FIELD OF THE INVENTION
The present invention relates to an apparatus for efficient removal of disulphide oil produced during catalytic regeneration of caustic. The present invention further relates to a process for the efficient removal of disulphide oil from caustic using said apparatus. The invention further relates to a process for the treatment of hydrocarbon including Liquefied Petroleum Gas for the extraction of sulfur impurities.
BACKGROUND OF INVENTION
The extraction of sulfur compounds from hydrocarbon streams (and LPG) is widely practiced in the petroleum refining industry and is probably performed in most of the world's major petroleum refineries. It is described in basic reference sources such as Volume 15 of the second edition of the Kirk-Othmer Encyclopedia of Chemical Technology. This reference shows the basic mercaptan extraction process in which a hydrocarbon feed stream is passed through an extraction column countercurrent to a descending stream of lean aqueous alkaline solution normally referred to in the art as caustic. The treated product is removed from the top of the extraction column.
A mercaptan-containing caustic solution referred to as a "rich" caustic solution is removed from the bottom of the extraction column and passed into an oxidation zone in admixture with air. An oxidation catalyst dissolved in the caustic solution promotes the oxidation of the extracted mercaptans to disulfide compounds within the oxidation zone. The effluent stream of the oxidation zone is passed into a phase separation vessel from which the disulfide compounds are decanted. This procedure serves to remove the mercaptan compounds from the rich caustic stream and is therefore referred to as "regeneration" of the caustic. The resultant "lean" caustic is removed from the separation vessel and recycled to the extraction column.

(Formula Removed)

The disulfide oils are soluble in hydrocarbon streams. Therefore, disulfide compounds present in the regenerated caustic being fed to the top of extraction column will become dissolved in the hydrocarbon stream which is being treated. This will raise the sulfur content of


the treated hydrocarbon stream and may be totally unacceptable. It is known to one skilled in the art that a good separation of the disulfide compounds from the caustic solution is required in order to minimize the content of disulfides in the caustic being re-circulated to the extraction zone. It is known in the art to counteract this effect by removing disulfide compounds from the regenerated caustic.
Prior art conventional process of removal of disulphide compounds is shown in Figure 1. In the conventional process caustic is regenerated in a bubble column or a packed bed reactor using air in the presence of catalyst. Regenerated caustic containing fine droplets of disulphide compounds is either physically separated using a porous media or in a contactor containing fibre bundles with hydrocarbon solvent for e.g. naphtha or kerosene. DSO droplets are solubilized in the solvent as DSO is completely miscible in solvent. While physical separation using a porous medium is not very efficient for large commercial scale operation, use of hydrocarbon solvent in a fibre contactor is expensive as large volume of solvent is to be used. Fibre based contactors are also quite expensive.
US Patent 2853432 discloses the regeneration of used alkaline reagents by oxidizing same using a phthalocyanine catalyst. For example, mercaptides contained in a caustic solution were oxidized to disulfides, which were then withdrawn from the regeneration zone by skimming or by dissolving in a suitable solvent such as naphtha. The regenerated caustic or regenerated aqueous alkaline solution may therefore be processed as in US Patent 2921020 which describes the use of a disulfide removal zone 26. In this zone, the regenerated caustic is contacted with a hydrocarbon distillate such as pentane or hexane. The caustic solution is then passed into the extraction zone.
US Patents 2853432 and 3408287 also describe the regeneration of alkaline solution through the oxidation of dissolved mercaptans to disulfide compounds are presented. These patents references illustrate the common practices of admixing air with a mercaptan-containing extract stream to support the catalyzed oxidation of the mercaptans, the subsequent passage of the oxidation zone effluent stream into a phase separation zone from which excess air is vented and a regenerated alkaline solution is removed, and the usage of the preferred oxidation catalyst. The references also describe the overall flow of the alkaline stream including the


removal of this stream from the phase separation zone and its recirculation to the extraction zone.
US Patent 3574093 relates to a multi-step process wherein the spent caustic generated by treating a low-boiling hydrocarbon stream for mercaptan removal is thereafter used in a second treating step wherein a higher boiling sour distillate is sweetened. In the sweetening step, the mercaptans in the sour distillate are oxidized to disulfides. The disulfides exit the treating stage in the hydrocarbon stream along with those mercaptides which had been previously extracted from the low boiling hydrocarbon stream. Thus, the higher boiling stream is sweetened and the partially spent alkaline stream is regenerated at the same time.
US Patents 3758404, 3977829 and 3992156 are directed to methods and apparatus for liquid-liquid mass transfer between immiscible liquids. US Patents 4040947 and 4104155 illustrate the methods of handling a side stream of alkaline solution which is processed in a contacting zone for the purpose of water removal. US Patent 4362614 relates to a multi-step process for the extraction of mercaptans from hydrocarbon streams with an alkaline solution, followed by the regeneration of the mercaptide-containing alkaline solution resulting from such extraction by oxidation in the presence of a catalyst in an oxidation zone, followed by the separation of the disulfides and the alkaline solution by decantation within a phase separation zone. From this process, the alkaline solution is recycled.
US Patent 4562300 discloses contacting a hydrocarbon stream including organic
mercaptans with sodium hydroxide to free the hydrocarbons from the organic mercaptans. The
caustic solution rich in mercaptans is oxidized with a catalyst and the organic mercaptans are
converted to organic disulfides. US Patent 5244643 discloses a process whereby a
hydrocarbonaceous gas stream including mercaptan sulfur, air and aqueous alkaline solution
including a mercaptan oxidation catalyst are mixed in a mixing vessel in which mercaptans are
converted to disulfides. US Patent 7326333 Apparatus and process for extracting sulfur
compounds from a hydrocarbon stream. A prewash section for converting hydrogen sulfide to
sodium sulfide by reaction with an alkali such as caustic communicates with an extractor
section disposed directly above the prewash section for converting mercaptans to mercaptides
by reaction with alkali. Hydrocarbon product exits the extractor section through a coalescer that
prevents alkali from exiting with the hydrocarbon product stream.

OBJECTS OF THE INVENTION
An important object of the present invention is to provide an apparatus for efficient removal of disulphide compounds produced during catalytic regeneration of caustic, the apparatus comprising of bubble column reactor Bl, column reactor B2, eductor B3, reactor B4, pumps B5, B6, B7 and B8 and associated control system.
Another object of the present invention is to provide a process for efficient removal of disulphide compounds produced during catalytic regeneration of caustic by said apparatus.
Yet object of the present invention is to provide a process for the treatment of hydrocarbon including Liquefied Petroleum Gas.
Still another object of the invention is to provide an apparatus and a process for efficient removal of disulphide compounds in an effective way at reduced cost.
SUMMARY OF THE INVENTION
The above and other objects of the present invention are achieved by an apparatus for the efficient removal of disulphide oil produced during the catalytic regeneration of caustic to be used in the extraction of sulfur impurities in the treatment of hydrocarbons comprising: a bubble column reactor Bl, a column reactor B2, an eductor B3, reactor B4, pumps B5, B6, B7 and B8 and associated control system.
Preferably said bubble column reactor Bl has a set of specially designed sieve trays.
Preferably said column reactor B2 contains a metal sheet structured packing.
Preferably said eductor B3 joins bubble column reactor Bl and column reactor B2.
Preferably said reactor B4 receives rich caustic and catalyst.
Preferably said pump B6 re-circulates fresh hydrocarbon in B2.
Preferably said pump B7 withdraws regenerated caustic from B2.
Preferably said pump B8 withdraws rich solvent and adds fresh solvent in B2.
In another embodiment of the invention, a process for efficient removal of disulphide oil produced during the catalytic regeneration of caustic by the apparatus as claimed in preceding claims comprising the steps of distributing rich caustic and hydrocarbon in column Bl; feeding

caustic and hydrocarbon into eductor B3; feeding mixed phase of caustic and hydrocarbon in B2; recirculating hydrocarbon in B2 through pump B6 and withdrawing rich solvent and adding fresh solvent through pump B8.
Preferably said rich caustic is distributed at the top and said hydrocarbon is distributed at the bottom of the reactor.
Preferably said hydrocarbon solubilizes the disulphide oil produced in the reaction and reduces the foaming produced by the impurities thereby increasing the capacity of the reactor.
Preferably said hydrocarbon is selected from the group consisting of naphtha, kerosene, etc.
Preferably said Bl has trays to reduce back mixing of liquid thereby increase effectiveness of the reactor.
Preferably said eductor provides intimate mixing of caustic containing droplets of disulphide oil and hydrocarbon resulting in the diffusion of disulphide oil from caustic phase to the hydrocarbon phase.
Preferably said eductor in operated with hydrocarbon as motive fluid.
Preferably said eductor maintains pressure balance between bubble column reactor Bl and column reactor B2.
Preferably said B2 has structured packing which generates a thin film of caustic facilitating transport of fine droplets of disulphide oil to the hydrocarbon phase from caustic phase.
In still embodiment of the invention, a hydrocarbon and caustic separator cum distributor provided at top of column B2 is provided to ensure uniform distribution of caustic and hydrocarbon over the structured packing.
In yet embodiment of the invention, a coalescer bed is provided at the bottom of the column B2 to remove hydrocarbon from caustic.
In still another embodiment of the invention, the hydrocarbon to be treated is selected from the group consisting of Liquefied Petroleum Gas, etc.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIGURE 1: Apparatus for the conventional process of removal of disulphide compounds.
FIGURE 2: The apparatus of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to an improved apparatus and process for removal of disulphide oil in the regeneration of caustic in the treatment of hydrocarbons.
Figure 1 shows the apparatus for the conventional process of removal of disulphide compounds. In the conventional process, rich caustic from reactor B4 is fed to a bubble column or a packed bed reactor Bl using air in the presence of catalyst. Caustic from Bl is sent to column B2 and then to B3. Fresh solvent is added through pump B7 and spent catalyst is withdrawn through pump B5. The regenerated caustic is withdrawn from pump B6.
Figure 2 illustrates the apparatus of the invention. In the present invention, disulphide oil is extracted in three stages to ensure maximum removal. Rich caustic from the hydrocarbon including LPG treating unit containing sulphide and mercaptides of sodium is distributed to the bubble column reactor Bl through reactor B4. A measured quantity of air is also sparged through a distributor. Desired quantity of soluble catalyst is added into the reactor. In the presence of dissolved oxygen, mercaptides and sulphides of sodium react. In the presence of impurities, foaming may start and this may drastically reduce capacity of the reactor. To reduce foaming tendency hydrocarbon e.g. naphtha or kerosene is distributed at the bottom of the reactor. Hydrocarbon also solubilizes disulphide oil produced in the reaction. To improve performance of the bubble column reactor Bl, a set of specially designed sieve trays are provided. These trays help in reducing back mixing of liquid and thereby increase effectiveness of the reactor.
Lean caustic along with hydrocarbon and disulphide oil leaves the oxidizer and flows to Eductor B3, for which motive fluid is re-circulated hydrocarbon e.g. naphtha or kerosene, circulated by pump B6. In the eductor, both lean caustic and large quantity of hydrocarbon interact and generate droplets of caustic in hydrocarbon. This helps in better contacting and

disulphide oil diffuses from caustic phase to the hydrocarbon phase. Bigger droplets get solubilized in the hydrocarbon.
The mixed phase from the eductor exit is fed to the column B2. This column contains sheet metal structured packing. Caustic wets the packing element and hydrocarbon flows around it. Due to spreading of caustic phase over large surface area of structured packing thin film of caustic is generated. This facilitates transport of fine droplets of disulphide oil to the hydrocarbon phase from caustic phase. Rich hydrocarbon is re-circulated through pump B6. A small quantity of rich solvent is withdrawn using pump B8 and equal amount of fresh solvent is added. Lean caustic from bottom of B2 is released to the LPG treating process.
ADVANTAGES OF THE PRESENT INVENTION
• Hydrocarbon is introduced into the bubble column reactor Bl to reduce foaming tendency thereby increase in capacity of the apparatus of the present invention.
• Sieve trays are provided in bubble column reactor Bl at selected location to reduce back mixing of liquid in the reactor.
• Eductor B3 is provided to ensure proper mixing of solvent hydrocarbon with lean caustic such that droplet distributed in the caustic phase is solubilized by hydrocarbon through effective contacting of hydrocarbon and caustic phase.
• Hydrocarbon & caustic separator cum distributor is provided at top of column B2 to ensure uniform distribution of caustic and hydrocarbon over the structured packing.
• A proprietary structured packing bed is provided in column reactor B2 for proper contacting of hydrocarbon with caustic solution.
• A coalescer bed is provided at the bottom of the column reactor B2 to prevent carry over of hydrocarbon phase along with caustic.
• The process of the present invention is used for the treatment of hydrocarbons including Liquefied Petroleum Gas.

We claim:
1. An apparatus for the efficient removal of disulphide oil produced during the catalytic regeneration of caustic to be used in the extraction of sulfur impurities in the treatment of hydrocarbons comprising: a bubble column reactor Bl, a column reactor B2, an eductor B3, reactor B4, pumps B5, B6, B7 and B8 and associated control system.
2. The apparatus as claimed in claim 1, wherein said bubble column reactor Bl has a set of specially designed sieve trays.
3. The apparatus as claimed in claim 1, wherein said column reactor B2 contains a metal sheet structured packing.
4. The apparatus as claimed in claim 1, wherein said eductor B3 joins bubble column reactor Bl and column reactor B2.
5. The apparatus as claimed in claim 1, said reactor B4 receives rich caustic and catalyst.
6. The apparatus as claimed in claim 1, said pump B5 recycles fresh solvent in bubble column reactor Bl and column reactor B2.
7. The apparatus as claimed in claim 1, said pump B6 re-circulates fresh hydrocarbon in B2.
8. The apparatus as claimed in claim 1, said pump B7 withdraws regenerated caustic from B2.
9. The apparatus as claimed in claim 1, said pump B8 withdraws rich solvent and adds fresh solvent in B2.
10. A process for efficient removal of disulphide oil produced during the catalytic regeneration of caustic by the apparatus as claimed in preceding claims comprising the steps of -
(a) distributing rich caustic and hydrocarbon in bubble column reactor Bl;
(b) feeding caustic and hydrocarbon from step (a) into eductor B3;
(c) feeding mixed phase of caustic and hydrocarbon of step (b) in B2;
(d) recirculating hydrocarbon in column reactor B2 through pump B6;
(e) withdrawing rich solvent and adding fresh solvent through pump B8.

11. The process as claimed in claim 10, wherein said rich caustic of step (a) is distributed at the top and said hydrocarbon is distributed at the bottom of the reactor.
12. The process as claimed in claim 11, wherein said hydrocarbon solubilizes the disulphide oil produced in the reaction and reduces the foaming produced by the impurities thereby increasing the capacity of the reactor.
13. The process as claimed in claim 12, wherein said hydrocarbon is selected from the group consisting of naphtha, kerosene, etc.
14. The process as claimed in claim 10, wherein said bubble column reactor Bl of step (a) has trays to reduce back mixing of liquid thereby increase effectiveness of the reactor.
15. The process as claimed in claim 10, wherein said eductor B3 of step (b) provides intimate mixing of caustic containing droplets of disulphide oil and hydrocarbon resulting in the diffusion of disulphide oil from caustic phase to the hydrocarbon phase.
16. The process as claimed in claim 15, wherein said eductor B3 is operated with hydrocarbon as motive fluid.
17. The process as claimed in claim 15 or 16, wherein said eductor B3 maintains pressure balance between bubble column reactor Bl and column reactor B2.
18. The process as claimed in claim 10, wherein said column reactor B2 of step (b) has structured packing which generates a thin film of caustic facilitating transport of fine droplets of disulphide oil to the hydrocarbon phase from caustic phase.
19. The process claimed in claim 10, wherein a hydrocarbon and caustic separator cum distributor provided at top of column reactor B2 ensures uniform distribution of caustic and hydrocarbon over the structured packing.
20. The process as claimed in claim 10, wherein coalescer bed provided at the bottom of the column reactor B2 remove hydrocarbon from caustic.
21. The process as claimed in claims 10 to 20, wherein the hydrocarbon to be treated is selected from the group consisting of Liquefied Petroleum Gas, etc.