GB2151151A - Heavy oil distillation system - Google Patents

Abstract

A continuous crude petroleum distillation system wherein heated petroleum feedstock is introduced to a main distillation tower at a lower portion thereof and a vapor stream containing entrained particles passes upwardly into a phase separation zone 4 wherein the entrained particles are recovered from the vapor stream in a contaminated liquid overflash stream, is characterised by an improved residuum flashdown system comprising a multi-stage auxiliary tower 10; means 7 for feeding hot residual petroleum distillation bottoms from the main distillation tower to the auxiliary tower between upper and lower stages thereof; means 5 for feeding hot liquid overflash from the main distillation tower to the auxiliary tower at an upper stage thereof; means 15 for recovering a substantially uncontaminated gas oil fraction from the auxiliary tower; and means 11, 24 for stripping residual petroleum in a lower stage of the auxiliary tower, thereby increasing total distillate product yield of the distillation system. <IMAGE>

Description

SPECIFICATION Heavy Oil Distillation System This invention relates to an improved petroleum distillation system. In particular it relates to a vacuum residua flashdown technique for treating heavy oil fractions to increase recovery of distillate.

Conventional processing of crude petroleum oil to recover fractions suitable for upgrading in various refinery processing operations employs multi-stage distillation towers. Crude oil is first distilled or -fractionated in an atmospheric distillation tower, with residual material from the bottom of the distillation tower being further separated in a vacuum distillation tower. In this combination operation, gas and gasoline are recovered as overhead products of the atmospheric distillation tower, heavy naphtha, kerosene and gas oils are taken off as distillate side streams and the residual material is recovered from the bottom of the tower as reduced crude. Steam may be introduced to the bottom of the tower and various side strippers used to remove light material from withdrawn heavier liquid products.The residual bottoms fraction is usually charged to a vacuum distillation tower. The products of vacuum distillation include vacuum gas oils and a heavy residual material known as vacuum reduced crude. Usually, the vacuum charge is heated by furnace means in order to varporize a portion of the charge to produce the vacuum gas oil products. The preheated charge normally enters a lower portion of the vacuum tower and the vapors therefrom rise through the tower where they are cooled in selected stages producing successively lighter liquids which are separately withdrawn as sidestream products. Excess liquid runback known as overflash material may be combined with the liquid portion of the charge to form the vacuum reduced crude. A typical prior vacuum distillation system is disclosed in U.S. Patent 2,713,023 (Irvine).

As a result of introducing hot oil into the lower vacuum tower stages, which may be operated at a pressure of about 510 kPa (3575 mmHg) under vacuum, lower boiling components are vaporized in a flashing zone and pass upwardly to a vapor-liquid contact stage. It is common practice to first separate entrained crude particles from the upward vapor stream in a chimney tray separator or similar apparatus.

Vapor deentrainment apparatus is shown in U.S. Patent 3,501,400 (Brody) for overflash recovery. The overflashed crude particles are recovered as a contaminated liquid stream together with at least a portion of heavy distillate oil from higher contact stages. The presence of metallic impurities, asphaltenes and the like, may render this overflash stream, sometimes called a slop-cut stream, unsuitable for further catalytic processing. The heavy oil overflash stream is usually combined with tower bottoms to form the net vacuum reduced crude. Typical prior art distillation techniques are also disclosed in U.S. Patents 3,886,062, 4,239,618 and 4,261,814, and in "Crude Distillation", Petro/Chem Engineer, May, 1969, pp.9-11, incorporated herein by reference.

It is an object of the present invention to improve conventional vacuum distillation techniques for fractionating crude petroleum to obtain increased yield of useful and valuable distillate range fractions. It is another object to obtain heavy oil petroleum fractions substantially free of overflash contaminants.

A multi-stage fractionation technique has been developed for separating heavy petroleum feedstock containing non-volatile metallic and/or asphaltene contaminants to obtain increased distillate yield. The system includes a main vacuum distillation tower for fractionating the atmospheric tower bottoms; a multi-zone auxiliary distillation tower; means for recovering substantial amounts of distillate range hydrocarbons from the vacuum tower residuum and overflash streams; means for introducing the overflash stream at elevated temperature to the auxiliary distillation tower between an upper fractionation zone and intermediate fractionation zone; means for introducing the vacuum residuum at elevated temperature to the auxiliary tower between the intermediate zone and a lower stripping zone; means for contacting stripped distillate vapor from the lower zone with the overflash stream in the intermediate zone, thereby vaporizing a portion of the overflash stream; means for fractionating the distillate containing vapor from the lower and intermediate zones in the upper zone; means for withdrawing uncontaminated distillate liquid product from the upper zone; and means for recovering stripped residuum with contaminants from the lower zone of the auxiliary tower. Preferably the system has flashing means for maintaining the auxiliary tower at reduced pressure below the main vacuum distillation tower using a residuum flashdown technique. By flashing the residuum and overflash stream under low vacuum, increased vaporization of volatile distillate components is obtained.

The Figure is a process flowsheet showing a heavy oil vacuum distillation system with a main tower operatively connected to an auxiliary tower to effect the residuum flashdown technique.

Referring to the drawing, atmospheric residuum or similar heavy oil is preheated in furnace 1 and fed into a lower fractionation zone of main vacuum distillation tower, where the crude feedstock flashes and separates in zone 3 into a downward liquid stream and upward vapor stream. The vapor is further fractionated in the upper tower in a conventional distillation step to produce vacuum gas oils. Immediately above the feedstock flashing zone is a chimney type tray 4 wherein entrained overflash particles are separated and withdrawn along with the liquid descending from overhead in the main vacuum system (not shown). This overflash stream "slop-cut" is then withdrawn from main column 2 through outlet conduit means 5.The unvaponzed feedstock liquid is passed downwardly to a stream stripping fractionation zone 6 and the vacuum residuum is withdrawn through bottom outlet means 7.

Auxiliary tower 10 is operatively connected to receive the main column overflash stream from conduit 5 and pressure reduction valve 5A, which provides a means for flashing hot volatile components. The vacuum tower residuum is passed through conduit 7A, and if further heating is required, the feed to auxiliary column 10 may be passed through conduit 7B and furnace 8 to feed inlet 9. The auxiliary tower has a lower fractionation zone 11 disposed below the auxiliary tower feed inlet 9 and above the stripping steam inlet 12. An upper condensing zone 15, cooled by a pumparound stream 16 is located above the overflash stream inlet 17. An intermediate fractionation zone 20 is disposed between the residuum inlet 9 and overflash inlet 17.The slop-cut contaminants carried by the overflash stream pass downwardly through the auxiliary column and may be withdrawn from the bottom of auxiliary tower 10 via conduit 21 and combined with the feed prior to heating. A portion of the liquid from the bottom of lower zone 11 may be withdrawn through conduit 24 and cooled in heat exchanger 26 before being recycled to the bottom of auxiliary tower 10.

The clean distillate product is recovered as a liquid from the upper condensing zone 15. Reflux to the top zone 15 is provided by recirculating and cooling a portion 16 of this clean distillate. Lighter hydrocarbons and water are withdrawn through top vapor outlet 28 by motive steam and an appropriate eductor system 30.

While this embodiment has been described and shown as a typical vacuum main and auxiliary tower, it is understood that the operating pressure of each unit may be varied widely within the inventive concept. It is advantageous to maintain pressure in the auxiliarytowerabout4~6 kPa lower than the main vacuum tower flashing section, especially where the pressure drop contributes to vaporizing the residuum.

The inventive concept involved herein relates to the use of a contaminated distillate stream as reflux for the stripped residuum vapors, and can be called a residuum flash down (RFD) technique. The intermediate fractionation zone of the auxiliary tower assures economic recovery of clean distillate from otherwise lower value bottoms.

In the following example vacuum distillation system is employed to fractionate Arab light crude.

Utilizing the continuous system of a refinery scale unit (100,000 B/D), the main vacuum distillation tower is operated to produce 1810 barrels per day (B/D) of overflash/slop-cut and 12100 BID of vacuum resid. Stream conditions for the auxiliary tower are shown in Table I.

TABLE I Temperature Flow Rate Molecular Stream C/( F) (B/D) Wt. (avg.) Overflash* 397/(746) 1810 609 inlet Residuum 4151(780) 12100 765 inlet Steam inlet 3800 pounds/hr.

HGO Product 298/(568) 3860 603 HGO 106/(223) 4600 603 Pumparound Overhead 121/(250) Intermediate 393/(740) 650 691 Draw Off Residuum 371/(700) 10045 810 product *Contains~3~8 ppm V; 4-10 ppm Ni; 2-4 wt.% Conradson carbon residue.

In the above example, the main vacuum tower is operated under conditions sufficient to maintain absolute pressure in the flashing zone of about 3540 mmHg (or about 4~6 kPa), and the auxiliary tower vacuum system maintains a pressure of about 1-3 mmHg (or about 0.1-.5 kPa).

The distillate products recovered from the upper fractionation zone may be used as heating oil, diesel fuel, feedstock for a fluid catalytic cracking (FCC) unit, catalytic dewaxing, etc. Deleterious metals, especially V and Ni, and asphaltenes (as measured by carbon residue) are substantially removed, thus increasing the value and quality of distillate products from the overall distillation system.

In the above example, the HGO distillate recovered from the overflash and main tower residuum streams substantially increases the total HGO recovered from the system. By employing an overflash stream, which may contain a major amount of distillate-range hydrocarbons, to provide reflux to the auxiliary tower intermediate zone, a clean distilled product is obtained from the residuum and contaminated slop-cut streams. This could otherwise be achieved only by refluxing a portion of the main column HGO orthe like, resulting in a significant reduction in overall distillate yield.

Claims (8)

1. A multi-stage fractionation system for separating heavy petroleum feedstock containing non-volatile metallic and/or asphaltene contaminants to obtain increased distillate yield comprising: a main vacuum distillation tower for fractionating the feedstock; means for recovering a residuum bottoms stream and overflash stream containing substantial amounts of distillate range hydrocarbons; a multi-zone auxiliary distillation tower; means for introducing the overflash stream at elevated temperature to the auxiliary distillation tower between an upper fractionation zone and intermediate fractionation zone; flashing means for maintaining the auxiliary tower at reduced pressure below the main distillation tower whereby volatile components of the residuum stream are flashed to increase vaporization of volatile distillate components;; means for introducing the residuum stream from the main tower at elevated temperature to the auxiliary tower between the intermediate zone and a lower stripping zone; means for contacting stripped distillate vapor from the lower zone with the overflash stream in the intermediate zone, thereby vaporizing a portion of the overflash stream; means for condensing the distillate containing vapor from the lower and intermediate zones in the upper zone; means for withdrawing uncontaminated distillate liquid product from the upper zone; and means for recovering stripped residuum with contaminants from the lower zone of the auxiliary tower.

2. The system of claim 1 wherein said upper zone comprises a packed bed fractionation section and a pumparound section having means for withdrawing a fraction of heavy gas oil distillate and returning cooled distillate above the packed section for reflux.

3. The system of claim 1 comprising means for maintaining the main tower at about 5 to 10 kPa pressure, and the auxiliary tower at about 0.1 to .5 kPa.

4. The system of claim 1 comprising means for withdrawing contaminated liquid from the intermediate zone and reintroducing heated withdrawn liquid between the lower zone and intermediate zone.

5. In a continuous crude petroleum distillation system wherein heated petroleum feedstock is introduced to a main distillation tower at a lower portion thereof and a vapor stream containing entrained particles passes upwardly into a phase separation zone wherein the entrained particles are recovered from the vapor stream in a contaminated liquid overflash stream, the improvement which comprises: a multi-stage auxiliary tower; means for feeding hot residual petroleum distillation bottoms from the main distillation tower to the auxiliary tower between upper and lower stages thereof; means for feeding hot liquid overflash at reduced pressure from the main distillation tower to the auxiliary tower at an upper stage thereof; means for recovering a substantially uncontaminated gas oil fraction from the auxiliary tower; and means for stripping residual petroleum in a lower stage of the auxiliary tower, thereby increasing total distillate product yield of the distillation system.

6. The system of claim 5 wherein the main distillation tower is a vacuum tower.

7. The system of claim 6 wherein the auxiliary tower comprises means for maintaining pressure about 4~6 kPa below the main tower flashing zone and further comprises an upper packed bed fractionation stage with pumparound cooling means for a portion of gas oil liquid; and means for recovering a liquid stream from an intermediate fractionation stage.

8. The system of claim 6 further comprising means for recycling at least a portion of the liquid stream recovered from the intermediate stage for combining with the residual bottoms and means for heating the combined residual bottoms and intermediate stage liquid stream prior to feeding the combined stream to the auxiliary tower.