Abstract:
Methods and apparatus for removing sulfur compounds from a hydrocarbon stream are disclosed. In one exemplary embodiment, a method for removing sulfur compounds from a hydrocarbon stream includes the steps of steam stripping a mixed hydrocarbon stream to form a steam stripped overhead stream comprising naphtha and lighter hydrocarbons and a steam stripped bottoms stream comprising naphtha and heavier hydrocarbons; fractionating the steam stripped bottoms stream to form a fractionated overhead stream comprising naphtha hydrocarbons; combining the steam stripped overhead stream with a portion of the fractionated overhead stream and an H2-rich makeup gas stream; and hydrodesulfurizing the combined stream to form an HDS reaction effluent stream.

Description:
TECHNICAL FIELD 
       [0001]    The technical field relates generally to hydrocarbon processing methods and apparatus. More particularly, the technical field relates to methods and apparatus for removing sulfur compounds from a hydrocarbon stream. 
       BACKGROUND 
       [0002]    Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel, and other products known as middle distillates, as well as lower boiling hydrocarbonaceous liquids such as naphtha and gasoline, by hydrocracking a hydrocarbon feedstock derived from crude oil. Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered or derived from crude oil by distillation or by thermal or catalytic processes. A typical heavy gas oil includes a substantial portion of hydrocarbon components boiling above about 371° C. (about 700° F.), usually at least about 50% by weight boiling above 371° C. (about 700° F.). A typical vacuum gas oil normally has a boiling point range between about 315° C. (about 600° F.) and about 565° C. (about 1050° F.). 
         [0003]    Hydrocracking is generally accomplished by contacting the gas oil or other feedstock with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield a product containing a distribution of hydrocarbon products desired by the refiner. The operating conditions and the hydrocracking catalysts chosen within a hydrocracking reactor influence the yield of the hydrocracked products. 
         [0004]    There is thereby produced a reaction zone effluent stream that includes an admixture of the remaining hydrogen which is not consumed in the reaction, light hydrocarbons such as methane, ethane, propane, butane, and pentane formed by the cracking of the feed hydrocarbons, reaction by-products such as hydrogen sulfide and ammonia formed by hydrodesulfurization and hydrodenitrification reactions that occur simultaneously with the hydrocracking reaction. The reaction zone effluent will also contain the desired product hydrocarbons boiling in the naphtha, gasoline, diesel fuel, kerosene, or fuel oil boiling point ranges and some unconverted feed hydrocarbons boiling above the boiling point ranges of the desired products. The effluent of the hydrocracking reaction zone therefore includes an extremely broad and varied mixture of individual compounds. 
         [0005]    The hydrocracking reaction zone effluent is typically removed from contact with the catalyst bed, heat exchanged with the feed to the reaction zone, and then passed into a vapor-liquid separation zone normally referred to as a high pressure separator. Additional cooling can be done prior to this separation. In some instances a hot flash separator is used upstream of the high pressure separator. The use of cold separators to remove condensate from vapor removed from a hot separator is another option. The liquids recovered in these vapor-liquid separation zones are passed into a product recovery zone containing one or more fractionation columns Product recovery methods for hydrocracking are well known and conventional methods may be employed. 
         [0006]    Gas oils and other feedstocks may also include some level of impurities, such as sulfur and nitrogen. These impurities are often undesirable, and their removal from the hydrocracked product is typically sought. For example, due to environmental concerns and newly enacted rules and regulations, saleable fuels must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from diesel. For example, the ultra-low sulfur diesel (ULSD) requirement is typically less than about 10 wppm sulfur. 
         [0007]    With regard to the removal of sulfur impurities, sulfur guard beds are often specified to treat hydrocracked naphtha where the sulfur concentration in the naphtha (as mercaptans arising from recombination reactions in the hydrocracking beds) must be guaranteed as suitable for naphtha reforming and/or isomerization processing downstream. These beds are non-regenerable, and the spent material is a very hazardous waste (carcinogenic, pyrophoric) and requires periodic disposal, which adds undesirable costs to the process. 
         [0008]    Accordingly, it is desirable to provide improved methods and systems for removing sulfur impurities from hydrocracked hydrocarbon streams. Furthermore, it is desirable to provide such methods and systems that reduce the costs associated with sulfur impurity removal. Still further, it is desirable to provide such methods and system that do not require the use of non-regenerable components and that do not produce hazardous / toxic waste by-products. Embodiments of the present disclosure have the feature that the catalyst employed may be periodically regenerated, in situ or ex situ, at the same time that the system is periodically shut down for regularly scheduled maintenance. Furthermore, other desirable features and characteristics of the presently disclosed embodiments will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background. 
       BRIEF SUMMARY 
       [0009]    Methods and systems for removing sulfur compounds from a hydrocarbon stream are disclosed. In one exemplary embodiment, a method for removing sulfur compounds from a hydrocarbon stream includes the steps of steam stripping a mixed hydrocarbon stream to form a steam stripped overhead stream comprising naphtha and lighter hydrocarbons and a steam stripped bottoms stream comprising naphtha and heavier hydrocarbons; fractionating the steam stripped bottoms stream to form a fractionated overhead stream comprising naphtha hydrocarbons; combining the steam stripped overhead stream with a portion of the fractionated overhead stream and an H2-rich makeup gas stream; and hydrodesulfurizing the combined stream to form an HDS reaction effluent stream. 
         [0010]    In another exemplary embodiment, an apparatus for removing sulfur compounds from a hydrocarbon stream that includes a steam stripper that steam strips a mixed hydrocarbon stream to form a steam stripped overhead stream comprising naphtha and lighter hydrocarbons and a steam stripped bottoms stream comprising naphtha and heavier hydrocarbons; a fractionation column that fractionates the steam stripped bottoms stream to form a fractionated overhead stream comprising naphtha hydrocarbons; a means for combining the steam stripped overhead stream with a portion of the fractionated overhead stream and an H2-rich makeup gas stream; and a hydrodesulfurization (HDS) reactor that hydrodesulfurizes the combined stream to form an HDS reaction effluent stream. 
         [0011]    In yet another exemplary embodiment, a process for removing sulfur compounds from a hydrocarbon includes the steps of steam stripping a mixed hydrocarbon stream to form a steam stripped overhead stream comprising naphtha and lighter hydrocarbons and a steam stripped bottoms stream comprising naphtha and heavier hydrocarbons, fractionating the steam stripped bottoms stream to form a fractionated overhead stream comprising naphtha hydrocarbons, and combining the steam stripped overhead stream with a portion of the fractionated overhead stream and an H2-rich makeup gas stream that comprises from 50-250 volume parts per million hydrogen sulfide. The process further includes hydrodesulfurizing the combined stream to form an HDS reaction effluent stream, wherein hydrodesulfurizing comprises contacting the combined stream with a Ni(Co)Mo γ-alumina supported catalyst and debutanizing the reaction effluent stream to form an H2, H2S stream, a C3/C4 product stream, and a naphtha bottoms stream. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The various embodiments will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
           [0013]      FIG. 1  is a schematic illustration of a system and a method for removing sulfur compounds from a hydrocarbon stream in accordance with an exemplary embodiment; and 
           [0014]      FIG. 2  is a schematic illustration of a system and a method for removing sulfur compounds from a hydrocarbon stream in accordance with another exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The following detailed description is merely exemplary in nature and is not intended to limit the various embodiments or the application and uses thereof. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
         [0016]    Various embodiments contemplated herein relate to methods and systems for sulfur compound removal (such as sulfur impurity compounds) from a hydrocarbon stream, such as a hydrocracked hydrocarbon stream. The embodiments described herein replace the sulfur guard bed solution (described above) with a conventional naphtha hydrodesulfurization step, which is positioned upstream of the debutanizer column in the typical hydrocracking/ fractionation flow-scheme. The disclosed hydrodesulfurization step requires only once-through hydrogen contact, and the reactor is small relative to the upstream hydrocracking reactor. The hydrodesulfurization step can be achieved at a pressure only slightly higher than the operating pressure of the debutanizer itself, thus minimizing the need for additional compressor duty to be added to the system. The reactor charge heating and cooling needs are not significant and can be easily integrated with the fractionation flowscheme. 
         [0017]    In the present disclosure, various values of temperature, pressure, flow rates, number of stages, feed entry stage number, etc. are recited in association with the specific examples described below; those conditions are approximate and merely illustrative, and are not meant to limit the described embodiments. Additionally, for purposes of this disclosure, when the terms “middle”, “top” or “lower” are used with respect to a column, these terms are to be understood as relative to each other, i.e. that withdrawal of a stream from the “top” of the column is at a higher position than the stream withdrawn from a “lower” portion of the column. When the term “middle” is used it implies that the “middle” section is somewhere between the “upper” and the “lower” section of the column However, when the terms “upper”, “middle” and “lower” have been used with respect to a distillation column it should not be understood that such a column is strictly divided into thirds by these terms. 
         [0018]      FIG. 1  is a schematic illustration of a system  10  employing a method for removing sulfur compounds from a hydrocarbon stream in accordance with an exemplary embodiment of the present disclosure. A hydrocracked hydrocarbon stream is introduced to system  10  via line  11 . As noted above, the hydrocracked hydrocarbon stream may include an admixture of the remaining hydrogen which is not consumed in the reaction, light hydrocarbons such as methane, ethane, propane, butane, and pentane formed by the cracking of the feed hydrocarbons, reaction by-products such as hydrogen sulfide and ammonia formed by hydrodesulfurization and hydrodenitrification reactions that occur simultaneously with the hydrocracking reaction. Line  11  may also contain the desired product hydrocarbons boiling in the naphtha, gasoline, diesel fuel, kerosene, or fuel oil boiling point ranges and some unconverted feed hydrocarbons boiling above the boiling point ranges of the desired products. In  FIG. 1 , line  11  is introduced into steam stripped fractionation column  12 . Steam is introduced by line (not separated illustrated) in a lower portion of steam stripped fractionation column  12  and travels upwardly through contacting zone  13  in order to strip volatile components from the downwardly flowing hydrocarbon feedstock. A hydrocarbon stream having a reduced concentration of volatile components is removed from steam stripped fractionation column  12  via line  15  and recovered. A vapor stream containing lower boiling hydrocarbons, which have been stripped from the feedstock, and steam is removed from steam stripped fractionation column  12  by line  16  and is introduced into heat-exchanger  17 . A cooled and partially condensed stream containing steam condensate, liquid hydrocarbons and normally gaseous hydrocarbons is removed from heat-exchanger  17  via line  18  and introduced into stripper receiver  19 . A liquid hydrocarbon stream is removed from stripper receiver  19  via line  20  and at least a portion is transported via line  14  and introduced into steam stripped fractionation column  12  as reflux and another portion is transported to further processing via line  21 . The stripper receiver is designed to separate condensate water from the liquid hydrocarbons. 
         [0019]    Hydrocarbon stream  15  continues to product fractionator  30 . The process of separation and fractionation may be conducted in accordance with well known techniques. One exemplary process of separation and fractionation may be described as follows. The fractionator  30  may separate the hydrocarbon stream  15  into a plurality of product streams, including for example a light hydrocarbons and naphtha stream  31  (as an overhead product), a liquid stream  32  including unconverted kerosene (as an upper side cut), a liquid stream  33  including unconverted diesel (as a lower side cut), and a bottoms stream  34  from the fractionation column  30  that can include residual uncracked diesel fuel which can be recycled back to the hydrocracking stage or be used as fuel in the process heaters. This process can be carried out at a pressure between about 6 MPa and about 17 MPa, for example, between about 9 MPa and about 12 MPa, such as about 10 MPa. 
         [0020]    The term “diesel fuel” is defined in accordance with the specifications defined in the American Society for Testing and Materials (ASTM). Specification D 975 and refers to a petroleum fraction comprised primarily of C10-C24 hydrocarbons (about 75 mass %), typically paraffins including straight-chained, branched, and cycloparaffins, and of aromatic hydrocarbons (about 25 mass %), such as alkylbenzenes and naphthalenes, and having distillation temperatures of about 260° C. at the 10% recovery point and about 340° C. at the 90% recovery point. The average net chemical formula for common diesel fuel is typically C12H26. The term “kerosene” is defined as a petroleum fraction containing hydrocarbons that are slightly heavier than those found in gasoline and naphtha, such as C9-C16 hydrocarbons, and having a boiling point at ambient pressure between about 150° C. and about 300° C. Further, the term “naphtha” refers to a petroleum fraction with an approximate boiling range between about 50° C. and about 200° C. As further known in the art, the terms “diesel fuel,” “kerosene,” and “naphtha” may be defined with regard to their well-known T5/T95 ranges. 
         [0021]    Each of the above-described streams  31 ,  32 , and  33  may include associated heat exchangers, condensers, or other means to produce a reflux stream and a product stream. Thus, the reflux stream from stream  31  is shown as stream  34   a  and the product stream therefrom is shown as stream  35 . The reflux stream from stream  32  is shown as stream  36  and the product stream therefrom is shown as stream  37 . Further, the reflux stream from stream  33  is shown as stream  38  and the product stream therefrom is shown as stream  39 . 
         [0022]    With continued reference to  FIG. 1 , the de-watered, steam stripper overhead liquid product  21  is mixed with a portion of the fractionator overhead product stream  35  (not all, can also be none) and/or another available feed naphtha (e.g. straight-run, etc.). This naphtha feed blend is then mixed with a hydrogen-rich gas stream  41 , which preferably contains a small, but non-zero concentration of hydrogen sulfide (e.g. 50-250 vol. ppm), the blend of which shall be referred to as reactor charge stream  40 . The small, non-zero concentration of hydrogen sulfide will help preserve the sulfide state of the downstream catalyst&#39;s active sites. The reactor charge is then heated preferably to a temperature of about 350-650° F., at about 100-500 psig, and more preferably to a temperature of about 450-550° F. at about 250-350 psig before entering a hydrodesulfurization (“HDS”) reactor  42 . The HDS reactor is filled principally with a Group VI metal and Group VIII metal/γ-alumina catalyst, with volume sufficient for 5-15 1/h liquid hourly space velocity processing of the reactor charge. The HDS reactor effluent stream  43  is then cooled to the required temperature for feeding a debutanizer column downstream. 
         [0023]      FIG. 1  shows that the effluent stream  43  from the HDS may be sent to a debutanizer column  50 . The debutanizer column  50  is in downstream communication with the reactor  42 . The debutanizer column  50  may fractionate a portion of the HDS reactor effluent to provide a C4− overhead stream  51  and C5+ bottoms stream  52 . A portion of the debutanizer bottoms in line  52  may be split between line  53  carrying debutanized naphtha as reflux, and line  54 , which may direct debutanized naphtha to a naphtha splitter  60 . The overhead stream  51  may be further separated into an H2, H2S stream  55  and a C3/C4 stream  56 , as is known in the art. The naphtha splitter  60  may separate the debutanized naphtha stream  54  into light and heavy naphtha product streams  61  and  62 , respectively, as is also known in the art. 
         [0024]      FIG. 2  provides an alternative embodiment of the present disclosure, which is similar to the embodiment shown in  FIG. 1 , except as noted herein. As shown in  FIG. 2 , the HDS reactor  42  is integrated after the debutanizer column  50 . The stream  21  may provide the sole feed to the debutanizer column  50 , and the stream  35  bypasses the debutanizer column  50 . The bottom product stream  54  from the debutanizer column  50  combines with stream  35  and the H2 makeup to form the stream  40 , which as noted above is the feed to the HDS reactor. Accordingly, in this embodiment, the H2/H2S stream  55  is removed after the naphtha splitter  60 , instead of after the debutanizer column  50  as in  FIG. 1 . 
         [0025]    Accordingly, the foregoing disclosure provides embodiments of methods and systems for sulfur compound removal (such as sulfur impurity compounds) from a hydrocarbon stream, such as a hydrocracked hydrocarbon stream. The embodiments described herein replace the sulfur guard bed solution (described above) with a conventional naphtha hydrodesulfurization step, which is positioned upstream of the debutanizer column in the typical hydrocracking/fractionation flow-scheme. The disclosed hydrodesulfurization step requires only once-through hydrogen contact, and the reactor is small relative to the upstream hydrocracking reactor. The hydrodesulfurization step can be achieved at a pressure only slightly higher than the operating pressure of the debutanizer itself, thus minimizing the need for additional compressor duty to be added to the system. The reactor charge heating and cooling needs are not significant and can be easily integrated with the fractionation flowscheme. 
         [0026]    While at least one exemplary embodiment has been presented in the foregoing detailed description of the disclosure, 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 disclosure 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 disclosure. It being 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 disclosure as set forth in the appended claims.