Abstract:
An improved process for the delayed coking of a heavy residual hydrocarbon feedstock to reduce the coking induction period and to enhance the coking process relative to the processes of the prior art is achieved by mixing a sufficient volume of a paraffinic solvent having the formula C n H 2n+2 , where n=3 to 8 with the heavy feedstock to disturb the equilibrium of asphaltenes in the solution of maltenes in order to flocculate substantially all of the solid asphaltenes particles to thereby increase the yield and quality of valuable liquid products and minimize undesirable cracking reactions that result in high molecular weight polymers and the formation of coke.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/513,369 filed Jul. 29, 2011, the disclosure of which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an improved process for the delayed coking of heavy residual hydrocarbons that reduces the coking induction period and thereby enhances the coking process. 
         [0004]    2. Description of Related Art 
         [0005]    A coking unit is an oil refinery processing unit that converts the low value residual oil, or residua, from the vacuum distillation column or the atmospheric distillation column into low molecular weight hydrocarbon gases, naphtha, light and heavy gas oils, and petroleum coke. The process thermally cracks the long chain hydrocarbon molecules in the residual oil feed into shorter chain molecules. Coking is the preferred option for processing vacuum residues containing high level of metals because metals end up in the coke by-product and are disposed of more easily and economically in this solid form. The liquid coker products are almost free of metals. The processing of heavy crude oils having high metals and sulfur content is increasing in many refineries, and as a result the coking operations are of increasing importance to refiners. The increasing concern for minimizing air pollution is another incentive for treating vacuum residues in a coker, since the coker produces gases and liquids having sulfur in a form that can be relatively easily removed from the product stream. 
         [0006]    The most commonly used coking unit is a delayed unit, or a “delayed coker”. In a basic delayed coking process, fresh feedstock is introduced into the lower part of a fractionator. The fractionator bottoms including heavy recycle material and fresh feedstock are passed to a furnace and heated to a coking temperature. The hot feed then goes to a coke drum maintained at coking conditions where the feed is cracked to form light products while heavy free radical molecules form heavier polynuclear aromatic compounds, which are referred to as “coke.” With a short residence time in the furnace, coking of the feed is thereby “delayed” until it is discharged into a coking drum. The volatile components are recovered as coker vapor and returned to the fractionator, and coke is deposited on the interior of the drum. When the coke drum is full of coke, the feed is switched to another drum and the full drum is cooled and emptied by conventional methods, such as by hydraulic means or by mechanical means. 
         [0007]    Typical coking unit feedstocks are vacuum residues derived from fossil fuels. Selected properties and characteristics of vacuum residue samples derived from crude oils from the various geographical regions indicated are shown in Table 1. As can be seen from Table 1, vacuum residues have low American Petroleum Institute (API) gravities in the range of from 1 to 20 degrees and a sulfur content that ranges from 0.2 to 7.7 W %. In addition, vacuum residues are rich in nitrogen and can contain metals such as nickel and vanadium in relatively high concentrations which make them difficult to process in other refinery unit operations. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Taching 
                 Brent 
                 Kirkuk 
                 Safaniya 
                 Athabasca 
                 Boscan 
                 Rospomare 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Specific Gravity 
                 0.932 
                 0.984 
                 1.021 
                 1.04 
                 1.038 
                 1.035 
                 1.065 
               
               
                 API Gravity 
                 20.3 
                 12.3 
                 7.1 
                 4.6 
                 4.8 
                 5.2 
                 1.4 
               
               
                 Viscosity @ 100° F. 
                 175 
                 380 
                 870 
                 4000 
                 1300 
                 4000 
                 3500 
               
               
                 Sulfur 
                 0.2 
                 1.6 
                 5.2 
                 5.4 
                 4.9 
                 5.6 
                 7.67 
               
               
                 Nitrogen 
                 3800 
                 4700 
                 4000 
                 4300 
                 5700 
                 7800 
                 4200 
               
               
                 Conradson Carbon 
                 9.4 
                 16.5 
                 18 
                 24.6 
                 16.7 
                 19.3 
                 26.3 
               
               
                 Residue (CCR) 
               
               
                 C 5 -Insolubles 
                 0.8 
                 3.5 
                 15.7 
                 23.6 
                 17.9 
                 23.2 
                 35.2 
               
               
                 C 7 -Insolubles 
                 0.3 
                 1 
                 7.7 
                 13.6 
                 10.2 
                 14.1 
                 23.9 
               
               
                 Nickel (Ni) ppmv 
                 10 
                 11 
                 52 
                 44 
                 101 
                 121 
                 71 
               
               
                 Vanadium (V) ppmv 
                 7 
                 38 
                 125 
                 162 
                 280 
                 1330 
                 278 
               
               
                 Ni + V ppmv 
                 17 
                 49 
                 177 
                 206 
                 381 
                 1451 
                 349 
               
               
                   
               
             
          
         
       
     
         [0008]    Vacuum residues also contain asphaltenes in the range 0.3 to 35 W %, depending upon the source of the crude oil. Asphaltenes are defined as the particles precipitated by addition of a low-boiling paraffin solvent such as normal-pentane. It is commonly accepted that asphaltenes exist in solution in the petroleum. Asphaltenes are commonly modeled as a colloid, with asphaltenes as the dispersed phase and maltenes as the continuous phase. Petroleum residua can be modeled as ordered systems of polar asphaltenes dispersed in a lower polarity solvent phase, and held together by resins of intermediate polarity. 
         [0009]    As schematically illustrated in  FIG. 1 , it is known to the prior art that asphaltenes are dispersed by resin molecules, or maltenes, while small molecules such as aromatics act as a solvent for the asphaltenes-resin dispersion and hydrocarbon saturates act as a non-solvent. If crude oil is separated into fractions and then mixed together with less resin content, asphaltenes will only be present as flocculates in solution. Addition of the maltenes or resins brings the asphaltenes back into solution until the equilibrium is disturbed by addition of hydrocarbon saturates, in which case asphaltenes will again start to flocculate. 
         [0010]    It is well known and accepted that coke formation is delayed when the asphaltenes are in solution in the petroleum. This delay in coke formation is also referred as the “induction period” which immediately precedes the formation of coke. During this period, valuable lighter components and/or secondary products formed by coking of feedstocks are subject to continued thermal cracking and recombine to form undesirable high molecular weight polymeric compounds. 
         [0011]    It is also known from independent studies of the thermal cracking of bitumens that the yield of gaseous products increases with the residence time in the coking unit and that liquid yields are correspondingly reduced. 
         [0012]    It is also desirable to produce a coke having a volatile matter content of not more than about 15 W %, and preferably in the range of 6 to 12 W %. 
         [0013]    It is therefore an object of this invention to address the problem of how to reduce the coking induction period so that the residence time of the feed in the coke drum is shortened. This will maximize the desired yield of liquids and minimize the coke yield. 
         [0014]    As used herein, the terms “coking unit” and “coker” refer to the same apparatus, and are used interchangeably. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention comprehends an improved process for the delayed coking of heavy residual hydrocarbons that reduces the coking induction period and enhances the coking process by injecting a paraffinic solvent having the formula C 1 H 2n+2 , where n=3 to 8 into the feedstock. The improved delayed coking process includes the steps of: 
         [0016]    a. introducing a fresh heavy hydrocarbon feedstock containing asphaltenes for preheating into the lower portion of a coking product fractionator; 
         [0017]    b. discharging a bottoms fraction that includes the preheated fresh hydrocarbon feedstock from the fractionator as a coking unit combined feedstream; 
         [0018]    c. introducing a paraffinic solvent having the formula C n H 2n+2 , where n=3 to 8, into a mixing zone with the coking unit combined feedstream in a ratio of solvent-to-feedstream of from 0.1:1 to 10:1 by volume to solvent-flocculate all or substantially all of the asphaltenes present in the coking unit combined feedstream; 
         [0019]    d. introducing the coking unit combined feedstream containing the flocculated asphaltenes into a coking unit furnace for heating to a predetermined coking temperature; and 
         [0020]    e. passing the heated combined feedstream containing the solvent-flocculated asphaltenes and paraffinic solvent to a delayed coking drum to produce a delayed coking product stream having an increased portion of liquids and depositing a reduced amount of coke on the interior of the drum, as compared to the amount of coke deposited in the absence of the addition of the paraffinic solvent to the same heavy hydrocarbon feedstock. 
         [0021]    In accordance with another embodiment of the invention, the improved delayed coking process comprehends the steps of: 
         [0022]    a. introducing a fresh heavy hydrocarbon feedstock containing asphaltenes for preheating into the lower portion of a coking product fractionator; 
         [0023]    b. discharging a bottoms fraction that includes the preheated fresh hydrocarbon feedstock from the fractionator as a coking unit combined feedstream; 
         [0024]    c. introducing the coking unit combined feedstream into a coking unit furnace for heating to a predetermined coking temperature; 
         [0025]    d. mixing downstream of the coking furnace a paraffinic solvent having the formula C n H 2n+2 , where n=3 to 8, with the furnace heated coking unit combined feedstream in a ratio of solvent-to-feedstream of from 0.1:1 to 10:1 by volume to form solvent-flocculated asphaltenes in the heated coking unit combined feedstream; 
         [0026]    e. passing the heated coking unit combined feedstream containing the solvent-flocculated asphaltenes and paraffinic solvent to a delayed coking drum to produce a delayed coking product stream having an increased proportion of liquids and depositing a reduced amount of coke on the interior of the drum, as compared to the amount of coke deposited in the absence of the addition of the paraffinic solvent to the same heavy hydrocarbon feedstock. 
         [0027]    The mixing in step (d) referred to in the embodiment described immediately above occurs in a mixing zone upstream of the coking unit or inside the coking drum. In the latter case, paraffinic solvent is injected directly into the coking drum to mix with the incoming feedstream. Where a separate mixing zone is established upstream of the furnace, a rotating disk contactor apparatus can advantageously be employed. Feedstock and solvent can be introduced into the top of the unit and the flocculated portion can be sent to the coking unit from the bottom. This arrangement will prevent or minimize fouling of the mixing apparatus. 
         [0028]    The processes and systems of the invention described provide the following benefits:
       1. The paraffinic solvent added to the feedstream disturbs the equilibrium of the asphaltenes in the maltenes solution to flocculate the solid particles of asphaltenes. The coking induction period is therefore reduced.   2. The injected paraffinic solvent facilitates the removal of reacted and/or unreacted lighter liquid compounds from the coking drum, and prevents undesirable secondary cracking reactions that form additional free radicals.   3. The residence time for coking reactions is reduced. This minimizes the coking of resin molecules boiling in the vacuum gas oil range to thereby increase the yield of more valuable liquid products.       
 
         [0032]    As residence time increases, the liquids in the feed are subjected to further cracking to produce gaseous products. Since the coke induction period is eliminated by the addition of solvent in accordance with the present invention, the residence time in the coke drum will be shortened and the liquids produced will not be subjected to further cracking. Accordingly, the present improved process yields more liquid and less gaseous products than the same coking process conducted without the addition of a solvent. 
         [0033]    The process has been described above and will be described further below with reference to the use of a paraffinic solvent. However, it should be understood that an embodiment of the invention employs as the solvent a portion of the light naphtha stream recovered from the coking product stream fractionator. That product stream includes olefins that are principally C 5  to C 8  compounds. For convenience and in the interest of brevity, the term paraffinic solvent is used in describing and claiming the invention with the understanding that its source can be the light naphtha that is produced in the process which also includes olefin compounds. 
         [0034]    Other aspects, embodiments, and advantages of the process of the present invention are discussed in detail below. Moreover, it is to be understood that both the foregoing summary and the following detailed description are merely illustrative examples of various aspects and embodiments, and are intended to provide an overview or framework for understanding the nature and character of the claimed features and embodiments. The accompanying drawings are included to provide illustration and a further understanding of the various aspects and embodiments. The drawings, together with the remainder of the specification, serve to explain principles and operations of the described and claimed aspects and embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    The foregoing summary, as well as the following detailed description will be best understood when read in conjunction with the attached drawings in which the same or similar elements are referred to by the same numeral, and where: 
           [0036]      FIG. 1  is schematic a model illustrating generally the nature of the colloidal dispersion of a petroleum mixture; 
           [0037]      FIG. 2  is a process flow diagram of an improved delayed coking system and process of the present invention; 
           [0038]      FIG. 3  is a process flow diagram of another embodiment of an improved delayed coking system and process in accordance with the present invention; and 
           [0039]      FIG. 4  is a process flow diagram of a further embodiment of an improved delayed coking system and process of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]    Referring now to  FIG. 2 , an improved delayed coking process and apparatus  10  is schematically illustrated. Apparatus  10  includes a fractionator  20 , a mixing zone  30 , a furnace  40  and a coking drum  50 . Fractionator  20  includes an inlet  27  for receiving fresh heavy hydrocarbon feedstock, an inlet  21  in fluid communication with a coking drum outlet  52  for receiving delayed coking product stream. Fractionator  20  also includes an outlet  22  for discharging a light naphtha fraction, an outlet  23  for discharging a heavy naphtha fraction, an outlet  24  for discharging a gas oil fraction, an outlet  25  for discharging a heavy gas oil fraction, and an outlet  26  for discharging a mixture of the bottoms fraction and preheated fresh heavy hydrocarbon feedstock. Mixing zone  30  includes an inlet  31  in fluid communication with a conduit  33  for introducing a paraffinic solvent and fractionator outlet  26  for receiving the combined stream of preheated fresh hydrocarbon feedstock and the fractionator bottoms fraction. Mixing zone  30  also includes an outlet  32  for discharging a combined stream containing solvent-flocculated asphaltenes and paraffinic solvent. Furnace  40  includes an inlet  41  in fluid communication with mixing zone outlet  32  and an outlet  42  for discharging heated combined stream. Coking drum  50  includes an inlet  51  in fluid communication with furnace outlet  42  and an outlet  52  in fluid communication with fractionator inlet  21  for receiving the delayed coking product stream. 
         [0041]    In the practice of the method of the invention, a fresh heavy hydrocarbon feedstock containing asphaltenes is introduced into the lower portion of the fractionator  20  via inlet  27 . The preheated feedstock is combined with the fractionator bottoms stream and passed to mixing zone  30  via inlet  31 . A paraffinic solvent is introduced into mixing zone  30  via conduit  33  in a ratio of solvent-to-feedstream of from 0.1:1 to 10:1 by volume to form solvent-flocculated asphaltenes in the combined stream. The combined stream containing solvent-flocculated asphaltenes and paraffinic solvent is discharged via outlet  32  and introduced into furnace  40  via inlet  41  where it is heated to a predetermined coking temperature in the range 480° C. to 530° C. The heated combined stream is discharged via outlet  42  and passed to coking drum  50  via inlet  51  to produce the delayed coking product stream having an increased portion of liquids and to deposit a reduced amount of coke on the interior of the drum. The delayed coking product stream is discharged via outlet  52  and passed to fractionator  20  where it is fractionated to produce a paraffinic light naphtha solvent boiling in the range 36° C. to 75° C. via outlet  22 , a heavy naphtha product boiling in the range 75° C. to 180° C. via outlet  23 , a light gas oil boiling in the range 180° C. to 370° C. via outlet  24 , a heavy coker gas oil boiling in the range 370° C. to 520° C. via outlet  25 , and a bottoms fraction boiling in the range above 520° C. via outlet  26 . Optionally, a portion of paraffinic light naphtha solvent is recycled back to conduit  33  to minimize the use of fresh paraffinic solvent. 
         [0042]    Referring to  FIG. 3 , an improved delayed coking process and apparatus  100  is schematically illustrated. Apparatus  100  includes a fractionator  120 , a mixing zone  130 , a furnace  140  and a coking drum  150 . Fractionator  120  includes an inlet  127  for receiving fresh heavy hydrocarbon feedstock, an inlet  121  in fluid communication with a coking drum outlet  152  for receiving delayed coking product stream. Fractionator  120  also includes an outlet  122  for discharging a light naphtha fraction, an outlet  123  for discharging a heavy naphtha fraction, an outlet  124  for discharging a gas oil fraction, an outlet  125  for discharging a heavy gas oil fraction, and an outlet  126  for discharging a mixture of the bottoms fraction and preheated fresh heavy hydrocarbon feedstock. Furnace  140  includes an inlet  141  in fluid communication with fractionator outlet  126  and an outlet  142  for discharging heated combined stream of bottoms fraction and fresh heavy hydrocarbon feedstock. Mixing zone  130  includes an inlet  131  in fluid communication with a conduit  133  for receiving a paraffinic solvent and furnace outlet  142  for receiving heated combined stream. Mixing zone  130  also includes an outlet  132  for discharging combined stream containing solvent-flocculated asphaltenes and paraffinic solvent. Coking drum  150  includes an inlet  151  in fluid communication with mixing zone outlet  132  and an outlet  152  in fluid communication with fractionator inlet  121  for receiving delayed coking product stream. 
         [0043]    A fresh heavy hydrocarbon feedstock containing asphaltenes is introduced into the lower portion of the fractionator  120  via inlet  127 . The preheated feedstock is combined with fractionator bottoms stream and passed to furnace  140  via inlet  141  where it is heated to a predetermined coking temperature in the range 480° C. to 530° C. The heated combined stream is conveyed to mixing zone  130  via inlet  131 . A paraffinic solvent is introduced into mixing zone  130  via conduit  133  in a ratio of solvent-to-feedstream of from 0.1:1 to 10:1 by volume to form solvent-flocculated asphaltenes in the combined stream. The combined stream containing solvent-flocculated asphaltenes and paraffinic solvent is discharged via outlet  132  and passed to coking drum  150  via inlet  151  to produce the delayed coking product stream having an increased portion of liquids and to deposit a reduced amount of coke on the interior of the drum, relative to the prior art process. The delayed coking product stream is discharged via outlet  152  and passed to fractionator  120  where it is fractionated to produce a light naphtha containing paraffinic solvent boiling in the range 36° C. to 75° C. via outlet  122 , a heavy naphtha boiling in the range 75° C. to 180° C. via outlet  123 , a light gas oil boiling in the range 180° C. to 370° C. via outlet  124 , a heavy coker gas oil boiling in the range 370° C. to 520° C. via outlet  125 , and a bottoms fraction boiling in the range above 520° C. via outlet  126 . Optionally, a portion of light naphtha containing paraffinic solvent is recycled back to conduit  133  to minimize the use of fresh paraffinic solvent. 
         [0044]    Referring to  FIG. 4 , an improved delayed coking process and apparatus  200  is schematically illustrated. Apparatus  200  includes a fractionator  220 , a furnace  240  and a coking drum  250 . Fractionator  220  includes an inlet  227  for receiving fresh heavy hydrocarbon feedstock, an inlet  221  in fluid communication with a coking drum outlet  252  for receiving delayed coking product stream. Fractionator  220  also includes an outlet  222  for discharging light naphtha fraction, an outlet  223  for discharging a heavy naphtha fraction, an outlet  224  for discharging a gas oil fraction, an outlet  225  for discharging a heavy gas oil fraction, and an outlet  226  for discharging a mixture of the bottoms fraction and preheated fresh heavy hydrocarbon feedstock. Furnace  240  includes an inlet  241  that is in fluid communication with a conduit  254  for receiving a paraffinic solvent and with fractionator outlet  226  and an outlet  242  for discharging heated combined stream of bottoms fraction and fresh heavy hydrocarbon feedstock. Coking drum  250  includes an inlet  251  in fluid communication with a conduit  253  for receiving a paraffinic solvent and furnace outlet  242  for receiving heated combined stream. Coking drum  250  also includes an outlet  252  for discharging delayed coking product stream. 
         [0045]    A fresh heavy hydrocarbon feedstock containing asphaltenes is introduced into the lower portion of the fractionator  220  via inlet  227 . The preheated feedstock is combined with fractionator bottoms stream and passed to furnace  240  via inlet  241  where it is heated to a predetermined coking temperature in the range 480° C. to 530° C. The heated combined stream is conveyed to coking drum  250  via inlet  251 . A paraffinic solvent is introduced into coking drum  250  via conduit  253  in a ratio of solvent-to-feedstream of from 0.1:1 to 10:1 by volume to form solvent-flocculated asphaltenes in the combined stream. Combined stream containing solvent-flocculated asphaltenes and paraffinic solvent is processed in coking drum  250  to produce the delayed coking product stream having increased portion of liquids and deposit a reduced amount of coke on the interior of the drum. The delayed coking product stream is discharged via outlet  252  and passed to fractionator  220  where it is fractionated to produce a light naphtha containing paraffinic solvent boiling in the range 36° C. to 75° C. via outlet  222 , a heavy naphtha boiling in the range 75° C. to 180° C. via outlet  223 , a light gas oil boiling in the range 180° C. to 370° C. via outlet  224  a heavy coker gas oil boiling in the range 370° C. to 520° C. via outlet  225 , and a bottoms fraction boiling in the range above 520° C. via outlet  226 . Optionally, a portion of light naphtha containing paraffinic solvent is recycled back to conduit  253  to minimize the use of fresh paraffinic solvent. 
         [0046]    The feedstocks for the improved delayed coking process described herein are heavy hydrocarbons derived from natural resources including crude oil, bitumen, tar sands and shale oils, or from refinery processes including atmospheric or vacuum residue, products from coking, visbreaker and fluid catalytic cracking operations. The heavy hydrocarbon feedstock has a boiling point in the range of from 36° C., this being the boiling point of pentane, up to 2000° C. Some heavy hydrocarbon feedstocks such as bitumens include little light hydrocarbons. In these cases, the feedstock can have an initial boiling point (IBP) of 180° C., e.g., the IBP of gas oils, or 370° C., e.g., the IBP of vacuum gas oil. 
         [0047]    The paraffinic solvent has the general formula of C n H 2n+2 , where n can be from 3 to 8. As noted above, a portion of the light naphtha stream from the fractionator can be used as the solvent that is mixed with the feedstream to the furnace or the coking drum. In accordance with the definition of light naphtha conventionally used in the art, octanes and olefin compounds, including pentenes, hexenes, heptenes and octenes, can also be present in the mixture. The presence of C 3  and C 4  compounds on the mixture will be dependent upon the prevailing pressure and temperature conditions in the coking unit and upstream. The C 5  to C 8  alkanes have boiling points in the range from about 28° C. to about 114° C., and the C 5  to C 8  olefins have initial boiling points in the range of from about 30° C. to about 121° C. The solvent is injected at a solvent battery limit temperature and a pressure of from 1 bar to 100 bars. 
         [0048]    The coking unit is a typical delayed coking unit with two drums operating alternatively. In general, the operating conditions for the coking drum include a temperature of from 425° C. to 650° C.; in certain embodiments from 425° C. to 540° C.; in further embodiments from 450° C. to 510° C.; and in additional embodiments from 470° C. to 500° C.; and at a pressure of from 1 bar to 20 bars; in certain embodiments from 1 bar to 10 bars; and in further embodiments from 1 bar to 7 bars. The coking cycle time can be from 8 hrs to 60 hrs; in certain embodiments from 24 hrs to 48 hrs; and in further embodiments from 8 hrs to 24 hrs. 
         [0049]    The method of the invention represents an improvement over the prior art processes by reducing the coking induction period by mixing a predetermined amount of paraffinic solvent with the heavy hydrocarbon feedstocks in order to disturb the equilibrium of the asphaltenes in the maltenes solution and to flocculate all, or substantially all of the solid asphaltenes particles. In the present process, the yield and qualities of valuable liquid products are increased while undesirable cracking and the formation of coke are minimized. 
         [0050]    The method and system of the present invention have been described above and in the attached drawings; however, modifications will be apparent to those of ordinary skill in the art and the scope of protection for the invention is to be determined by the claims that follow.