March 7, 1967 J. F. G. ELLIS 3,308,060

Transcription

1 March 7, 1967 J. F. G. ELLIS PETROLEUM DISTILLATION Filed Jan. 28, 1965 NVENTOR. JOHN FRANCIS GRIFFITH ELLS BY MORGAN, FINNEGAN, DURHAM 8, PINE ATTORNEYs

2 United States Patent Office Patented Mar. 7, PETROLEUM DISTILLATION John Francis Griffith Ellis, Ashford, Middlesex, England, assignor to The British Petroleum Company Limited, 5 London, England Filed Jan. 28, 1965, Ser. No. 428,821. Claims priority, application Great Britain, Feb. 21, 1964, 7,320/64 1 Claim. (C ) 10 This invention relates to the distillation of petroleum and particularly to the maximum separation of gas oil fractions, i.e. fractions boiling within the range C. on a TBP curve, from crude oils. In a conventional crude oil distillation column operat- 5 ing at a pressure of approximately three atmospheres, four side streams are commonly withdrawn, viz. naphtha, kerosine, light gas oil and heavy gas oil. Atmospheric residue is withdrawn from the column base, and a totally condensed overhead distillate from the column head. 20 The maximum yield of straight-run gas oil of a given pour point obtainable from a conventional crude oil distill lation column is limited by the degree of fractionation at the gas oil back end cut point. Thus, improved fractiona tion would increase the gas oil yield The internal reflux in a conventional distillation colu has its maximum value at the top of the column and de creases to almost nothing immediately above the flash zone of the column. Both the light and heavy gas oil frac tions are drawn from a zone in the column in which the 30 reflux flow is insufficient to provide the maximum theoreti cally possible yield of gas oil of a given quality. Additional reflux flow above the flash zone can be pro vided by "overflash," i.e. flashing the crude oil to a higher cut point than that demanded by the product require- 35 ments. A flow of liquid is then provided over the trays 2 immediately above the flash zone. The amount of addi tional reflux which can be obtained in this way is, how ever, limited and insufficient to effect maximum gas oil separation. According to the present invention, in a crude oil dis tillation process conducted in a conventional atmospheric column, the heavy gas oil side stream fraction is combined with at least a portion of the atmospheric residue, the combined fraction being refractionated to give an addi tional gas oil product. The term heavy gas oil, in this specification, means the distillate fraction which boils above 300 C. Preferably the heavy gas oil is combined with the front 10-40% weight portion of the atmospheric residue, which is preferably obtained by vacuum flashing the atmospheric residue to give a 10-40% weight overhead portion. This portion is subsequently combined with the heavy at mospheric gas oil, and the mixture fractionated under vacuum in a reboiled column. This method has the ad vantage that close control can be maintained over the frac tionation conditions. Alternatively the whole of the atmospheric residue may be combined with the heavy gas oil before vacuum flash ing the mixture in a tower containing fractionating trays, under flash conditions arranged to give the required reflux in the fractionation section. This method is not so closely controllable however, but may be suitable where mini mum plant alteration is essential. The invention is illustrated by the following example: Example 1 A crude oil of Middle East origin, when fractionated in a conventional atmospheric distillation unit at the rate of 55,800 b.p.s.d., resulted in a material balance over the unit as indicated in Table 1. TABLE 1. Stream TBP range, Sp. gr. at Percent wit, Lb.fhr. o C 60 F on crude Overhead distillate----- to ,066 Naphthal , 42 23,746 Kerosine , ,962 Light gas oil , ,984 Heavy gas oil ,035 Atmospheric residue--- > , 549 Crude charge ,342 The product properties from this operation are given in the following Table 2: - TABLE 2 Test Crude Overhead Naphtha Kerosine Light gas Heavy gas Atmospheric feed distillate oil oil residue Sp. gr. at 60 F ASTM distillation, C.: IBP a i a a a Total distillate, p Residue

3 3 4. TABLE 2 -Continued Test Crude Overhead Naphtha Kerosine Light gas Beavy gas Atmospheric feed distillate oil oil residue H2S, percent wt Negative Mercaptain Sulphur, percent wt Total sulphur, percent wt Reid vapour pressure, p.s.i Flash point, F- - - Saybolt colour Aniline point, C Kin. Viso. at 70F., cs Kin. visc. at i00 F., cs Kin. visc. at 122 F., cs----- Dieselindex w as a - an a w - w n m kin. visc. at 210 F, cs.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i.i In order to determine the maximum gas oil that could The experiment demonstrates that the yield of gas oil be recovered from the crude oil charge the following ex- boiling above a crude oil TBP cut point of 223 may be periment was performed in the laboratory. increased from 22.47% wt. on crude to 26.72% wt. on Heavy atmospheric gas oil was combined with at- 20 crude; the pour point of the total gas oil remains con mospheric residue in the proportions which they were stant at 15 F., while the specific gravity (measured at drawn from the crude unit. The resulting composite was 60 F.) increases from to At the same put in a 14 plate batch distillation column operating at time the cloud point decreases from 22 F. to 18 F., 2:1 external reflux with a column head pressure of 50 mm. 25 while the diesel index decreases from 58 to 55. Thus an Hg absolute. The distillation was continued until a 20% improvement in both specific gravity and cloud point is wt. (on feed) distillate was obtained. Inspection data on obtained, and although the diesel index decreases slightly the feeds and products are given in Table 3. the decrease is acceptably small. TABLE 3 Test Atmospheric Atmospheric 20% wt. 80% wt. heavy gas oil residue distillate residue Sp. gr. at 60 F Colour (ASTM) C 1.0 Total sulphur, percent wt. 1,12 Kin. visc. at 22 F., cs Cloud point, F Pour point, F Aniline point, C Diesel index /49 Distillation (ASTM), C.: IBP % recovery % %- 294, % % % % % % ! % % FBP Total distillate, percent vol Residue Loss Nil Bad match. Relevant data on the effect of (i) incorporating the Example 2 whole of the distillate product into the gas oil pool (i.e. The invention has been further demonstrated on exist light gas oil plus vacuum distillate) (ii) incorporating the 55 ing refinery equipment. whole of the heavy atmospheric gas oil into the gas oil A crude oil of Middle East origin when fractionated in pool (i.e. light gas oil plus atmospheric heavy gas oil) are a conventional atmospheric distillation unit, consisting of summarised in Table 4. a primary distillation column and a secondary distillation TABLE 4 Test Atmospherie Atmospheric Vacuum gas Blend of Blend of light gas oil heavy gas oil oil HGO--LGO VGO--LGO Percent wt. On crude Sp. gr. at 60' E Colour (ASTM) L CO L L 1.0 Total sulphur, percent w , , 70 F. c.s F., cs Cloisi Pour point, F f2. d / / Bad match.

4 5 column, at a rate of 82,000 b.p.s.d., resulted in a material balance over the unit as indicated in Table 5. A further two experiments were carried out in which the heavy gas oil stream was shut in and the yield of atmospheric residue increased by the amount that the heavy gas oil was decreased. In both these experiments the crude oil was processed at 82,000 b.p.s.d. and the temperature and pressure at the inlet to the secondary column were maintained constant, and at the same values as were observed for the first experiment. During these latter two experiments a portion of the atmospheric residue was passed to an adjacent vacuum distillation unit operat ing at 21,550 b.s.p.d. and the material balances resulting from the operation of the atmospheric and vacuum distill lation units in the above described manner are indicated in Tables 6 and 7. The product properties resulting from these three ex periments are given in Tables 8, 9 and 10. Relevant data on the effect of (i) incorporating the whole of the heavy atmospheric gas oil into the gas oil pool (i.e. light gas oil plus heavy gas oil) (ii) incorporat ing the whole of the vacuum gas oil into the gas oil pool (i.e. light gas oil plus vacuum gas oil) are summarised in Table 11. The experiments demonstrate that improved fractiona tion enhances both gas oil yield and gas oil quality, viz: (a) Experiment 3 shows that for constant diesel index the yield of gas oil boiling above a crude oil TBP cut point of 235/236 C. is increased from 21.51% wt. on crude (experiment 1) to 23.09% wt. on crude, i.e. an increase of 7.35% wt. on product; at the same time the pour point is reduced from 25 F. to 20 F., whilst the cloud point also shows a significant reduction from 32 F. to 20 F. (b) Experiment 2 shows that for substantially constant diesel index and cloud point the yield of gas oil boiling above a crude oil TBP cut point of 235/236 C. is in creased from 21.51% wt. on crude (experiment 1) to 26.38% wt. on crude, i.e. an increase of 22.7% wt. on 20 product; although the pour point is adversely raised from 25 F. to 30 F. TABLE 5-MATERIAL BALANCE Stream SG 60/60 F. Percent wt. Lb./hr. IGPH Percent vol. TBP range on crude, C. Crude oil feed ,019, , Gas and PFD ,628 i 5, To ,623 18, ,907 6, ,715 14, Light gas oil , , Heavy gas oil ,03 71,686 8, Atmospheric residue , , >357.5 Total ,019, , Calculated by difference, TABLE 6.-MATERIAL BALANCE Stream SG 6060 F. Percent wt. Lb./hr. IGPE Percent vol. TBP range on crude, C. Crude oil feed ,022, , Gas and PFD n ,752 16, To 46 SRB ,859 17, Naphtha ,770 6, Kerosine , , Light gas oil ,420 19, Heavy gas oil Nill Nil Atmospheric residue , , Total ,022, , Atmospheric residue feed ,570 31, >328.5 Vacuum gas oil ,314 6, Waxy distillate , ,483 13, Wacuum residue ,773 11, >543 Total ,570 31, Calculated by difference. TABLE 7-MATERIAL BALANCE Stream SG 60/60 F. Percent wt. Lb./hr. IGP Percent vol. TBP range on crude, C. Crude oil feed ,024, , Gas and PFD , , To 42.5 SRB ,721 17, Naphtha ,295 6, Kerosine ,084 15, Light gas oil , , Heavy gas oil Ni Nill Atmospheric residue ,078 56, >320 Total ,024, , Atmospheric residue feed ,390 31, >320 Vacuum gas oil ,177 5, Waxy distillate ,907 14, Vacuum residue ,306 11, >543 Total ,390 31, Calculated by difference.

5 TABLE 8-INSPECTION DATA, ON PRODUCTS Sample Agha, Jari SRB Naphtha, Kerosine Light Heavy Atm. crude oil gas oil gas oil residue Specific gravity 60/ S , Specific gravity Distillation test, C.: Recovery, percent Residue, percent LOSS, percent Total sulphur, percent wt Aniline point, C Diesel index Cloud point, F Pour point (upper), F Carbon residue (Con), percent w 100 F., cs F., cs. 140 F., cs. F. F ESF: I Calculated from value at 140 F. TABLE 9.--INSPECTION DATA, ON PRODUCTS Sample Agha Jari SRB Naphtha Kerosine Light Atm. Vacuum Waxy Wacuum crude oil gas oil residue gas oil distillate residue Specific gravity ,883 0, Specific gravity Distillation test, C.: Recovery, percent Residue, percent LOSS, percent.-- Recovered at 40 y Total sulphur, percent wit Aniline point, C Diesel index.--- Cloud point, F--- Pour point (upper), Carbon Residue (Con), 100 E., cs. A. 122 F., cs F., cs / Calculated from value at 140 F.

6 9 10 TABLE 10.-INSPECTION DATA, ON PRODUCTS Sample Agha Jari SRB Naphtha Kerosine Light Atm. Wacuum Waxy Vacuum crude oil gas oil residue gas oil distillate residue Specific gravity 60/ , , ) Specific gravity Distillation IBP test, C % vol. recovered % , % % % % % % % % % FBP Recovery, percent Residue, percent-- LOSS, percent Total sulphur, percent wt Aniline point, C Diesel index Cloud point, F Pour point (upper), F Carbon residue (Con), percent wit F., cs F., cs. 140 F., cs F., es F., cs Wax content (methy percent wt Melting point of wax Calculated from value at 140 F. TABLE 1.-SUMMARIZING TABLE Blend Light gas oil Heavy gas oil Vacuum gas oil of So Blend of LGO al EIGO and WGO Experiment Yield on crude, percent wt Nil Front end TBP cut pt., C Back end TBP cut pt., C i o Specific gravity 60/ i Total sulphur, percent wt I Cloud point, F Pour point, F O 0 O Aniline point, C Dieselindex / / /55 57 Distillation test, C IBP o % vol. recovered % , % , , % , , % , % % , %- 282, 5, , , 5 70% , % , % FBP Recovery, percent Residue, percent Recovered at 400 C., percent The invention is further illustrated with reference to the distillation column 2 via steam stripper 11 and line the accompanying drawing, which is a schematic flow 12. The mixture is passed to reboiled distillation col diagram of a plant suitable for operating the invention, 60 umn 14, which operates at a moderate vacuum so as to the design of which is based on the information obtained reduce cracking to a minimum, via line 13, and a gas oil in the above Example 1. overhead fraction is recovered via line 15. The yield In the drawing, heated crude oil is fed via line 1 into of gas oil product expressed as a percentage of atmos conventional atmospheric distillation column 2. Atmos- pheric heavy gas oil together with atmospheric residue pheric residue from the base of the column is fed via line 65 is 20% wt., while on rerun column feed it is equivalent 3 to a vacuum column 4 operating at 96 mm. Hg. A to a 47.6% wt. distillate. Residue from this reboiled 33.9% weight (based on feed) flash distillate is re- column is combined via line 14 with that recovered from moved overhead from the vacuum column 4 via line 5 the column 4, so as to give a final residue of 80% weight to a collecting vessel 6, vacuum being drawn via line 7. calculated on atmospheric heavy gas oil and atmospheric Residue from the vacuum column 4, steam stripped at 70 residue feed to the recovery system. a rate of 0.25 lb./hr. steam per imperial gallon/hr. of The gas oil recovery unit will thus produce a good residue via line 8, is removed from the base of the col- quality gas oil with a pour point of 35 F. (as indicated umn via line 9 to storage. The flash distillate is passed in Table 3) at a yield on crude of 10.88% wt., so that via line 10 from the collecting vessel 6 and, after heat when blended with the light gas oil drawn from the exchange, is combined, with heavy gas oil drawn from 75 crude unit (pour point -5 F., yield on crude 15.84%

7 wt.) the resulting gas oil has a pour point of 15 F. at a yield on crude of 26.72% wt. Other standard items of equipment, e.g. pumps, heat exchangers and the like, are shown in the drawing. Equipment of the type described above could be added to an existing crude distillation unit to increase gas oil recovery. If units already exist for vacuum distill lation of only a part of the atmospheric residue, the above equipment could be integrated with the existing units. In a new refinery where vacuum distillation of atmospheric residue is required, say to produce feed stock for another process, then the above equipment could be integrated with the vacuum distillation unit with corresponding reduction in capital and operating costs. I claim: In a crude oil distillation process, combining the heavy gas oil side stream fraction obtained in an atmospheric O 15 2 pressure fractionation zone with the front 10-40% weight portion of the atmospheric residue obtained in that Zone, said portion being a product of the vacuum flashing of said residue, followed by redistilling of the combined fraction under vacuum in a reboiled fractionation zone, to give an additional gas oil product. References Cited by the Examiner UNITED STATES PATENTS 1,954,839 4/1934 Youker ,675 4/1935 Bahlke et al ,850,436 9/1958 Beuther et al ,234,118 2/1966 Chen DELBERT E. GANTZ, Primary Examiner. H. LEVINE, S. P. JONES, Assistant Examiners.