preferred embodiment of our invention, i. e., that described above as the iirst embodiment. Example Il is illustrative of the practice in the art prior to our invention.

EXAMPLE I A mixture of West Texas and Texas Panhandle crude oils was charged to a crude oil distillation unit which included a conventional vacuum tower. A portion of the topped crude oil was subjected to mild cracking in a visbreaking unit. A portion of the cracked topped crude, after admixture with an aromatic gas oil (identified further below) was charged to a thermal cracking unit (A in Table II below) where it was severely cracked under non-coki'ng conditions. The severely cracked topped crude was then separated into a fuel oil fraction and a second fraction containing gas oil and lighter. The fraction containing gas oil and gasoline was fractionated to yield a fractionator gas oil, a portion of which was adrnixed with the cracked topped crude being charged to the severe thermal cracking step as a part of the said admixed gas oil. The fuel oil fraction was vacuum distilled to yield an improved petroleum pitch and an overhead heavy aromatic vacuum still gas oil. A portion of said heavy aromatic vacuum still gas oil containing high molecular weight components was admixed with the cracked topped crude being charged to the severe thermal cracking step as a part of said admixed aromatic gas oil. Thus, the said diluent was comprised of a mixture of said fractionator gas oil and said heavy aromatic vacuum still gas oil in the volumetric ratio of approximately 10:1. The volumetric ratio of the volume of topped crude to the volume of total diluent in the combined charge to the severe thermal cracking step was 8:1.

EXAMPLE Il A portion of the cracked topped crude prepared in Example I was charged to a second thermal cracking unit (Unit B in Table Il below) and cracked in the absence of heavy aromatic diluent recovered from the vacuum distillation of the fuel oil from said unit. The remainder of the processing steps were the same. A portion of the fractionator gas oil was returned to the cracking unit charge as in Example I. The ratio of the volume of said fracticnator gas oil to the volume of the cracked topped crude charged to the unit was 8:1.

Tables I and Il below compare operating conditions, yields and properties of the products from the above examples.

TABLE I Cracking of cracked topped crude tBased on charge to unit excluding the fractionator G. O. recycle and Wa er.

TABLE Il Vacuum dlstzllmlon of fue] oil from Table l Pitch Recovered From From Unit A Unit B Fuel Oil Fuel Oil Vac, Still temperature, F 600 610 Vac. Still pressure, abs., min. Hg... 5 5 Yield, LV Percent l 40 32 Specific gravity, (iO/60 F 1,154 1.155 softening Point, R. dz. B., 231 243 Penetrations:

gms. 5 secs.-

F 10 7 Soluble in CS2, Percent 98. 5 98. 3 Soluble in Benzene, Percent 97. 5 98. 2 Soluble in CCli, Pereent 86. 5 84. 9 Soluble in Acetone, Percent 65.4 63. 0 Volatile Combustible Matter, Percent 48. 3 47.4 Ash, Percent 0.1 0.1 Fixed Carbon, Percent 51.6 52. 5

TABLE II-Continucd Gas Oil Recovered From From Unit A Unit B Fuel Oil Fuel Oil Yield, LV Percent l i7 9 Gravity, API 0. 4 2. 6 SU Viscosity at; 130 F., secs 141 3(4) Carbon Residue, Conradson Percent 1. 7 c, Vacuum Distillation: F., 760 ni r 1st dropggg 67@ iecoveie 670 630 71o 673 30:: 74o 704 40 771 728 50 790 748 60 808 779 Correlation Index-B/M 121 116 1 Based on original charge to cracking step, Table I.

Figure 2 is a plot of the pitch'penetration data given in Table II. One desirable characteristic of a pitch is the property of lending itself to crushing, grinding or other mechanical handling without flow or fusion. Coal tar pitch is usually superior to petroleum pitch in this respect. Penetration is a measure of this property. and a pitch which possesses a flat penetration curve having a sharp break is a superior pitch. Such a penetration curve indicates a. material having more uniform chemical characteristics because it is approaching what would be termed a melting point of a pure chemical component. It is to be noted that the penetrating curve `of the pitch from Unit A has a shaiper break thanthat of the pitch from Unit B. It will be remembered that the added aromatic oil charged to Unit A included, as a portion thereof, the heavy aromatic oil containing high molecular Weight cornponents. p

The correlation index referred to in Table II is a cor relation index developed by the Bureau of Mines and is used to denote aromaticity of an oil; a higher numerical index denotes a more aromatic oil. The index is calculated from the formula c rQ-rirarG-esss where:

C. I.=Bureau of Mines correlation index K=Average boiling point (K.) of the fraction G=Specic gravity @60 F./60 F.

In the specification and in the appended claims the term non-coking conditions, as applied to the severe cracking step, refers to those operating conditions which in the absence of the added aromatic gas oil would be coking conditions.

Reasonable variation `and modification are possible within the scope of the foregoing disclosure and the i appended claims to the invention, the essence of which is:

the process of severely cracking a cracked residual oil, under non-coking conditions in the presence of an aromatic diluent and recovering an improved petroleum pitch; a portion of said aromatic diluent, in a preferred embodiment, being a heavy aromatic gas oil containing components of high molecular weight.

We claim:

l. A process for the cracking of ar esidual oil recovered from a process in which an oil has been cracked to convert the same into higher boiling and lower boiling oil portions which comprises; obtaining a residual cracked oil from said cracked oil, further cracking said residual cracked oil in the presence of a broad boiling range aromatic oil containing aromatic components of relatively high molecular weight and aromatic components of relatively low molecular weight under conditions which in the absence of said aromatic oil would be coking conditions for said obtained cracked residual oil, and recovering a cracked cracked residual oil substantially more aromatic than said obtained residual cracked oil, said cracked cracked residual oil containing components boiling higher and components boiling lower than said obtained cracked residual oil.

2. A process for the conversion of a residual oil which comprises; subjecting said residual oil to a thermal cracking step, recovering from the resulting cracked residual oil a fraction containing gasoline and gas oil and a second fraction substantially comprised of a cracked residue, adding a broad boiling range aromatic oil containing aromatic components of relatively high molecular weight and aromatic components of relatively low molecular weight to said cracked residue, subjecting said cracked residue to severe thermal cracking under conditions which in the absence of said aromaic oil would be coking conditions, recovering a fraction containing gasoline and gas oil from said severely cracked residue and recovering a residual fuel oil from said severely cracked residue.

3. A process for the conversion of a residual oil which comprises; subjecting said residual oil to a thermal cracking step, recovering from the resulting cracked residual oil a first fraction containing gasoline arid gas oil and a second fraction substantially comprised of a cracked resi due, adding a broad boiling range aromatic gas oil containing aromatic components of relatively high molecular weight and aromatic components of relatively low molecular weight to said cracked residue, severely cracking said cracked residue in a severe cracking zone under conditions Which in the absence of said aromatic oil would be coking conditions, recovering a fraction containing gasoline and gas oil from said severe cracking zone, passing residual fuel oil from said severe cracking step to a vacuum distillation zone, and recovering an improved petroleum pitch as bottoms product from said vacuum distillation zone.

4. A process for the conversion of a residual oil which comprises; subjecting said residual oil to a thermal cracking step, recovering a first fraction containing gasoline and gas oil and a second fraction comprised substantially of a cracked residue from `said thermal cracking step, adding a broad boiling range aromatic gas oil containing aromatic components of relatively high molecular weight and aromatic components of relatively low molecular weight to said cracked residue, severely cracking said cracked resid ual oil under non-coking conditions in a severe cracking zone, recovering a fraction containing gasoline and gas oil from said severe cracking zone, passing residual fuel o il from said severe cracking zone to a vacuum distillation zone, vacuum distilling said fuel oil recovering from said vacuum distillation zone a heavy aromatic `gas oil having an initial boiling point in excess of 550 F. and a 50 per cent distillation point in excess of 780 F. at atmospheric pressure, returning said heavy aromatic gas 011 to said severe cracking step as a portion of said aromatic gas oil, and recovering an improved petroleum pitch having a softening point of at least 230 F. from said vacuum distillation zone.

5. The process of claim 3 wherein said aromatic oil is added in volumetric ratio to said cracked residue of at least 6:1 and said improved petroleum pitch has a softeni ing point of at least 230 F.

6. process foi the conversion of a residual oil which comprises; subjecting said residual oil to a thermal cracking step, recovering a fraction containing gasoline and taining gasoline and gas oil from said severe cracking zone, passing residual fuel oil from said severe cracking zone to a vacuum distillation zone, vacuum distilling said fuel oil, recovering an improved petroleum pitch from said vacuum distillation zone, recovering a gas oil as overhead product from said vacuum distillation zone, separating said overhead gas oil product into a portion of relatively low molecular weight and a portion of relatively high molecular weight, and returning said portion of relatively high molecular weight to said sever cracking zone as a portion of said diluent therein.

7. The process of claim 6 wherein said separation of said overhead gas oil product into said portions of relatively low molecular weight and relatively high molecular weight is accomplished in a second vacuum distillation zone.

8. The process of claim 2 wherein said residual oil is topped crude, and said residual fuel oil is passed to a vacuum distillation zone, recovering a heavy aromatic gas oil containing components of high molecular weight as overhead product from said vacuum distillation zone, returning said heavy aromatic gas oil overhead product to said severe cracking zone as a portion of said added aromatic gas oil, and recovering an improved petroleum pitch having a softened point of at least 230 F. from said second vacuum distillation zone.

9. The process for the conversion of a topped crude oil which comprises; subjecting said topped crude to mild thermal cracking in a mild cracking zone, recovering a fraction containing gasoline and gas oil from said mild cracking zone, passing a second fraction comprised substantially of a cracked residue to a vacuum distillation zone, removing a gas oil product overhead from said vacuum distillation zone, passing a vacuum reduced cracked topped crude as bottoms product from said vacuum distillation zone to a severe thermal cracking zone, adding a broad boiling range aromatic gas oil containing aromatic components of relatively high molecular weight and aromatic components of relatively low mo* lecular weight to said vacuum reduced cracked topped crude, severely cracking said vacuum reduced cracked topped crude under condition which in the absence of said aromatic gas oil would be coking conditions, recovering a fraction containing gasoline and gas oil from said severe cracking zone, recovering a residual fuel oil from said severe cracking zone, passing said residual fuel oil to a separation zone, and recovering an improved petroleum pitch from said separation zone.

10. The process of claim 9 wherein said residual fuel oil is passed to a second vacuum distillation zone, recovering a heavy aromatic gas oil containing components of high molecular weight as overhead product from said second vacuum distillation zone, returning said heavy aromatic gas oil overhead product to said severe cracking zone as a portion of said added aromatic gas oil, and recovering an improved petroleum pitch from said second vacuum distillation zone.

1l. A process for the conversion of a cracked residual oil which comprises; cracking a gas oil in a cracking zone to form said cracked resid tal oil containing aromatic components of relatively high molecular weight and aromatic components of relatively low molecular weight, withdrawing said cracked residual oil from said cracking zone, adding a broad boiling range aromatic gas oil to said cracked residual oil, severely cracking said cracked residual oil in a severe cracking zone under conditions which in the absence of said added aromatic gas oil would be coking conditions, passing a cracked residue from said severe cracking zone to a vacuum distillation zone, vacuum distilling said cracked residue, and recovering an improved petroleum pitch from said vacuum distillation zone.

12. A process for the conversion of a topped crude oil which comprises, in combination, the steps of: subjecting said topped crude to mild thermal cracking in a mild cracking zone; recovering a fraction containing gasoline and gas oil from said mild cracking zone; withdrawing a cracked residual oil from said mild cracking zone; adding an aromatic gas oil diluent to said cracked residual oil in a volumetric ratio of gas oil to residual oil of at least 6:1; severely cracking said cracked residual oil in the presence of said aromatic oil in a severe cracking zone under conditions which in the absence of said aromatic oil would be coking conditions; recovering a fraction containing gasoline and gas oil from said severe cracking zone and passing said last mentioned fraction to a fractionation zone; recovering a fractionator gas oil from said fractionation zone and passing same to said severe cracking zone as a portion of said aromatic gas oil diluent; passing a cracked residual fuel oil fraction from said severe cracking zone to a vacuum distillation zone; vacuum distilling said residual fuel oil; recovering a heavy aromatic gas oil having an initial boiling point in excess of 550 F. and a 50 per cent distillation point in excess of 780 F., at atmospheric pressure, from said vacuum distillation zone; returning said heavy aromatic gas oil to said severe cracking zone as another portion of said aromatic gas oil diluent, the volumetric ratio of said fractionator gas oil to said heavy aromatic gas oil being about 10:1; and recovering an improved petroleum pitch having a softening point of at least 230 F. from said vacuum distillation zone.

References Cited in the file of this patent UNITED STATES PATENTS 2,182,599 Seguy Dec. 5, 1939 2,288,262 Atwell June 30, 1942 2,312,445 Ruthrul Mar. 2, 1943 2,516,134 Molique July 25, 1950 2,542,608 Winkler Feb. 20, 1951 2,663,675 Ewell Dec. 22, 1953 2,666,022 Johnson Jan. 12, 1954 2,702,782 Little Feb. 22, 1955

Claims

1. A PROCESS FOR THE CRACKING OF A RESIDUAL OIL RECOVERED FROM A PROCESS IN WHICH AN OIL HAS BEEN CRACKED TO CONVERT THE SAME INTO HIGHER BOILING AND LOWER BOILING OIL PORTIONS WHICH COMPRISES; OBTAINING A RESIDUAL CRACKED OIL FROM SAID CRACKED OIL, FURTHER CRACKING SAID RESIDUAL CRACKED OIL IN THE PRESENCE OF A BROAD OILING RANGE AROMATIC OIL CONTAINING AROMATIC COMPONENTS OF RELATIVELY HIGH MOLECULAR WEIGHT AND AROMATIC COMPONENTS OF RELATIVELY LOW MOLECULAR WEIGHT UNDER CONDITIONS WHICH IN THE ABSENCE OF SAID AROMATIC OIL WOULD BE COKING CONDITIONS FOR SAID OBTAINED CRACKED RESIDUAL OIL, AND RECOVERING A CRACKED CRACKED RESIDUAL OIL SUBSTANTIALLY MORE AROMATIC THAN SAID OBTAINED RESIDUAL CRACKED OIL, SAID CRACKED CRACKED RESIDUAL OIL CONTAINING COMPONENTS BOILING HIGHER AND COMPONENTS BOILING LOWER THAN SAID OBTAINED CRACKED RESIDUAL OIL.

6. A PROCESS FOR THE CONVERSION OF A RESIDUAL OIL WHICH COMPRISES; SUBJECTING SAID RESIDUAL OIL TO A THERMAL CRACKING STEP, RECOVERING A FRACTION CONTAINING GASOLINE AND GAS OIL FROM SAID THERMAL CARCKING STEP, PASSING A SECOND FRACTION COMPRISED SUBSTANTIALLY OF A CRACKED RESIDUE FROM SAID THERMAL CARCKING STEP TO A SEVERE THERMAL CARCKING ZONE, ADDING A BROAD BOILING RANGE AROMATIC GAS OIL CONTAINING AROMATIC COMPONENTS OF RELATIVELY HIGH MOLECULAR WEIGHT AND AROMATIC COMPONENTS OF RELATIVELY LOW MOLECULAR WEIGHT TO SAID CRACKED RESIDUE AS A DILUENT THEREFOR, SEVERELY CRACKING SAID CRACKED RESIDUE UNDER CONDITIONS WHICH IN THE ABSENCE OF SAID ADDED AROMATIC GAS WOULD BE COKING CONDITIONS, RECOVERING A FRACTION CONTAINING GASOLINE AND GAS OIL FROM SAID SEVERE CRACKING ZONE, PASSING RESIDUAL FUEL OIL FROM SAID SEVERE CRACKING ZONE TO A VACUUM DISTILLATION ZONE, VACUUM DISTILLING SAID FUEL OIL, RECOVERING AN IMPROVED PETROLEUM PITCH FROM SAID VACUUM DISTILLATION ZONE, RECOVERING A GAS OIL AS OVERHEAD PRODUCT FROM SAID VACUUM DISTILLATION ZONE, SEPARATING SAID OVERHEAD GAS OIL PRODUCT INTO A PORTION OF RELATIVELY LOW MOLECULAR WEIGHT AND A PORTION OF RELATIVELY HIGH MOLECULAR WEIGHT, AND RETURNING SAID PORTION OF RELATIVELY HIGH MOLECULAR WEIGHT TO SAID SEVERE CRACKING ZONE AS A PORTION OF SAID DILUENT THERIN.