Abstract:
Methods and apparatus for making naphtha substantially free of H 2 S are described. The method includes stripping an incoming stream containing naphtha and H 2 S in a fractionator into at least an overhead stream containing the naphtha and H 2 S and a bottoms stream, and introducing the overhead stream from the fractionator into a separator to form a naphtha stream substantially free of H 2 S and an overhead stream containing H 2 S.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to fractionation columns, and more particularly, to apparatus and methods for removing H 2 S and moisture from the naphtha overhead of a fractionator. 
     BACKGROUND OF THE INVENTION 
     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, H 2 S, and, in some cases, water. 
     The H 2 S generated during desulfurization reactions in the hydroprocessing zone is removed predominantly in the stripper column. Although the stripper column is designed to remove H 2 S to the level of parts per billion (ppb) in the stripper bottoms stream, small amounts of H 2 S slip through into the fractionator. The H 2 S 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 H 2 S in the overhead naphtha stream. An H 2 S 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 H 2 S must be removed from the naphtha overhead stream using a secondary processing system. 
     In many units, the H 2 S 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 H 2 S 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 H 2 S from naphtha. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to a method of making naphtha substantially free of H 2 S. In one embodiment, the method includes stripping an incoming stream containing naphtha and H 2 S in a fractionator into at least an overhead stream containing the naphtha and H 2 S and a bottoms stream, and introducing the overhead stream from the fractionator into a separator to form a naphtha stream substantially free of H 2 S and an overhead stream containing H 2 S. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates one embodiment of a process utilizing the present invention. 
         FIG. 2  illustrates another embodiment of a process utilizing the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 H 2 S can be removed, and the naphtha can be made substantially free of H 2 S. 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 H 2 S”, we mean the H 2 S 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 H 2 S 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 H 2 S. In this case, the separator can be a coalescer which is installed to remove the water, and hence the H 2 S. 
     The selection of the type of separator, such as a vacuum dryer or a coalescer, depends on the amount of H 2 S slipping through into the naphtha overhead stream and how low the moisture content needs to be to meet the Doctor Test. 
       FIG. 1  illustrates one embodiment of a process utilizing the present invention. The feed  5  can be any hydrocarbon feed stream(s) predominantly boiling between about 240° C. and about 600° C. The feed  5  is hydroprocessed in the hydroprocessing zone  10 . The effluent  15  can be subjected to one or more separation processes where at least a portion of the gas is removed and the remaining liquid/gas effluent proceeds, as is known in the art (not shown), if desired. The remaining effluent  15  from the hydroprocessing zone  10  is sent to a stripper column  20 , where it is separated into a stripper overhead stream  25  containing at least one of light naphtha, light petroleum gas, light hydrocarbons, and H 2 S, and a stripper bottoms stream  30  containing light and heavy naphtha, other hydrocarbons heavier than naphtha (e.g., kerosene, diesel, vapor gas oil, unconverted oil, and the like, depending on the feed and the hydroprocessing zone), and H 2 S. The stripper bottoms stream  30  is sent to a fractionator  35 . Stripping medium  40  is introduced into the fractionator  35 . The stripper bottoms stream  30  is separated into a fractionator bottoms stream  45  containing unconverted oil, a heavy gas oil (HGO) stream  50 , a light gas oil (LGO) stream  55 , and a fractionator overhead stream  60 . The HGO stream  50  and LGO stream  55  can be further processed and/or recovered, if desired. 
     The fractionator overhead stream  60  contains primarily naphtha, and H 2 S. Although most of the H 2 S is removed in the stripper column  20 , the remaining H 2 S 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 H 2 S. 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  FIG. 1 , or a coalescer  130 , as shown in  FIG. 2 . Sufficient H 2 S is removed in the vacuum dryer  95  so that the naphtha in product stream  100  is substantially free of H 2 S. An overhead stream  105  from the vacuum dryer  95  contains H 2 S. 
     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 H 2 S so that the naphtha in product stream  100  is substantially free of H 2 S. 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 H 2 S 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  FIG. 2 . The coalescer  130  removes the water as stream  140  from the naphtha product  135 . Because of the high solubility of H 2 S in water, the H 2 S would be removed with the water. Typical operating conditions for the coalescer include operating at the temperature of stream  90 . 
     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.