Patent Application
20150034529
The technical field generally relates hydrocarbon processing apparatuses and processes for removing mercaptans from hydrocarbon feed streams, and more particularly relates to processes and apparatuses for removing polymeric precursors from mercaptan solvent streams.
The extraction of mercaptans from hydrocarbon streams is widely practiced in the petroleum refining industry. A basic mercaptan extraction process passes a hydrocarbon feed stream through an extraction column countercurrent to a descending stream of lean aqueous alkaline solution normally referred to 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.
Good separation of the disulfide compounds from the caustic solution is required in order to minimize the content of disulfides in the caustic being recirculated to the extraction zone. 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 in the art to counteract this effect by removing disulfide compounds from the regenerated caustic. The regenerated caustic or regenerated aqueous alkaline solution may therefore be processed to remove disulfides. The caustic solution is then passed into the extraction zone.
When caustic extraction of mercaptans is applied to petrochemical feed stocks with higher concentrations of butadiene, polymeric material is found in the regeneration section and extraction sections of mercaptan removal units. This polymeric material causes decreased extraction efficiency, caustic carry over, increased reentry sulfur, and level control problems. Ultimately, this causes problems with downstream units by producing off-specification feed than can spend guard beds and neutralize expensive acidic catalysts by titrating their acid sites with caustic.
Accordingly, it is desirable to provide improved processes and apparatuses for removing mercaptans from hydrocarbon streams. Further, it is desirable to provide improved processes and apparatuses for removing polymeric precursors from mercaptan solvent streams. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.
Processes and apparatuses for removing mercaptans from feed streams are provided. In accordance with an exemplary embodiment, a process for removing mercaptans from a feed stream includes extracting mercaptans and a polymeric precursor from the feed stream with a caustic solvent to form a polymeric precursor rich spent caustic solvent. The polymeric precursor rich spent caustic solvent is contacted with a polymeric precursor-deficient hydrocarbon stream in a multistage vessel and a polymeric precursor is extracted from the polymeric precursor rich spent caustic solvent to form a treated spent caustic solvent. The process contacts the treated spent caustic solvent with oxygen and oxidizes the mercaptans to disulfides to form an oxidized caustic solvent stream. The process includes separating the oxidized caustic solvent stream into an offgas stream, a disulfide stream, and a caustic solvent stream.
In accordance with another exemplary embodiment, a process for removing mercaptans from a feed stream is provided. The process includes extracting mercaptans and butadiene from the feed stream with a solvent stream to form a product stream and a spent solvent stream. The method further includes contacting the spent solvent stream with a hydrocarbon stream in a multistage countercurrent liquid-liquid contacting vessel and extracting butadiene from the spent solvent stream to form a treated spent solvent stream. The method further includes converting the mercaptans to disulfides and separating the treated spent solvent stream into a disulfide stream and a clean solvent stream. Also, the method introduces the clean solvent stream and a fresh hydrocarbon stream to a contacting zone to remove remaining disulfide and to form the hydrocarbon stream and the solvent stream.
In accordance with another exemplary embodiment, an apparatus for removing mercaptans from a feed stream is provided. The apparatus includes a mercaptan extractor configured to remove mercaptans from the feed stream with a caustic solvent to form a product stream and a butadiene rich spent caustic solvent. The apparatus further includes a multistage countercurrent liquid-liquid contacting vessel configured to contact the butadiene rich spent caustic solvent and a butadiene deficient hydrocarbon stream to extract the butadiene from the butadiene rich spent caustic solvent to form a treated spent caustic solvent. The apparatus also includes an oxidizer configured to receive the treated spent caustic solvent and to oxidize the mercaptans to disulfides to form an oxidized caustic solvent stream. The apparatus includes a disulfide separator configured to separate the oxidized caustic solvent stream into an offgas stream, a disulfide stream, and a caustic solvent stream.
Embodiments of processes and apparatuses for removing mercaptans from hydrocarbon streams and for removing polymeric precursors from mercaptan solvent streams will hereinafter be described in conjunction with the following drawing FIGURE wherein:
The FIGURE is a schematic illustrating an apparatus for removing mercaptans from a feed stream and for removing a polymeric precursor from a mercaptan solvent stream in accordance with an embodiment herein.
The following detailed description is merely exemplary in nature and is not intended to limit the processes and apparatuses for removing mercaptans from feed streams. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background or brief summary, or in the following detailed description.
Most distillate hydrocarbon streams produced from crude oil contain some amount of sulfur in one form or another unless these streams have been subjected to extensive sulfur removal procedures such as hydrotreating. Often a major amount of this sulfur is present in the form of mercaptans. Mercaptan sulfur content must be reduced in the hydrocarbon distillate stream in order to meet certain product specifications such as a limitation on the total sulfur content of a product. It may also be desirable to remove mercaptan compounds from a hydrocarbon stream for the purpose of eliminating the rather malodorous mercaptan compounds and thereby improve or reduce the odor associated with the hydrocarbon stream. A third reason for removing mercaptan compounds from a hydrocarbon stream would be to eliminate the passage of sulfur-containing compounds into a catalyst bed which is sensitive to the presence of sulfur. It may therefore be necessary to remove mercaptans from a hydrocarbon distillate stream for the purpose of preserving the activity of a catalyst employed in a downstream conversion unit.
Mercaptans may be removed from hydrocarbon streams through the use of an extraction process in which the hydrocarbon stream is brought into contact with an aqueous alkaline solution, i.e., a mercaptan solvent. It is contemplated herein that the regeneration of a mercaptan solvent stream can be improved by removing polymeric precursors, such as butadiene, from the mercaptan-rich solvent stream in a multistage vessel having a plurality of liquid-liquid contacting stages. The entrance of butadiene into the regeneration oxidation zone will result in a significant amount of polymerization occurring within the oxidation zone. This polymerization is undesirable as it may form polymers or polymeric deposits which can clog the equipment employed in the process and in other ways interfere with or degrade the performance of the overall treating process.
The processing described herein provides for improved removal of butadiene, for example to levels of less than about 20 parts per million (ppm). While the description may at times refer only to butadiene, it is understood that any polymeric precursors may be removed as described in relation to butadiene. Under the processing described herein, the formation of emulsions in the mercaptan-rich solvent stream is also inhibited.
In accordance with the various embodiments herein, the FIGURE illustrates an apparatus 10 for processing a feed stream 12, such as a C4 rich hydrocarbon stream including propylene and contaminated with mercaptans, to produce a product stream 14 that is substantially free of mercaptans. As shown, the apparatus 10 includes an extraction unit 15 that receives the feed stream 12 and an aqueous alkaline solution or caustic solvent 18. During contact between the feed stream 12 and the caustic solvent 18, mercaptans in the feed stream 12 are preferentially dissolved in the caustic solvent 18 and are thereby extracted from the feed stream 12. As a result of this extraction, the product stream 14 is formed with little or no mercaptans and a mercaptan-rich spent caustic solvent stream 20 is formed. The spent caustic stream 20 includes butadiene, because butadiene and other similar unsaturated hydrocarbons and acetylene hydrocarbons are more soluble in the caustic solvent 18 than are the saturated hydrocarbons remaining in the product stream 14. Thus, such hydrocarbons are extracting from the feed stream 12 into the spent caustic stream 20. An exemplary spent caustic solvent stream 20 has a butadiene content of about 0.5 wt % to about 3 wt %.
In order to remove polymeric precursors from the spent caustic solvent stream 20, the exemplary process feeds the spent caustic solvent stream 20 to a countercurrent liquid-liquid contacting vessel 25 having a plurality of contacting stages or packing. As shown, the countercurrent liquid-liquid contacting vessel 25 also receives a hydrocarbon stream 26 that is deficient in polymeric precursors, such as butadiene. For example, the hydrocarbon stream 26 may include no butadiene. In other embodiments, the hydrocarbon stream 26 includes between about zero and about 100 ppm butadiene, or other polymeric precursors.
In the exemplary embodiment, the spent caustic solvent stream 20 is delivered to the countercurrent liquid-liquid contacting vessel 25 at a temperature of about 32° C. to about 49° C., such as at about 42° C. Further, the hydrocarbon stream 26 is delivered to the countercurrent liquid-liquid contacting vessel 25 with a boiling point of less than about 350° C. The spent caustic solvent stream 20 and the hydrocarbon stream 26 are contacted in the countercurrent liquid-liquid contacting vessel 25. Through the establishment of equilibrium between the spent caustic solvent stream 20 and the hydrocarbon stream 26 at the plurality of stages in the countercurrent liquid-liquid contacting vessel 25, butadiene is extracted from the spent caustic solvent stream 20 and is absorbed by the hydrocarbon stream 26 to form a butadiene rich hydrocarbon stream 28. Further, the extraction process forms a treated spent solvent stream 30 with a butadiene content of less than about 20 ppm.
The treated spent solvent stream 30 retains its mercaptan content and is delivered to a mercaptan removal or regeneration zone that processes and regenerates a caustic solvent 18. Specifically, the treated spent solvent stream 30 is mixed with air 32 and is delivered to an oxidation reactor 35. In the oxidation reactor 35, a catalytic oxidation reaction converts the mercaptan compounds present in the treated spent solvent stream 30 into hydrocarbon soluble disulfide compounds. The effluent stream 36 of the oxidation reactor 35, which comprises an admixture of any residual oxygen or other vapors, the caustic solvent and disulfide compounds, is passed into a three-phase or disulfide separator 45. At separator 45, off gases 46 such as residual oxygen and nitrogen from the added air 32 are vented off. Disulfides are relatively insoluble in the caustic solvent and may therefore be separated by decantation and withdrawn from the process as disulfide stream 48. A remaining clean caustic solvent stream 50 is formed with a reduced mercaptan content and is fed to a contacting zone 55 such as a disulfide scrubber.
As shown, a fresh hydrocarbon stream 56 is also delivered to the contacting zone 55. The fresh hydrocarbon stream 56 may be formed by a processing unit 65. In an embodiment, the processing unit 65 is a hydrotreating unit that hydrotreats a feed 66 such as a naphtha feed and forms the fresh hydrocarbon stream 56 with from about zero to about 100 ppm butadiene. In another embodiment, the processing unit 65 is a butadiene extraction unit that forms a butadiene rich extract stream 68 and forms a butadiene-free raffinate stream that serves as the fresh hydrocarbon stream 56.
The clean solvent stream 50 and fresh hydrocarbon stream 56 are contacted in the contacting zone 55 to remove any remaining disulfides therein and to form the caustic solvent 18 and the clean hydrocarbon stream 26 that are delivered to the extraction unit 15 and the countercurrent liquid-liquid contacting vessel 25, respectively. As shown, the butadiene rich hydrocarbon stream 28 and the disulfide stream 48 are combined to form stream 70 that is fed to a filter 75, such as a disulfide sand filter. The disulfide sand filter 70 is used to remove any entrained caustic and water from the disulfide. As shown, a wash oil stream 76 of disulfide, wash oil, and polymeric precursor such as butadiene is formed and exits the disulfide sand filter 75. Further, a stream 78 of water and caustic are also formed and exit the disulfide sand filter 75. Stream 70 is filtered to ensure that caustic is not sent to catalytic processing apparatuses, like a hydrotreater, where the wash oil stream 76 can be reclaimed as useful product.
In view of the apparatus and process of the FIGURE, mercaptans are removed from hydrocarbon feed streams with a caustic solvent. The spent caustic solvent is thereafter processed in a multistage liquid-liquid contacting apparatus to remove butadienes and/or other polymer precursors from the spent caustic solvent before the spent caustic solvent is regenerated by oxidizing the mercaptans to form disulfides and those disulfides are separated from a regenerated caustic solvent. As a result, the regeneration and separation apparatuses are not fouled by polymers. Further, separation efficiency in the regeneration zone is not hindered by the formation of emulsions by the butadiene. Specifically, the multistage liquid-liquid contacting apparatus does not require shear to separate the butadienes from the spent caustic solvent that can cause emulsification.
While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the claimed subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the processes without departing from the scope defined by the claims, which includes known equivalents and foreseeable equivalents at the time of filing this patent application.
