US2944958A - Process of making pitch - Google Patents

Description

Unite PROCESS OF MAKING PITCH Filed Feb. 14, 1958, Ser. No. 715,366

3 Claims. (Cl. 208-39) This invention relates to a method for the production of petroleum pitches and is particularly concerned with the preparation of petroleum pitches from crude oils of low asphaltene content.

In the refining of petroleum oils it is desirable to obtain the highest possible yields of distillate oils such as gasoline, kerosene and heating oils because of the higher prices received for those products. The yield of the distillate oils is increased by making deep cuts into the crude by vacuum distillation to separate as much of the distillable po'rtion of the crude as possible from the residual fractions. The heaviest distillate fractions separated in the vacuum distillation are the heavy gas oils ordinarily used as charge stock to catalytic cracking units for conversion to lighter fractions, principally gasoline. Residues from vacuum distillation are used in the preparation of heavy fuel oils. As higher flash temperatures are used in the distillation, the residual pro'ducts become heavier and more viscous. In order to produce a heavy fuel oil meeting the A.S.T.M. viscosity specifications for a No. 6 fuel oil, the residual products from deep vacuum distillation must be blended with lighter oils, designated as cutter oils. Because of the low market value of the heavy fuel oil, the blending of cutter oils with residual products to produce a heavy fuel oil results in a reduction of the ultimate market value of the cutter oils.

One method of increasing the market value of the products obtained from the crude oil is to prepare a solid petroleum pitch as a residual product. In such processes no cutter oil is required; hence, the ultimate yield of distillate products is increased. The petroleum pitches are of principal value as a solid fuel and when used as such are often subjected to substantial pressures and temperatures above about 130 F. in storage. To prevent fusion of the pitch into a solid mass while in storage, a pitch having a ring and ball (A.S.T.M. E28-51T) softening point of about 300 F. and preferably 325 F. or higher has been required when prepared by the processes heretofore available. Those processes were either restricted to the treatment of crude oils of high asphaltene content, or demanded extremely severe operating conditions to produce a pitch having the necessary fusion characteristics from crude oils of low asphaltene content. The extremely severe conditions needed to produce the high softening point pitches from the crude oils of low asphaltene content favor increased coking and fouling of equipment and thus impair the operability of the process.

This invention resides in a process of excellent operability for the production of a petroleum pitch having a high incipient fusio'n temperature by visbreaking a bottoms fraction of a crude oil of low asphaltene content, fractionating the visbreaker product to produce a visbreaker tar and then subjecting the visbreaker tar to a second visbreaking step performed in the absence of virgin petroleum fractions. The final petroleum pitch product is then obtained as a bottoms product from the atmospheric distillation of the eflluent from the second States Patent "ice visbreaking operation. An important advantage of the process of this invention is the higher incipient fusion temperature of a pitch of a given ring and ball softening point prepared by this method compared with pitches of corresponding ring and ball softening point prepared by the processes heretofore available. Because of this unique relationship between the incipient fusion temperature and the ring and ball softening point, pitches satisfactory for use as solid fuels can have ring and ball softening points as low as 250 F. when prepared by this invention.

In the drawings:

Figure 1 is a diagrammatic flow sheet of the process for the preparation of a petroleum pitch by the method of this invention.

Figure 2 is a graph showing the incipient fusion temperature of petroleum pitches of given ring and ball softening points as compared with pitches produced by processes heretofore available.

The charge stock for the process of this invention can be any petroleum crude oil which ordinarily would be used for the preparation of pitches. This process is particularly valuable for the preparation of pitches suitable for use as solid fuels from low asphaltene content crude oils, such as those obtained from Kuwait, West Texas and Mid-Continent fields. Other methods, for example, the method described in application Serial No. 416,634 of Harold Beuther and Richard G. Goldthwait, filed March 16, 1954, now US. Patent No. 2,850,436, are preferred for the preparation of petroleum pitches from crude oils of high asphaltene content.

Referring to Figure l of the drawings, a topped crude oil from an atmospheric tower, not shown, in which, for example, a furnace oil and lighter fractions have been removed, is delivered through line 10 to a vacuum tower l2. Vacuum tower 12 is operated at a flash temperature, corrected to atmospheric pressure, above about 940 F. and may range up to a corrected temperature of about 1050 F. Higher corrected flash temperatures can be used, but ordinarily are not used because of the increased costs incurred in obtaining them without excessive cracking in the distillation operation. A heavy gas oil suitable for use as a catalytic cracking charge stock top of the vacuum tower 12 and a bottoms fraction containing virgin heavy residual oils is delivered through line 16 from the bottom of the vacuum tower 12. Ordinarily steam is injected through a line such as line 18 into the bottom of the vacuum tower to aid in the stripping of volatile components from the bottoms fraction.

The vacuum reduction of the topped crude oil that occurs in vacuum tower 12 is a normal operation for the preparation of a visbreaking charge stock in modern refinery practice. The depth of reduction of some crude oils may be limited by the maximum metals and carbon residue permissible in the gas oil delivered overhead from the vacuum tower. In the vacuum distillation of some crude oils, the metals co'ntent or the carbon residue of the overhead from the vacuum tower will increase rapidly as the depth of the cut into the crude oil is increased. Extremely deep vacuum reduction of such crude oils is not feasible. Propane deasphalting of the topped crude oil also can be used to produce a highly asphaltenic residue for charging to the first visbreaker.

The bottoms fraction from the vacuum tower is delivered through line 16 to the coils 20 of a first visbreaker furnace 22 in which the bottoms fraction is heated to a furnace outlet temperature of about 825 to 1000 F. The pressure in the coils 20 of visbreaker furnace 22 is in the range of about 50 to 1000 p.s.i.-g. and the volume of the coil above 750 F. in the range of about 0.016 to 0.050 cubic foot per barrel of through- 3 put per day. The first visbreaking operation is a oncethrough cracking operation of the residual oils at a severity such that about 8 to 17% debutanized 400 and point gasoline is produced. Recycle of intermediate fractions of the first visbreaker product, for example, light gas oils, may be employed.

The product from the first vibreaker furnace 22 is delivered through line 24 into an intermediate distillation tower 26 in which a heavy bottoms fraction, herein designited as a visbreaker tar is separated from lighter oils for further treatment. The intermediate distillation may vary widely from a mild atmospheric distillation to a vacuum distillation. In the mild atmospheric distillation the visbreaker tar is essentially a furnace oil-free residue, produced as a bottoms product and discharged from the tower 26 through line 28. The mild atmospheric distillation may be defined as one adapted to remove a furnace oil 90% of which boils below about 600 F. Ordinarily the flash temperature in the mild atmospheric distillation will be in the range of 650 to 700 F. Steam can be injected into the distillation tower 26 to aid in the stripping of light fractions from the bottoms removed from the tower. Similarly, in a mild atmospheric distillation a reboiler may be used to increase stripping in the lower part of the tower 26. I

A deep atmospheric distillation also may be performed as an intermediate distillation between the first and second visbreaking step. In a deep atmospheric distillation a distillate gas oil fraction having a 90% boiling point in the range of 650 to 800 F. can be obtained. In a deep atmospheric distillation, steam is usually added at a moderate rate up to about 50 pounds per barrel of first visbreaker product, and preferably about pounds per barrel, to improve the stripping.

The intermediate distillation also can be a vacuum distillation step. The vacuum distillation of the effluent from the first visbreaker 22 has certain advantages in the reduction of tendencies to form coke in the second visbreaking step but suffers the disadvantage of increased cost because of the size of the unit and steam requirements for a vacuum distillation. Generally, the incipient fusion temperature of a petroleum pitch of a given ring and ball softening point prepared with an intermediate vacuum distillation is higher than of pitches of the same ring and ball softening point prepared with other intermediate distillation steps. In the vacuum distillation a flash temperature of 940 F., corrected to 760 mm. of mercury absolute pressure, up to about 1030 F. is used.

Regardless of the manner of performing the intermediate distillation, the visbreaker tar charged to the second visbreaking step is a very heavy residue which has had a single pass through a visbreaker. Furnace oil, or gas oil, and lighter fractions have been separated from the visbreaker tar. The gravity of the visbreaker tar, even when the first visbreaker product is subjected to only a mild atmospheric distillation, is less than about 5' A.P.I.

The visbreaker tar from the distillation tower 26 is delivered through line 28 to the coil 30 of a second visbreaker 32 in which the tar is heated to a coil outlet temperature in the range of 820 to 1000 F. The severity of the visbreaking in coil 30 is normal for conventional visbreaking of vacuum reduced crudes. About 5 to of the charge to the second visbreaker coil is converted to 400 F. end point gasoline. The temperature, pressure, and coil volume for the second visbreaking step are substantially the same as corresponding conditions in the first visbreaking step.

The product from the visbreaker coil 30, herein referred to as the second visbreaker product, is delivered through line 34 into an atmospheric flash distillation tower 36. Gas oil and lighter fractions are removed as distillate products from tower 36 and the petroleum pitch is discharged as a bottoms product through line 38. The flash temperature in tower 36 ranges from about 700 F.

up w a maximum of about 825 F. and o to so, preferably 25 to 50, pounds of steam per barrel of oil are charged to the tower. The maximum flash temperature used in tower 36 is limited by the formation of coke in the tower. A portion of the steam may be injected through quench line 40 to quench the second visbreaker product discharged from coil 30 and the remainder of the steam is added directly through line 42 into the bottom of the tower 36. A portion of the distillate fractions separated in tower 36 may be recycled through visbreaker 30, or used as qu ench, but the visbreaker operation is once-through for the residual fraction. A heavy gas oil, suitable for blending with No. 6 fuel oils, and lighter fractions are obtained as products in addition to the pitch.

An important advantage of this invention is the higher incipient fusion temperature of pitches of a given ring and ball softening point as compared with incipient fusion temperature of pitches of the same ring and ball softening point prepared by processes in which there is a single visbreaking step. The incipient fusion temperature is defined as the minimum temperature at which adhesion of pitch particles occurs when the pitch particles are subjected to a pressure corresponding to the pressure exerted by a pile of pitch 40 feet high. Determinations of the incipient fusion temperature were made by placing pitch particles in a cylinder and applying a pressure of 13.3 p.s.i. to a piston movable in the cylinder. The cylinder was placed in a bath and the temperature raised gradually until incipient fusion occurred. At the incipient fusion temperature there was adhesion of the pitch particles at the interface between the pitch and the piston only, but not throughout the cylinder. The adhesion was predominantly to the piston, and, at most, extended about Va inch into the pitch. Inst below the temperature of incipient fusion, there was no adhesion of pitch particles. Just above the incipient fusion temperature, at a temperature of about 5 to 10 F. higher than the incipient fusion point, adhesion occurred throughout the cylinder but when the pitch particles were cooled and broken up, separation occurred predominantly at the original interfacial surfaces.

Referring to Figure 2 of the drawings, curve 46 shows the relationship between the incipient fusion temperature and the ring and ball softening point of pitches prepared from Kuwait crude oil by the two visbreaking step method of this invention. Slight variations from curve 46 will occur and will depend upon the type of intermediate distillation used. The curve identified by reference numeral 48 shows the relationship between the incipient fusion temperature and the ring and ball softening point of pitches prepared by a single visbreaking of the residue from the vacuum reduction of Kuwait crude oil followed by the deep vacuum reduction of the visbroken residue to produce the pitch as a bottoms product.

It will be seen from Figure 2 that the incipient fusion temperature of a pitch prepared by the two visbreaking step process of this invention is about 50 higher than the incipient fusion temperature of a pitch prepared by a process employing a single visbreaker and having the same ring and ball softening point. Conversely, a pitch prepared according to this invention having an incipient fusion temperature of 130 F. has a ring and ball softening point above F. lower than the ring and ball softening point of a pitch of the same incipient fusion temperature prepared in a process using a single visbreaking step.

EXAMPLE 1 A Kuwait crude oil was topped in an atmospheric distillation tower and the bottoms fraction from the atmoshperic distillation flashed in an equilibrium distillation at a temperature corrected to 760 mm. of mercury absolute pressure, of approximately 1030" F. The bottoms fraction from the vacuum flash distillation was charged to a visbreaker coil maintained at a pressure of about 275 p.s.i. The outlet temperature of the vis- 5 breaker coil was about 915 F. and the coil volume above 750 'F. was about 0.027 cubic foot per barrel of throughput per day. The visbreaking step produced 11.4%, by volume of the charge stock the visbreaker, of 400 F.

distillation. The visbreaker tar was cracked in a second visbreaker step at a pressure of 275 p.s.i.g., a furnace coil volume above 75 0 F. of 0.027 cubic foot per barrel of throughput per day, and a furnace outlet temperaend point gasoline. ture of 875 F. The product from the second visbreaker The visbreaker product was distilled in an atmospheric was flashed at a temperature of 800 F. in an atmosdistillation to separate a furnace oil and lighter fracpheric distillation employing 47.9 pounds of steam per tions from a visbreaker tar having a gravity of 2.9 A.P.I. barrel of reduced crude charge to the second visbreaker. The furnace oil had a distillation curve such that 90% The bottoms from the atmospheric distillation was a of the furnace oil distilled over at 591 F. A portion pitch having a ring and ball softening point of 285 F. of the furnace oil-free visbreaker tar was then passed and an incipient fusion temperature of 170 F. through a second visbreaker gt a furnace outlet tem- EXAMPLE 5 perature of approximately 875 F. A furnace pressure of 275 p.s.i.g. and a furnace coil volume above 750 F. A p n f he v sbre ker tar from the vacuum disof 0,027 cubic foot per barrel of throughput per day tillation Of the fiI St visbreaker product described in E X- were employed in the second visbreaker. The eifiuent ample 4 was subiected to a second vlsbreakmg t p at from the visbreaker was flashed at a temperature of 803 the salhe furnace Pressure and C volume above 750 F. in an atmospheric distillation tower using a total of as 111 p 4 but at a l p 0f 47.8 pounds of steam per barrel of reduced crude charge The Product h f eeeehd Vlshfeakel was flashed to the second visbreaker, A pitch having a ring d an atmospheric distillation at a temperature of 801 F., ball softening point of 292 F. and an incipient fusion 1151118 Pounds of Steam p barrel of reduced Cr temperature f 170 F, was d charge to the second visbreaker to produce as a bottoms EXAMPLE 2 product a pitch having a ring and ball softening point of 349 F. and an incipient fusion temperature of 230 F. A sample of the furnace oil-free visbreaker tar of Example 1 Was subjected to a second visbreaking step EXAMPLE 6 with th sam dition 35 E l 1 except h the A portion of the visbreaker tar from the vacuum disoutlet temperature was approximately 915 F, Th dtillation of the product of the first visbreaker described uct from the visbreaker was flashed at a temperature 111 Example 4 was Suhlected to a Second vishl'eakihg p of 801 F. using a steam rate of 49.1 pounds per barrel at a telTlPel'etllfe 0f and at the Same Co ditions of reduced crude charge to the second visbreaker. The 9 furnace Pressure and ell V0111Ine above as pitch produced had a ring and ball softening point of 111 Example The secohd Y h h Product was then 346 F, d an i i i f i temperature f 230 F. flashed in an atmospheric distillation at a temperature of 803 F. using 31.6 pounds of steam per barrel of EXAMPLE 3 reduced crude charge to the second visbreaker. The A sample of the first visbreaker product described bottoms product from the atmospheric distillation was in Example 1 was subjected to a deep atmospheric disa PItch h a and ban Softehlhg P of tillation to produce a -1.0 A.P.I. visbreaker tar as a and an mclplent fuslon temperature 5 bottoms product and separate thcrefrom a gas oil hav- The results 0f the pfoeesses described In Examples 1 ing a 90% distillation point at 655 F., and lighter 40 through 6 are Presented In Table T able 1 Volume ASTM Percent E28-51T Incipient 400 F. Flash Steam, Ring and Fusion Example No. Charge to2nd visbreaker End Temp., lb./bbi. Ball Point,

Point F. Softening F. Gasoline Temp,

Furnace Oil-Free Tar 6.2 803 47.8 292 170 do 9. 6 801 49.1 346 230 Deep Atmospheric Distilled Tar 5. 5 795 51.6 305 175 Vacuum Distilled Tar 6. 2 800 47. 9 285 170 do 9.8 801 50.6 349 230 .do 9.9 803 31.6 351 255 EXAMPLE 4 A sample of the product from the first visbreaker step described in Example 1 was subjected to a vacuum distillation under conditions to separate a visbreaker tar having a specific gravity corrected to 77 F./77 F. of 1.089. A gas oil, 88% of which boiled below 1000 F., corrected to 760 min. of mercury, and lighter fractions were separated from the visbreaker tar in the vacuum This invention provides a method for the preparation of petroleum pitches from petroleum crude oils of low asphaltene content. Pitches having high ring and ball softening points can be prepared without using such severe visbreaking and vacuum flash distillation conditions that excessive amounts of coke are formed. Hence, the process of this invention has excellent operability characteristics, i.e. it can be operated for long periods without shutting down to clean coke from the equipment. Moreover, the very high incipient fusion temperatures of the pitches prepared by the process of this invention makes it unnecessary .for the pitches to have extremely high ring and ball softening points to be useful as solid fuels.

We claim:

1. A process for the preparation of a petroleum pitch having a ring and ball softening point above about 250 F. and an incipent fusion temperature above about F. comprising flashing a topped crude oil under vacuum fraction at conditions to produce about 8 to 17% 400 F. end point gasoline, distilling the visbreaker product in an atmospheric distillation to separate furnace oil and lighter fractions and produce a visbreaker tar as a residual fraction, passing the visbreaker tar in the absence of virgin petroleum fractions through a second visbreaker in which about to 15% 400 F. end point gasoline is formed, flashing the second visbreaker product in an atmospheric distillation at a temperature in the range of 700 to 825 F. in the presence of steam to produce the pitch as a bottoms product.

2. A process as set forh in claim 1 in which the distillation of the first visbreaker product produces a visbreaker tar having a gravity lower than about 5 A.P.I.

3. A process for the preparation of petroleum pitch having a ring and ball softening point above about 250 F. and an incipent fusion temperature above about 130 F., comprising reducing a topped crude by a process selected from the group consisting of vacuum distillation and propane deasphalting to produce a heavy, highly asphaltenic residue, passing the residue through a visbreaker coil in a once-through, coil-only visbreaking of the residue at a temperature in the range of 825 F. to 1000 F. and a pressure in the range of about to 1000 p.s.i.g. to produce about 8 to 17% 400 F. end point gasoline, distilling the visbreaker'product in an atmospheric distillation to produce as a bottoms fraction a visbreaker tar having a gravity less than 5 A.P.I., passing the visbreaker tar in the absence of virgin residual petroleum fractions through a second visbreaker in which about 5 to 15% 400 F. end point gasoline is formed, and flashing the second visbreaker product in an atmospheric distillation at a temperature in the range of 700 to 825 F. to produce the pitch as a bottoms product.

References Cited in the file of this patent UNITED STATES PATENTS 2,305,440 Noel Dec. 15, 1942 2,687,986 Jennings et al. Aug. 31, 1954 2,796,388 Beuther et al. June 18, 1957 2,847,359 Beuther et al. Aug. 12, 1958 2,865,835 Owen Dec. 23, 1958

Claims (1)

1. A PROCESS FOR THE PREPARATION OF A PETROLEUM PITCH HAVING A RING AND BALL SOFTENING POINT ABOVE ABOUT 250* F. AND AN INCIPENT FUSION TEMPERATURE ABOVE ABOUT 130* F. COMPRISING FLASHING A TOPPED CRUDE OIL UNDER VACUUM AT A FLASH TEMPERATURE, CORRECTED TO 760 MM. OF MERCURY ABSOLUTE PRESSURE, ABOVE ABOUT 940*F. TO PRODUCE A HEAVY BOTTOMS FRACTION, VISBREAKING THE HEAVY BOTTOMS FRACTION AT CONDITIONS TO PRODUCT ABOUT 8 TO 17% 400* F. END POINT GASOLINE, DISTILLING THE VISBREAKER PRODUCT IN AN ATMOSPHERIC DISTILLATION TO SEPARATE FURNACE OIL AND

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