Patent Application
20140048446
This invention relates generally to fractionation columns, and more particularly, to apparatus and methods for removing H2S and moisture from the naphtha overhead of a fractionator.
Hydrocarbon feeds can be reacted in a hydroprocessing zone where a number of reactions take place, including hydrocracking, hydrotreating, hydrogenation, and desulfurization. The hydroprocessing zone is typically followed by a stripper column, where the hydroprocessing zone effluent is separated into a stripper overhead stream and a stripper bottoms stream. In some processes, the stripper column bottoms is sent to a fractionation column, where it is separated into a fractionation column bottoms stream and a naphtha overhead stream. Other streams, such as light gas oil and heavy gas oil streams, can also be separated out in the fractionator, if desired. The naphtha overhead stream is recovered. The naphtha overhead stream includes naphtha, H2S, and, in some cases, water.
The H2S generated during desulfurization reactions in the hydroprocessing zone is removed predominantly in the stripper column. Although the stripper column is designed to remove H2S to the level of parts per billion (ppb) in the stripper bottoms stream, small amounts of H2S slip through into the fractionator. The H2S becomes concentrated to a level of parts per million (ppm) in the fractionator overhead liquid stream. ASTM D-4952-09 (Doctor Test) is often used as an indicator for the presence of H2S in the overhead naphtha stream. An H2S level of 1 weight ppm (wppm) can result in the naphtha not meeting the Doctor Test. If the naphtha does not meet the Doctor Test, it cannot be sent directly to the naphtha pool for storage. Consequently, the H2S must be removed from the naphtha overhead stream using a secondary processing system.
In many units, the H2S is removed using a caustic (NaOH) wash and a sand filter. However, many refiners do not want to use caustic because of the hazards associated with handling it and problems related to disposing of the spent caustic.
Alternatively, the naphtha may be sent to a downstream stabilizer/splitter combination for removal of light petroleum gas. The H2S can be removed along with the light petroleum gas. However, this equipment increases the cost of the process.
Therefore, it would be desirable to provide alternative processes for removing H2S from naphtha.
One aspect of the present invention relates to a method of making naphtha substantially free of H2S. In one embodiment, the method includes stripping an incoming stream containing naphtha and H2S in a fractionator into at least an overhead stream containing the naphtha and H2S and a bottoms stream, and introducing the overhead stream from the fractionator into a separator to form a naphtha stream substantially free of H2S and an overhead stream containing H2S.
Another aspect of the invention is an apparatus for making naphtha. In one embodiment, the apparatus includes a hydroprocessing zone having an inlet and an outlet. The inlet of a stripper column is in fluid communication with the outlet of the hydroprocessing zone. The inlet of the stripping fractionator is in fluid communication with the bottoms outlet of the stripper column. The apparatus includes a separator having an inlet, a product outlet, and an overhead outlet. The inlet of the separator is in fluid communication with the overhead outlet of the stripping fractionator.
By installing a separator, including but not limited to, vacuum dryers or coalescers, on the naphtha overhead stream from the fractionator column to the product line, the H2S can be removed, and the naphtha can be made substantially free of H2S. By “naphtha,” we mean C5 hydrocarbons up to hydrocarbons having a boiling point of about 150° C. (i.e., hydrocarbons having a boiling point in the range of about 30° C. to about 150° C.). By “substantially free of H2S”, we mean the H2S content is undetectable by ASTM test method UOP 163 and the naphtha passes the Doctor Test, ASTM D4952. This eliminates the need for the caustic/sand filter arrangement or the downstream stripper/stabilizer. In some embodiments where the separator is a vacuum dryer, the liquid portion of the vacuum dryer overhead can be recycled back to the stripper.
The solubility of H2S in steam is quite high in columns which are steam stripped. Since this “sour water” remains in the overhead naphtha and is not totally removed, the naphtha may test positive for H2S. In this case, the separator can be a coalescer which is installed to remove the water, and hence the H2S.
The selection of the type of separator, such as a vacuum dryer or a coalescer, depends on the amount of H2S slipping through into the naphtha overhead stream and how low the moisture content needs to be to meet the Doctor Test.
The fractionator overhead stream 60 contains primarily naphtha, and H2S. Although most of the H2S is removed in the stripper column 20, the remaining H2S is concentrated in the fractionator overhead stream 60. Fractionator overhead stream 60 is sent to receiver 65 wherein it is separated into a receiver overhead gas stream 70, a sour water stream 75, and a liquid naphtha stream 80. The liquid naphtha stream 80 can contain small amounts of water and H2S. The liquid naphtha stream 80 is split into a reflux stream 85, which is sent back to the fractionator column 35, and stream 90, which is sent to a separator. Suitable separators include, but are not limited to, a vacuum dryer 95, as shown in
The vacuum dryer is operated under vacuum. The level of vacuum is not limited; however, it is desirably the lowest level that will remove sufficient H2S so that the naphtha in product stream 100 is substantially free of H2S. The vacuum dryer can be operated at any suitable temperature. The temperature of operation is related to the level of vacuum generated in the dryer (i.e., the higher the level of vacuum, the lower the temperature needs to be).
The vacuum dryer overhead stream 105 is sent to an ejector receiver 110, where it is separated into ejector stream 115, which is condensed steam, a non-condensible vapor stream 120, and a condensable stream 125. Ejector stream 115, non-condensible vapor stream 120, and condensable stream 125 will have some H2S in them. Condensable stream 125 can be recycled to the stripper column 20, if desired.
When steam is used as the stripping medium 40, a coalescer 130 could be used, as illustrated in
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It should be understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.
