US 3110663 A
Description (OCR text may contain errors)
Nov. 12, 1963 J. N. MILLER 3,110,653
PROCESS AND APPARATUS F OR DISTILLING AND VISBREAKING REDUCED CRUDE Filed Dec. 50, 1959 2 Sheets-Sheet 1 ATMOSPHERIC I I TOWER 25 Cruds Oil Virgin Vacuum 38 29b Reduced Crude Visbroken Vacuum Reduced Crude Oi I TAR SEPARATOR 357 Visbroken Reduced Crude Containing Gas Oil INVENTOR. F," I James M. Miller his AT T 'ORN E Y Quench J. N. MILLER PROCESS AND APPARATUS FOR DISTILLING AND VISBREAKING REDUCED CRUDE Nov. 12, 1963 2 Sheets-Sheet 2 Filed Dec. 30, 1959 nlHluu III.
INVENTOR. James N. Miller his ATTORNEY United States Patent 3,119,663 PRGCESS AND APPARATUS FGR DISTILLING AND VESEBREAKENG REDUCED CRUDE James N. Miller, Canonshurg, Pa, assignor to Gulf Oil Corporation, Pittsburgh, 1 21., a corporation of Pennsylvania Filed Dec. 3%, 1959, Ser. No. 862,876 2 Claims. (ill. 2ll893) This invention relates to the preparation of gas oil charge stock for catalytic cracking and more particularly to such a process employing vacuum distillation and visbreaking and to novel apparatus employed in the process.
In conventional processing of crude petroleum oil to recover fractions suitable as charge stock for catalytic cracking, the crude is first distilled at substantially atmospheric pressure. Gas and gasoline are recovered as overhead products, naphtha and perhaps a light gas oil are taken oii as side streams and the residual material is recovered from the bottom of the tower as atmospheric reduced crude. The residual fraction from the atmospheric tower is then passed to a vacuum distillation tower. The products of vacuum distillation include gas oil and a heavy residual fraction or vacuum reduced crude. The gas oil fraction employed as catalytic cracking charge stock, which can be a mixture of fractions obtained by atmospheric and vacuum distillation, is a liquid distillate that boils in the range of about 500 to 1,000 F.
To obtain additional catalytic cracking charge stock it is also conventional to subject petroleum fractions heavier than gas oil, including residual fractions from atmospheric and vacuum distillation, to a thermal cracking procedure known as viscosity breaking, or more commonly, as visbreaking. This is essentially a single-pass, mild thermal cracking operation in which the heavy oil is passed at rather short residence time through a coil heated to a temperature in the range of about 850 to 950 F. The product is then separated to recover a gas oil cracking stock.
The present invention provides an improved process and apparatus for recovery of catalytic cracking charge stock from petroleum crude oil by vacuum distillation and visbreaking of heavy fractions thereof. The process of the invention in general comprises subjecting a residual fraction obtained by atmospheric distillation of crude oil to distillation at subatrnospheric pressure in a first vacuum distillation zone which is positioned above a second vacuum distillation zone. The first zone is sealed against liquid communication with the second zone but has vapor communication from the second zone. From the first vacu um distillation zone a catalytic cracking charge stock distillate fraction and a vacuum reduced crude residual fraction are recovered, the distillate fraction including distillate from the second zone and the residual fraction being free of unvaporized products from the second zone. The vacuum reduced crude from the first zone is subjected to visbreaking. A residual fraction of the visbreaking prod-' not which comprises gas oil and visbroken reduced crude is subjected to distillation at subatmosp'heric pressure in the second vacuum distillation zone. All of the vapor derived from the visbroken charge to said second zone is passed upwardly into said first zone to be recovered with the distillate product of said first zone. All of the unvapo-rized portion of the charge to the second zone is withdrawn as the residual product of said second zone free of products from the first zone. In the preferred modifica tion of the process the distillation conditions in the second vacuum zone are controlled to limit the amount of Vapor passed from the second zone into the first zone and thus to provide a gas oil product from said first zone of suitable composition for catalytic cracking charge stock, including low content 'of metals.
The apparatus of the invention comprises a novel vacuelongated vertical shell having an upper flash section and an upper stripping section, a lower flash section and a lower stripping section, each of said stripping sections being provided with vertically spaced bubble trays and said upper stripping section being separated from said lower flash section by a sealing tray. Each flash section is provided with a feed line entering above the topmost tray of the corresponding stripping section. A vapor conduit extends upwardly from the lower flash section and extends into the upper flash section above the feed line therefor. Above the upper flash section is the fractionating section of the vacuum tower which is provided with a series of vertically spaced bubble trays, at least one line for withdrawing distillate product and at least one reflux line. At the lower end of the upper stripping section is a line for withdrawing residual product and a line for introducing steam. Below the sealing tray and above the top most bubble tray of the lower stripping section is a feed line for the lower flash section and at its lower end the lower stripping section is provided with a line for withdrawing residual product and a line for introducing steam. In a preferred embodiment the vacuum tower is provided with separate feed heaters for the feed line to the upper section and the feed line to the lower section. The tower is employed in combination with a visbreaking heater, with fractionation means for separating the vis-' breaking product into at least one distillate fraction and a residual tfraction, with a line for delivering residual product from the upper flash section to the visbreaking heater, and with a line for delivering residual product from the visbreakingproduct separator to the feed heater [for the lower flash section of the vacuum tower.
The process and apparatus of the invention will be described in more detail by reference to the drawings of which:
FIGURE 1 is a flow diagram of an embodiment of the process of the invention and a diagrammatic representation of the apparatus;
FIGURE 2 is an elevation of a portion of the vacuum tower, partly in section and with partsbroken away;
FIGURE 3 is an elevation in section of a portion of the tower taken along line IIIIII of FIGURE 2;
FIGURE 4 is a .plan view in section taken along line IVIV of FIGURE 3; and
FIGURE 5 is a plan view in section taken along line VV of FIGURE 3.
FIGURE 1 illustrates apparatus comprising a vacuum tower 10 which is provided with an upper flash zone or section 11, an upper stripping section 12, a lower flash zone or section 13, a lower stripping section 14, and a blind or sealing tray 15 which separates the lower flash section 13 from the upper stripping sect-ion 12 and which permits the passage of vapor from the lower flash section to the upper flash section while preventing the passage of liquid between the two sections. The structure of the two separate flash sections of the vacuum tower will be described in more detail hereinafter with reference to FIGURES 2-5.
The structure of vacuum tower 10 above the upper flash section is conventional. Thus, section 16 of the tower is a conventional fractionation or rectification sec tion provided with a series of vapor-liquid contacting means or bubble trays 17 of conventional design and with an overhead line 18 leading to a single vacuum producing means 19 such as a conventional vacuum system compris ing a barometric condenser and steam jets. The tower is also provided with one or more lines for withdrawing distillate fractions such as lines 23 and 24 and one or more lines for returning top or side reflux streams to the tower,
such as lines 27 and 28.
The vacuum tower with its two separate flash sections and feed lines and its single vacuum system is preferably employed in combination with separate heating means for the feed lines to the two flash sections. Thus, a heater 29 can be provided for line 39 and a separate heater 43 for line 44. Instead of two heaters, a single furnace or heater, which has separate heating sections for each feed line, the temperature for each section being separately controllable, can be employed. The vacuum tower is also employed in combination with a visbreaking heater 32 to which the residual fraction from the upper stripping section 12 is delivered, with means such as the short fractionating column 33 for separating the visbreaking product into at least one distillate fraction and a residual fraction and with a line 35 for delivering the residual fraction of the visbreaking product to the feed heater 43 for the lower flash section 13 of column 10.
The fresh feed to the unit is aresidual fraction from atmospheric distillation of petroleum crude oil. This charge, which is also called atmospheric reduced crude, is introduced to the unit by line 38. The charge is heated in the charger heater 29 to a temperature, e.g., about 780 F., suitable for vacuum distillation at a vacuum tower pressure, for example, of 4 to 5 inches Hg. The heated feed is passed by line 39 into the upper flash section 11 of tower 16. As a result of the elevated temperature, the low pressure and the introduction of stripping steam by line 40, the reduced crude introduced by line 39 is at least partially vaporized and the conventional vapor-liquid contacting mechanism of rectificationstripping columns occurs, with vapor rising upwardly in the tower and liquid descending. Steam and uncondensed hydrocarbon vapors pass overhead by line 18 to the vacuum system 19. Suitable distillate fractions are withdrawn from the side of the tower. For example, a light gas oil fraction is withdrawn by line 23, a portion thereof being refluxed to the top of the tower by line 27 as required A heavy gas oil fraction is withdrawn by line 24, a portion thereof being returned to the column as side reflux by line 28 as required.
A vacuum reduced crude oil product from the upper stripping section 12 is withdrawn from tray by line 41. This fraction is charged to the visbreaking furnace 32. During passage through the coil'of visbreaker 32 the oil is heated to an outlet temperature, for example, of 920 F. and mild thermalcracking occurs, the extent of cracking depending upon the specific temperature range of the visbreaker and the residence time of the oil therein.
The visbroken product passes from the furnace 32 to a so-called tar separator 33. This is a short, atmospheric distillation column provided with a few bubble trays sufficient to achieve separation of the visbreaking product into an overhead and bottoms fraction. Preferably, a gas oil stream is introduced to the separator 33 by line 34 as a quench oil to cool the hot oil quickly and prevent coking in the separator.
A light distillate fraction composed of furnace oil and lighter material is recovered overhead by line 37 from separator 33 and a residual fraction comprising visbroken reduced crude that includes gas oil produced by visbreaking is withdrawn by line 35. The latter fraction forms the feed or charge stock for the lower flash section 13 of tower 10.
The visbreaking residium is heated by the charge heater 43 to a suitable temperature, e.g., 770 F., for vaporizing a desired portion of the gas oil content thereof at the pressure and steam ratio of the lower section of column 10. The lower stripping section 14 is provided with a few bubble trays or other vapor-liquid contacting means. The heated charge is introduced by line 44 to the lower flash section 13 above the topmost tray of lower stripping section 14. The lower flash section, which has vapor communication to the upper flash section 11, is at essentially the same subatmospheric pressure as upper sec tion 11. The pressure in the lower section may be somewhat higher than that of the upper section because of the pressure drop between the two sections. As a result of the low pressure, the temperature of the feed and the introduction of steam by line 45, gas oil and lighter hydrocarbons of the visbroken reduced crude are vaporized and passed upwardly through vapor conduits which pass through the sealing tray 15 into the upper flash section 11. Thus, the gas oil produced by visbreaking the vacuum reduced crude withdrawn from the upper stripping section 12 is passed into the upper flash section 11 and is recovered in the distillate fractions of lines 23 and 24. The visbrealrer gas oil is thus recovered with the hydrocarbons of similar boiling range obtained from the virgin atmospheric reduced crude which is charged to the upper flash section 11.
Steam lines 49 and 45 are each provided with valves and therefore can supply steam to stripping sections 12 and 14, respectively, at desired rates. Separate control of vaporization in the upper and lower sections is aided by separate control of the steam rate to each stripping section.
FIGURE 2 shows details of a preferred embodiment of the vacuum tower of the invention. The figure is a partly sectioned view in elevation with portions of the apparatus broken away and with certain conventional features omitted. In the embodiment of FIGURE 2 tower 10 has an upper flash section 11, an upper stripping section 12, a lower flash section 13' and a lower stripping section 14. The lower flash section and upper stripping section are separated by a blind tray or sealing tray 15. The upper and lower stripping sections are provided with a series of vertically spaced bubble trays. The uppermost tray of the upper stripping section 12 is tray 20. The upper stripping section extends from this tray downwardly to the sealing tray 15. The upper flash section 11 is the zone between the top stripping tray 25 and the lowermost tray 17 of fractionating section 16. The uppermost tray of the lower stripping section is tray 21. The lower stripping section extends from this tray downwardly to the bottom of the tower. The lower flash section is the zone between tray 21 and the sealing tray 15.
Liquid is introduced to the upper flash section and the top tray of the upper stripping section by the feed line 39. This top tray 24} of the stripping section also receives liquid from the downcomer 46 of the lowermost tray 17' of the fractionating section 16. Liquid is Withdrawn from the bottom of the upper stripping section, i.e., from the sealing tray 15, by the residual product drawoff line 4-1.
Two vapor conduits 47 rise from opposite sides of the sealing tray 15. Each vapor conduit 47 is formed by a vertical wall 43 in combination with a portion of the shell of tower it} to which the wall 4-8 is attached. The wall 48 and the associated portion of the tower shell form a vertical vapor passage or conduit 47 which extends through the sealing tray 15 and upwardly into the upper flash section :11.
The wall 48 and thus the'vapor conduit formed thereby, terminates in the upper flash section above the top tray of the upper stripping section and substantially above the level of feed line 39 and the outlet of downconier 46. All liquid is introduced to the upper flash section below the upper ends of the vapor conduits 47, thus preventing the introduction of liquid into the vapor conduits and into the lower flash section. As a further precaution suitable means, such as baffles 50 shown in FIGURE 1, can be placed above the vapor conduits. Such baffles will permit the flow of vapor from the conduits while reducing the possibility that liquid might splash into the vapor conduits.
The trays of the stripping sections can be of conventional design. For instance, they are provided with means such as bubble caps for achieving vapor-liquid contact and each tray is provided with means such as the weirs 51 and 51 to maintain a liquid level on the tray. Means delivering liquid from each tray to the next lower tray.
The liquid flow design for the upper stripping section in FIGURES 2-5 is the so-cailed split flow or twin flow design in which the liquid downflow changes alternately from the sides of the tray to the middle of the tray. However, any of the different types of tray layouts can be employed in the apparatus of the invention, it the struc ture is compatible with the provisiorrof a sealing tray to prevent liquid flow between the two sections of the tower and the provision of one or more vapor conduits to pass vapor from the lower flash section to the upper flash section. v
The described process and apparatus provide a number of advantages over conventional procedures and apparatus for producing catalytic cracking charge from crude oil by vacuum distillation and visbreaking. In accordance with the invention one vacuum tower does the work which conventionally would require two vacuum towers. Thus, the tower cost is reduced and only a single vacuum system is required.
Another advantage is that two distinct vacuum flash zones are provided, thus allowing separate flashing at two different sets of operating conditions for the virgin and visbroken feed stocks. In this way the carbon residue and the metals content of the gas oil cracking stocks recovered as products can be more carefully controlled than would be possible if both stocks were distilled in a single vacuum zone or were distilled in equipment that would permit admixture of the liquid stocks.
The visbroken reduced crude is derived from the heaviest portions of the crude oil and will normally have a higher concentration of metallic compounds than the virgin atmospheric reduced crude. In accordance with the invention a substantial proportion of the gas oil derivable from the vacuum reduced crude of line 31 by visbreaking can be recovered with the gas oil fraction of the virgin atmospheric reduced crude without causing the mixed gas oil product to have an excessive content of metals. The metals content of the gas oil fraction or fractions withdrawn from the vacuum tower is controlled by controlling the fractionation conditions in the lower flash section of the vacuum tower. Thus, the visbroken feed to the lower flash section is heated by the charge heater 43 to a temperature suitable to vaporize an amount of material that will not excessively contaminate with metals the distillate fractions of the upper section of the vacuum tower. Normally, the temperature for the lower flash section will be somewhat lower than that for the upper flash section. In any event, the operating conditions for the two flash zones can be changed separately and readily since each section is provided with a separate preheater (or with a heater having separately controllable sections for each feed line) and with separate lines for stripping steam.
The following example illustrates results obtainable in processing a West Texas crude of 33 API gravity in accordance with the invention. The yields are based on a charge of 100,000 barrels per day of crude to the atmospheric distillation column which prepares the feed stock for the upper flash section of the vacuum tower. In this example the visbreaking conditions include a visbreaker heater outlet temperature in the range of about 900 to 940 F. and an inlet pressure to the visbreaker heater in the range of about 250 to 300 pounds per square inch gauge. The temperatures of 780 F for the upper flash section of the vacuum tower and 770 F. for the lower flash section are selected to vaporize gas oil from 1 the lower section having about the same low content of metals (i.e., nickel and vanadium) as the gas oil vaporized in the upper section. Thus, the combined gas oil product recovered from the fractionating section of the.
vacuum tower has a satisfactorily low metals content for catalytic cracking charge. The quench oil introduced in 6 V the visbreaker product separator is not included in the product yields.
Example' Operating conditions: i v
Upper flash section of vacuum tower- Temperature 780 F. Pressure 4.5 in. Hg. lower flash section of vacuum tower- 'Ifcmperature 770 F.
Pressure 5 in. Hg. Yields:
Barrels Vol. per- Per Day cent of Crude 33 APT West Texas Crude to Atmospheric Distillation 100, 000 100.0 Atmospheric Reduced Crudc to Upper Flash Section of Vacuum Tower 34, 700 34. 7 Vacuum Reduced Crude to Visbreaker. 9, 990 9. 9 Visbroken Reduced Crude (Containing Gas Oil) to Lower Flash Section of Vacuum Tower 7, 300 7. 3 Visbroken Vacuum Reduced Crude (Residue from Lower Section of Vacuum Tower) 4, 700 4. 7 Virgin Gas Oil from Vacuun Tower (Contains approximately 0.2 ppm. Ni and 0.3 ppm. 24,800 24.8 Visbrokeu Gas Oil from Vacuum Tower (Contains approximately 0.25 ppm. Ni and 0.25 p.p.m. V) 2, 500 2.6
Visbroken Low Boiling Material from Vacuum Tower 0.1
Obviously many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof and therefore only, such limitations should be imposed as are indicated in the appended claims.
1. The process which. comprises heating atmospheric reduced crude to a temperature suitable for vacuum distillation thereof, subjecting the heated atmospheric reduced crude to vacuum distillation in a first vacuum distillation zone in a vertical vacuum distillation tower, recovering a vacuum reduced crude residual fraction from said first zone, subjecting said latter fraction to visbreaking, recovering a visbreaking product consisting of gas oil and heavier hydrocarbons, heating said visbreaking product to a temperature appropriate for vacuum distillation thereof, subjecting the heated visbreaking product to vacuum distillation in a second vacuum distillation zone having vapor communication to but sealed from liquid communication with said first zone said first vacuum distillation zone positioned above said second vacuum distillation zone, the temperature to which said vis- I breaking product is heated prior to introduction into said second zone being at least 10 F. lower than the temperature to which said atmospheric reduced crude is heated and being controlled to limit the amount of vapor passed from said second zone into said first zone so as to provide a gas oil product from said first zone of suitable composition including a low content of metals for a catalytic cracking charge stock, maintaining the liquids in said distillation zones entirely separate, passing the vapor derived from the visbreaking product in said second zone into said first zone, withdrawing the unvapo-rized portion of the charge to the second zone as the residual product thereof free of products from said first zone, and recovering a gas oil fraction from said first zone comprising the gas oil vaporized by vacuum distillation of the atmospheric reduced crude and the gas oil vaporized by vacuum distillation of the visbreaking product.
separate charge heating means for each of said flash sections, a v-isbreaking heater, means for separating visbreaking product, a feed line for introducing hydrocarbon liquid from one of said charge heating means to the upper flash section of said vacuum tower, a conduit extending from the lower flash section of said Vacuum tower into the upper flash section of said tower and above the feed line for said upper flash section, said conduit providing vapor communication from said lower flash section to said upper flash section, an overhead line from said vacuum tower, a single vacuum producing means associated with said overhead line for drawing a vacuum in the two flash sections of said tower, at least one side product withdrawal line for said tower above the upper flash section thereof, a line for withdrawing liquid residuum from the sealing tray at the bottom of the upper stripping section of sm'd tower and for delivering such residuum to said visbreaking heater, a line for delivering visbroken product to said visbreaking product separating means, a line for delivering a residual fraction from said visbreaking product separating means to the other of said charge heating means, a feed line for delivering heated charge from the latter charge heating means to said lower flash section, separate lines for delivering steam to the lower portion of each of said stripping sections, a line for withdrawing residual product from the bottom of said lower stripping section and a line associated with said latter line for recycling a portion of said residual product to said visbreaking heater.
References Cited in the file of this patent UNITED STATES PATENTS 1,145,728 Wiegand July 6, 1915 1,597,674 Doherty Aug. 31, 1926 1,964,686 Gard et a1 June '26, 1934 2,040,431 Dean et a1 May 12, 1936 2,220,020 Mofsinger Oct. 29, 1940 2,651,601 Taff et a1. Sept. 8, 1953 2,655,462 Morfit Oct. 13, 1953 2,658,863 Guala Nov. 10, 1953 2,768,940 Rupp Oct. 30, 1956 2,805,981 Cavin et a1 Sept. 10, 1957 2,865,835 Owen Dec. 23, 1958 FOREIGN PATENTS 91,142 Austria Mar. 15,1922