GB2040306A

GB2040306A - Removing aromatics from gas oils

Info

Publication number: GB2040306A
Application number: GB7850276A
Authority: GB
Original assignee: RTL Contactor Holding SA
Current assignee: RTL Contactor Holding SA
Priority date: 1978-12-29
Filing date: 1978-12-29
Publication date: 1980-08-28

Abstract

Aromatic constituents, and preferably also organic sulphur constituents, are extracted from gas-oil by contacting gas-oil with a substantially immiscible stream comprising a selective solvent for the aromatics and withdrawing a liquid stream comprising solvent and aromatics and a liquid stream comprising gas-oil of reduced aromatic content, which is then suitable as a feedstock for steam cracking to produce olefins.

Description

SPECIFICATION Process for the extraction of aromatic constituents from hydrocarbons This invention relates to the extraction of aromatic constituents from hydrocarbons, and is particularly concerned with the separation of aromatic fractions from gas-oil which may be used as the feedstock in steam cracking processes for olefin production.

Various hydrocarbon fractions, normally derived from the refining of petroleum oil, are used as feedstocks for steam cracking processes for the production of olefin petro-chemicals such as ethylene, propylene and butadiene. Typical feedstocks which have been used for steam cracking are naphtha, ethane and other hydrocarbon fractions derived from oil refining operations. A hydrocarbon fraction such as gas-oil (i.e. hydrocarbon fractions boiling in the range 1900C to 530"C aromatic content 12 to 25%, sulphur content 0.5 to 2.5%) has not been considered suitable for steam cracking operations because of the presence of aromatic fractions in the gas-oil and also the high sulphur content usually present in gas-oils.

The presence of aromatic fractions in gas-oils when used for steam cracking causes fouling of the plant and the formation of coke in the cracking furnace tubes and other downstream units of the plant. This lowers efficiency and results in the need for more frequent periodic shut down for cleaning and thus reduces the production of olefins from such a plant. The presence of excess sulphur content in the gas-oil also leads to corrosion of the cracking plant and the necessity for including efficient and expensive sulphur removal facilities. Further, lower quality olefins are produced which must be further purified before subsequent use. Yet again, the by product fuel oil obtained from the cracking will have an unacceptably high sulphur content.

We have found a suitable method for removing the aromatic fraction from gas-oil so that gas-oil can be used as an excellent feedstock for a steam cracking plant for the production of olefins.

According to this invention, we provide a process for the extraction of aromatic constituents from gas-oil, which comprises introducing to a liquidliquid contactor a stream of gas-oil and contacting the stream with a substantially immiscible stream comprising a selective solvent for the aromatic constituents of the gas-oil, and preferably also the organic sulphur constituents thereof, and withdrawing from the contactor a liquid stream comprising solvent and aromatic constituents extracted from the gas-oil, and a liquid stream comprising the gas-oil freed from at least a portion of its aromatic constituents and preferably also from the organic sulphur constituents. Preferably, after leaving the contactor, the gas-oil is freed from residual solvent contained therein: such recovered solvent can be recycled.

Preferably too, the stream comprising solvent and aromatic constituents is treated to recover the solvent which can also be recycled.

The invention also includes a steam cracking process for the production of olefins using a feedstock comprising gas-oil, the gas-oil having been at least partially freed of aromatic constituents as described above.

The contactor may suitably be of the type described in Patents Nos. 972035 and 1037573. Such a contactor is constituted by a vessel through which two substantially immiscible liquid phases of different specific gravities are caused to flow with an interface between the two phases and rotating conveying means within the vessel causing droplets of each phase to be conveyed into the other phase.

Normally the vessel is stationary and the rotating means is a rotor rotating relative to the vessel; but, instead the vessel and rotating means may turn together. A contactor of this type is described in British Patent Specification No. 972035, in which the vessel contains a rotor comprising a series of partitions which are spaced apart along the vessel to divide the interior into a plurality of compartments, and buckets, which are normally carried between the partitions and which, in each compartment, carry liquid of each phase into the other.The passage of the phases along the vessel is made possible by spacing the edges of each partition from the wall of the vessel and/or by having openings through the partitions at selected points.lt is preferred to have the two phases moving through the vessel in counter-current flow, although co-current flow is possible.

As each bucket passes from one phase to the other, it carries with it a quantitiy of the first phase, which becomes progressively displaced by the second phase. The first phase is thus released as droplets and enter the second phase and because of the difference in specific gravities, migrates back to the first phase in the form of droplets. In other words, as the buckets on one side of the rotor travel in an upward direction, they carry the heavier phase into the lighter one and discharge their contents in a shower of droplets which fall back into the heavier phase. Similarly as the buckets descend on the other side, they carry the lighter phase downwards into the heavier phase and the lighter phase discharged from the buckets floats upwards in a series of droplets, back into the lighter phase.

Suitable solvents for the aromatic constituents of the gas-oil are furfural, dimethyl formamide, certain alcohols, dimethyl sulphoxide, and Nalkylpyrrolidone. The ratio of solvent to gas-oil will depend on the amount of aromatics to be removed from the gas-oil, and on the solubility and viscosity characteristics of the two fluids.

The accompanying flow diagram illustrates the process in accordance with the invention. Reference numeral 1 denotes a liquid-liquid contactor of the type described in Patents Nos. 972035 and 1037573.

Gas-oil is introduced to the contactor on line 2. The gas-oil contains an aromatic fraction and organic sulpher compounds. The gas-oil stream may be preheated to a temperature between 20 and 90"C if desired. The solvent for the aromatic fraction is introduced to the contactor on line 3. A suitable solvent is furfural. The furfural and gas-oil form two separate phases within the contactor with the furfural below the gas-oil. However the action of the contactor as described above ensures sufficient and efficient contact for the extraction to take place.

Furfural containing aromatic constituents withdrawn from the gas-oil is continuously removed on line 4, and the product treated in unit 5 to recover the solvent, which is recycled on line 6 to line 3. The aromatic constituents are withdrawn on line 7 and may be used, for example, to reduce the viscosity of heavy fuel oils. The furfural may be recovered for recycling from the aromatic fraction by fractional distillation. Alternatively, a second stage selective solvent extraction may be employed, for example using a contactorsimilarto contactor 1. Furfural extracted into a second solvent may then be recovered by distillation.

In raffinate, comprising gas-oil from which at least some of the aromatic constituents and also some of the organic sulphur compounds have been withdrawn, but also containing some residual solvent, is withdrawn on line 8 and passed to a treatment unit 9 where furfural is removed from the gas-oil and recycled on line 10 to line 3. The furfural may be recovered in unit 9 by fractional distillation. Alternatively, a second stage selective solvent extraction may be performed, for example employing a contactor similar to contactor 1. Suitable extractants for the furfural are hexane and water. The furfural can then be recovered from the hexane or water by distillation.

Claims

1. A process for the extraction of aromatic consti tuentsfrom gas-oil, comprising introducing to a liquid-liquid contactor a stream of gas-oil and contacting the stream with a substantially immiscible stream comprising a selective solvent for the aromatic constituents of the gas-oil and withdrawing from the contactor a liquid stream comprising solvent and aromatic constituents extracted from the gas-oil, and a liquid stream comprising the gas-oil freed from at least a portion of its aromatic constituents.

2. A process according to claim 1, wherein the solvent is selective also for the organic sulphur constituents of the gas-oil and the liquid stream comprising the gas-oil freed from at least a portion of its aromatic constituents is also freed from at least a portion of the organic sulphur constituents.

3. A process according to claim 1 or 2, wherein after leaving the contactor, the liquid stream containing the gas-oil is freed from residual solvent contained therein.

4. A process according to any one of claims 1,2 or 3, wherein after leaving the contactor, the liquid stream comprising solvent and aromatic constituents is treated to recover the solvent.

5. A process according to claim 3 or 4, wherein recovered solvent is recycled.

6. A process according to any one of the preceding claims, wherein the solvent is selected from furfural, dimethyl, formamide, alcohols, dimethyl, sulphoxideand N-alkylpyrrolidones.

7. A process according to claim 6, wherein the solvent is furfural.

8. A process according to any one of the preceding claims, wherein the liquid-liquid contactor comprises a vessel through which the substantially immiscible streams flow in the form of substantially immiscible liquid phases of different specific gravities with an interface between the phases, and rotating conveying means are provided within the vessel so as to cause droplets of each phase to be conveyed into the other phase.

9. Asteam cracking processforthe production of olefins using a feedstock gas-oil which has been at least partially freed of aromatic constituents by the process of any one of the preceding claims.

10. A processforthe extraction of aromatic constituents from gas-oil, substantially as described herein with reference to the accompanying drawing.

11. Asteam cracking processforthe production of olefins according to claim 10, substantially as described herein.