CONTACTOR AND SEPARATION APPARATUS AND PROCESS OF USING SAME

Abstract

In an improved fiber-film type contactor/separator an enhanced coalescing zone is provided by the presence of a disengagement device connected to a shroud that contains a bundle of high surface area vertical hanging fibers, where the enhanced coalescing zone diverts a portion of an admixture of immiscible liquids to flow radially in a path not parallel to the vertical axis of the hanging fibers whereby the diverted portion of liquids contacts a coalescing surface to cause at least one of the liquids to coalesce into droplets. The immiscible liquids are allowed to settle into separate phase layers and first and second outlets selectively remove the higher density liquid from the lower density liquid.

Claims

1. A method of retrofitting an apparatus designed for separating an admixture of at least two immiscible liquids into separate phases, where the apparatus before retrofitting comprises a plurality of hanging fibers defining a vertical axis that are contained in a shroud that is configured to only allow the admixture of liquids to flow downward along the hanging fibers parallel to the vertical axis, where the retrofitting comprises: providing a vessel containing the shroud having an existing length and that encloses the vertical hanging fibers; opening the vessel to provide access to the shroud; and adding a disengagement device to the shroud to form an enhanced coalescing zone that allows a portion of the admixture of liquids to exit the disengagement device through one or more openings following a flow path that is not parallel to the vertical axis.

2. The process of claim 1 further comprising the step of extending the length of the shroud prior to or during the addition of the disengagement device.

3. The process of claim 2 wherein the shroud is lengthened an amount so that the free ends of the hanging fibers are contained within the shroud.

4. A method of retrofitting an existing separator designed originally for gravity separation of an admixture of at least two immiscible liquids into separate phases comprising: providing an existing gravity separator; and adding to the gravity separator a vertically extending shroud containing a plurality of hanging fibers, where a disengagement device is added to the shroud to form an enhanced coalescing zone that allows a portion of the admixture of liquids to exit the disengagement device through one or more openings following a flow path that is not parallel to the vertical axis.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0031] FIG. 1 schematically illustrates a process flow diagram for one prior art process and apparatus for performing an oxidation process followed by the separation of two immiscible liquids;

[0032] FIG. 2 schematically illustrates a process flow diagram for one possible embodiment of the invention that includes an oxidation process followed by the separation of two immiscible liquids; and

[0033] FIG. 3 schematically illustrates a cross-section of a portion of the coalescing zone that uses an annulus containing a coalescing surface.

DETAILED DESCRIPTION

[0034] A comparison of the instant invention with an apparatus and process known in the art is helpful in understanding the improvements in our invention. To that end, FIG. 1 illustrates a prior art process and apparatus for use in a process that uses catalytic oxidation of reactants that results in the formation of an admixture comprising at least two immiscible liquids. The reactants are fed to the process and introduced to vertical vessel 1 through inlet process line 2 where the reactants contact the uppermost portion of a plurality or bundle of hanging fibers 5 that define a vertical axis 7. Notably, vertical vessel 1 does not have a large horizontal section located below the hanging fibers. Preferably the reactants pass through a liquid distributor (not shown) that is positioned at the top of the bundle of fibers. The hanging fibers 5 are contained within a shroud 6 that forces the admixture of liquids to flow parallel to the vertical axis and to contact the hanging fibers. The oxidation reaction primarily takes place within the upper most portion of the shroud 6 defined as the upper contact zone. Further reaction and contact continues in the lower contact zone as the admixture of immiscible liquids continue to flow down the shroud in contact with the vertical hanging fibers.

[0035] As the admixture of liquids moves downward inside the shroud the oxidation reaction completes and the fibers begin to facilitate the separation of the liquids into distinct phases. This occurs primarily in the lower contact zone. However, as mentioned, in those processes where the interfacial tension of the liquids is low (i.e., below about 10 dynes/cm), there is a tendency for dispersion to occur that leads to a poorly defined phase interface between the liquids. This is illustrated in FIG. 1 as the piling of the dispersion 10. As the dispersion exits the open end 30 of shroud 6, it begins to accumulate in the interior space/volume 41 of vessel 1 and in particular can grow or enlarge to occupy the annular space between the outside wall of the shroud 6 and the inside wall of the vertical vessel 1. This interior space of the vessel is known as the gravity settling zone. The denser liquid attempts to settle into layer 8 and the less dense liquid flows upward into layer 9. The interface 11 between the two layers is not well defined and typically leads to poor level control. The liquid in layer 9 flows upwards and is removed via a first outlet or process line 3 and the denser liquid in layer 8 flows downwards and is removed via a second outlet or process line 4. If the dispersion is allowed to continue piling it will eventually be carried over into line 3 and will contaminate the less dense liquid. Alternately, if the interface level is controlled too low there is a high probability that there will not be enough dense liquid in the lower layer 8 and this will cause cavitation of pump 31. Additionally, as the interface level drops, the chance of the lighter liquid contaminating the heavy liquid removed via process line 4 is greatly increased. In some instances, an optional coalescing device 40 can be added to the interior of the vessel below the first outlet 3 in an attempt to prevent carryover of the more dense liquid.

[0036] Our invention directly addresses the situation where the admixture of liquids in the process is characterized as having low interfacial tension (IFT). Specifically, the invention eliminates the piling up or excessive accumulation of the dispersion that results in an ever-enlarging dispersion band and ultimately, carryover of the heavier liquid. FIG. 2 illustrates one possible embodiment of our invention where a disengagement device 13 is added to the bottom of the lower contact zone of shroud 6. Like reference numbers have the same meaning as previously described. The disengagement device 13 defines an enhanced coalescing zone (ECZ) and has an open end 32. In the particular embodiment shown in FIG. 2, the disengagement device 13 comprises an annulus 42, which becomes an extension of shroud 6 and provides side openings to allow radial flow of the admixture of liquids to exit the disengagement device in flow path that is not parallel to axis 7. A cross-section of this annulus 42 is illustrated in FIG. 3 showing the bundle of hanging fibers 5 being contained within the inner wall 22 that forms an interior volume 23.

[0037] Both the inner wall 22 and outer wall 21 of the annulus 42 contain one or more openings 14. Inside the annulus is located the coalescing surface 20. The preferred coalescing surface has a volumetric void fraction in the range of from about 90% to about 99%, more preferably from about 95% to 98%. A preferred coalescing material is a co-woven type material comprised of 316 stainless steel and polytetrafluoroethylene (Teflon) fiber filaments, with very fine fiber size and having an installed density of about 15 to about 30 lb/ft.sup.3. The amount of coalescing material added to annulus, or in the cases of a wrapped configuration, is sufficient such that there is enough residence or hold up time of the liquids in the material to cause the liquid to coalesce. The plurality of openings 14 allow a portion of the admixture of liquids to flow through the annulus in a radial flow path that is non-parallel to the vertical axis 7. Preferably, the one or more openings 14 represent at least a 50% open area in the walls of the annulus. These openings can be slots, holes, punctures, or perforations of any shape or dimension.

[0038] As the admixture passes through the annulus 13 it contacts the coalescing surface 20 whereby any dispersion in the admixture is collapsed to form droplets 15 (see FIG. 2). These droplets 15 continue to grow until they either fall through the coalescing material or re-enter the fiber bundle or exit through the openings 14 in the outer wall 21. In other words, the formation and growing of the droplets 15 is the coalescing of one of the liquids in the admixture, typically the more dense liquid. When the droplets are of the more dense liquid, they grow and fall, dropping downward to become part of the liquid in layer 8. Because little or none of the dispersion survives the coalescing surface 20, a distinct phase interface 12 is formed between the higher density liquid in layer 8 and the lower density layer 9. This eliminates carry over of the higher density liquid in overhead line 3 and allows for precise control of interface level 12, which avoids pump cavitation and the contamination of the lighter liquid in process line 4. An optional secondary coalescing device 40 can be added to the interior of the vessel below the first outlet 3 to provide an additional safeguard against carryover of the more dense liquid.

[0039] As mentioned, the improved contactor/separator of our invention can be used in treating a liquid hydrocarbon stream containing mercaptans by an oxidation process where the hydrocarbons are contacted with an oxygen containing gas and mixed with an aqueous treatment solution in a contactor to convert the mercaptans to disulfide oils, which remain in the hydrocarbon. An upgraded hydrocarbon stream (containing the disulfide oils) is separated from the aqueous treatment solution and removed from the process. In another embodiment, as disclosed more fully below, the process includes at least two stages of contacting, oxidation and separation.

[0040] Any hydrocarbon can be treated which contains acidic species such as mercaptans. Representative hydrocarbons include straight run or cracked or selectively hydrotreated, one or more of natural gas condensates, liquid petroleum gas (LPG), butanes, butenes, gasoline streams, jet fuels, kerosenes, diesels, naphthas, crude oil and the like. An example hydrocarbon is a cracked naphtha, such as FCC naphtha or coker naphtha, boiling in the range of about 35 C. to about 230 C. Another hydrocarbon is kerosene/jet fuel, which has a typical boiling range of about 300 to about 550 F. Such hydrocarbon streams can typically contain one or more mercaptan compounds, such as methyl mercaptan, ethyl mercaptan, n-propyl mercaptan, isopropyl mercaptan, n-butyl mercaptan, thiophenol and higher molecular weight mercaptans. The mercaptan compound is frequently represented by the symbol RSH, where R is normal or branched alkyl, or aryl. The mercaptan sulfur is present in the hydrocarbons in an amount ranging from about 20 ppm to about 4000 ppm by weight, depending on the liquid hydrocarbon stream to be treated. The mercaptans range in molecular weight upwards from about C.sub.4 or C.sub.5, and may be present as straight chain, branched, or both. Specific types of mercaptans which may be converted to disulfide material by the oxidation process of this invention will include methyl mercaptan, ethyl mercaptan, propyl mercaptan, butyl mercaptan, pentyl mercaptan, hexyl mercaptan, heptyl mercaptan, octyl mercaptan, nonyl mercaptan, decyl mercaptan, undecyl mercaptan, dodecyl mercaptan, tridecyl mercaptan, tetradecyl mercaptan, pentadecyl mercaptan, hexadecyl mercaptan, heptadecyl mercaptan, octadecyl mercaptan, nonadecyl mercaptan, various mercaptobenzothiazoles, hydroxy mercaptans such as mercaptoethanol, cysteine, aromatic mercaptans such as thiophenol, methyl-substituted thiophenol isomers, ethyl-substituted thiophenol isomers, propyl-substituted thiophenol isomers, etc.

[0041] A hydrocarbon to be treated by the process of the instant invention may be a cracked naphtha or distillate which is hydrotreated to remove undesirable sulfur species and other heteroatoms. An undesirable side effect of hydroprocessing id that hydrogen sulfide formed during hydrotreating reacts with thermally-formed olefins to form mercaptans, which are referred to as reversion or recombinant mercaptans to distinguish them from the mercaptans present in the cracked naphtha or distillate conducted to the hydrotreater. Such reversion mercaptans generally have a molecular weight ranging from about 90 to about 160 g/mole, and generally exceed the molecular weight of the mercaptans formed during heavy oil, gas oil, and residue cracking or coking, as these typically range in molecular weight from 48 to about 76 g/mole. The higher molecular weight of the reversion mercaptans and the branched nature of their hydrocarbon component make them more difficult to remove from the naphtha using conventional caustic extraction.

[0042] The oxidation process preferably uses an aqueous treatment solution containing at least one alcohol and an alkali metal hydroxide can treat a hydrotreated naphtha boiling in the range of about 55 C. to about 180 C. and containing reversion mercaptan sulfur in an amount ranging from about 10 to about 100 wppm, based on the weight of the hydrotreated naphtha. Likewise, our process can treat a selectively hydrotreated hydrocarbon, i.e., one that is more than 80 wt. % (more preferably 90 wt. % and still more preferably 95 wt. %) desulfurized compared to the hydrotreater feed but with more than 30% (more preferably 50% and still more preferably 60%) of the olefins retained based on the amount of olefin in the hydrotreater feed.

[0043] Although some variations of the process can use a two-phase treatment solution in the absence of oxygen, we prefer a process that uses an aqueous treatment solution in conjunction with an added oxygen-containing gas that causes the mercaptans in the hydrocarbon feed to oxidize to disulfide oils, which remain in the hydrocarbon phase. The treatment solution can be prepared by adding metal phthalocyanine catalyst to an aqueous solution of alkali metal hydroxide and at least one alcohol. Another preferred treatment solution further contains at least one carboxylic acid, such as naphthenic or ethylhexanoic acid.

[0044] The apparatus of our invention as shown in FIG. 2 can be used for performing the process of converting sulfur compounds found in a hydrocarbon stream. Preferably, a two-stage process is used where two of the vessels illustrated in FIG. 2 are connected in series. The bundle of vertical hanging fibers in vessel 1 provide a large surface area to allow the reactants to contact and to eventually separate the resultant immiscible liquids formed. The fibers typically comprise a mass of hanging thin filaments or ribbons of metal or other materials of construction allows mass transfer and separation in a non-dispersive manner. While contacting/separation temperature and pressure may range from about 0 C. to about 150 C. and from 0 psig to about 500 psig, preferably the contacting occurs at a temperature in the range of about 25 C. to about 100 C. and a pressure in the range of about 0 psig to about 300 psig. When the hydrocarbon feed has a low atmospheric boiling point, higher pressures during contacting may be desirable to ensure that the contacting with the hydrocarbon occurs in the liquid phase.

[0045] During the contacting step the mercaptans are oxidized by the treatment solution, catalyst and oxygen to disulfide oils that ultimately remain in the hydrocarbon phase. Once the admixture of liquids is separated the lighter phase is again contacted with oxygen and treatment solution in a second vessel of basically the same design as the first. Make-up fresh treatment solution and/or catalyst can be added to the process as need to achieve the desired level of oxidation.

[0046] The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various application such specific embodiments without departing from the generic concept, and therefore such adaptations and modifications are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation.

[0047] The means, materials, and steps for carrying out various disclosed functions may take a variety of alternative forms without departing from the invention. Thus, the expressions means to . . . and means for . . . , or any method step language as may be found in the specification above or the claims below, followed by a functional statement, are intended to define and cover whatever structural, physical, chemical or electrical element or structure, or whatever method step, which may now or in the future exist which carries out the recited function, whether or not precisely equivalent to the embodiment or embodiments disclosed in the specification above, i.e., other means or steps for carrying out the same function can be used; and it is intended that such expressions be given their broadest interpretation within the terms of the following claims.