Abstract:
A process for coking a heavy oil feedstock with elimination of recycle is disclosed. In a preferred embodiment, heavy hydrocarbon feed is directly passed to the coking vessels, coker overhead vapors are combined and passed directly to a fractionator and fractionator bottoms are recovered as product for further processing in other refining systems. Distillate coker product is not used to reduce the heavy hydrocarbon feed viscosity or to manage coke fouling in the coker furnace.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of heavy hydrocarbon refining by the delayed coking process.  
         BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to an improved method for delayed coking and involves elimination of all recycle, in particular, heavy coker gas oil and fractionator bottoms recycle to improve heavy hydrocarbon feed throughput. Delayed coking is a well-known oil refining process that is used to convert very heavy hydrocarbon feed stocks into useful liquid fuel products. In this process the heavy hydrocarbon feed is heated rapidly to cracking temperatures at elevated pressure and fed into a coke drum. The heated feed drops in pressure as it enters the coke drum causing lower boiling range components to vaporize. The larger molecules in the heated feed rapidly crack into lower boiling volatile components, leaving behind a solid carbonaceous material known in the art as petroleum coke or, simply, coke.  
           [0003]    The volatile component, a vaporous hydrocarbon mixture, is fed overhead to a fractionator vessel which serves the dual purpose of separating (fractioning) the volatile component into coker products and serving as a coker feed surge drum. Coker system fractionator products typically include process gas, light and heavy naphtha, light and heavy gas oil, and fractionator bottoms. Heavier components of the vapor mixture that condense before reaching the fractionator trays fall to the bottom of the fractionator where they are mixed with incoming fresh feed. These heavier components that condense into the fresh feed in the fractionator are known as “natural recycle”. Lighter components condense on the fractionator trays, are collected and fed to other refining systems for further processing. As this process continues, coke accumulates in the active drum until it is filled to a safe level, at which time the heated feed is diverted to a “sister” coke drum to continue the above process. The full drum is isolated from the coking system and the accumulated coke is removed to prepare the drum for repetition of the above described cycle.  
           [0004]    A conventional coking process is illustrated in FIG. 1. This conventional process utilizes two coke drums for coking a heavy feedstock, and a fractionator for separating a vapor product from the coke drums and for recovering one or more liquid distillate products. Fresh coker feedstock from line  10  passes through heat exchangers  12  and  14 , where it is preheated. The fresh coker feedstock may be unstable to heating, and, as such, may cause deposits to form on the heat exchange surfaces during heating. To minimize this coke deposition, a portion of coker distillate product  86  is combined with the fresh feed through line  88  prior to preheating in heat exchanger  12 . Amounts of distillate are generally from about 2 to about 50 parts by volume of distillate per 100 parts of fresh feed, and preferably about 5 to about 30 parts for most cases. The preheated feed is then introduced through line  16  to the bottom of coker fractionator  22 , where it is combined with the bottoms from the fractionator. Alternatively, a portion of the heavy coker gas oil  18  may be combined with the fresh feed  10  through line  80 .  
           [0005]    Fractionator bottoms  26  are heated in furnace  28  and passed from line  30  via valve  90  and into either coke drum  36  through conduit  32  or into coke drum  38  through conduit  34 . Feed to the coke drum  36  is a mixture of fresh feed  10 , heavy gas oil recycle product  20 , distillate product recycle  88 , and bottoms product from fractionator  22 . The heavy coker gas oil recycle  20  may be combined with the fractionator bottoms product either internal to the fractionator  22  (as shown), or externally into line  26 . The heavy gas oil comes from several sources. As shown in FIG. 1, heavy coker gas oil is withdrawn from fractionator  22  via line  18 , and a portion of the heavy gas oil is returned to the fractionator via line  20  where it is utilized to knock down entrained material and condense the heavier components of the vapor in fractionator  22 . Heavy gas oil  18  withdrawn from the fractionator  22  is also used to quench the vapor overhead product  50  through line  24  and to condense the heavier boiling material in the overhead product  50 .  
           [0006]    The feed for coking is thus passed from the fractionator through furnace  28  for heating the feedstock to coking temperatures and from there, alternatively, to the coke drums. The mixture  26  is heated in furnace  28  to temperatures normally in the range of about 850° F. to 1100° F., and preferably in the range of 900° F. to 975° F. A furnace that heats the mixture rapidly to such temperatures, such as a pipe still, is normally used. The mixture exits the furnace through line  30  at substantially the above-indicated temperatures and is introduced into the bottom of coke drum  36 . The mixture is charged to the coke drum at pressures usually ranging between about 20 to 200 psig, though higher pressures may be used if desired. The coke drum is insulated and may also be heated, such as by introduction of hot gas and vapor from a sister vessel into the drum, so as to maintain the drum&#39;s contents at a temperature in the range of about 800° F. to about 1200° F., more usually 750° F. to 950° F. Inside the drum the heavy hydrocarbon in the mixture thermally cracks to form cracked vapors and coke.  
           [0007]    The vapors are continuously removed overhead from the drum through line  40 . Coke accumulates in the drum until it reaches a predetermined safe level at which time the feed to the drum is shut off and switched to the alternate coke drum  38 . The operation of drum  38  is identical to that of drum  36 . This switching permits drum  36  to be taken out of service, opened, and the accumulated coke removed therefrom using conventional techniques. The hydrocarbon vapors that are taken overhead from the coke drum(s) are carried by line  50  to a fractionator  22 . Even though the coker vessels  36  and  38  are operated alternately the overhead hydrocarbon vapor products flow continuously via line  50  to the fractionator  22 .  
           [0008]    After the Coke drum  36  is filled with coke, the feed  30  is redirected to the alternate coke drum  38 , steam  92  is immediately introduced to drum  36  to strip out any remaining hydrocarbon liquid. The drum is stripped with steam and the resulting vapor is fed to the fractionator through line  50  for a period of time, then the vapors are redirected to the blowdown system  48  via line  44 . Heavier oils stripped out of the drum to the blowdown system are condensed. This condensed material is then pumped into the feed stream  16  through line  58  on the way to the fractionator  22  and, thus, represents a source of recycle.  
           [0009]    Coke in drum  36  is removed by a drilling operation. This process is well known, and does not require detailed explanation here. After drum  36  is drilled and is empty of coke, the drum is preheated, using product vapor from drum  38 . This may be accomplished by diverting a portion of the vapor from drum  38  through drum  36  in a reverse direction through line  96 . The resulting flow of condensed liquid and uncondensed vapor exiting through the bottom of the drum through line  52  is routed to a coke condensate drum  54 . Vapor leaves the coke condensate drum  54  through a balance line  56  with the fractionator  22 . Liquid  98  is routed from the coke condensate drum  54  into the blowdown system  48 , and from there via line  58  into the feed stream  16 . This represents another source of recycle.  
           [0010]    Likewise, after the Coke drum  38  is filled with coke, the feed  30  is redirected to the first coke drum  36 . Steam  94  is immediately introduced to drum  38  in order to strip out any remaining hydrocarbon liquid. The drum is stripped with steam to the fractionator through line  50  via line  42  for a period of time and then the drum vapors are redirected to the blowdown system  48  via line  46 . Preheating drum  38  using product vapor from drum  36 , with condensed liquid and uncondensed vapor out of the drum bottom of vessel  38  being routed to the coke condensate drum  54  is accomplished in the way described above for drum  36 . However, the product vapor stream (corresponding to line  96 ) and the condensed liquid and uncondensed vapor stream (corresponding to line  52 ) are not illustrated in FIG. 1.  
           [0011]    Products recovered from fractionator  22  include heavy Naphtha  86 , light naphtha  76  and a process gas overhead product  70 . In FIG. 1, a distillate stream in line  78  is recovered from fractionation, and stripped using stripper  82 . The bottoms from the stripper are cooled in exchanger  12  and recovered as distillate product  86 . A portion of the distillate product is combined with fresh feed via line  88 , using the method previously described. The overhead from stripper  82  is returned as reflux to fractionator  22  via line  84 . A fractionator overhead product  62  is cooled in exchanger  64  and passed, via line  66  to separation zone  68 . A portion of separation zone liquid  72  is returned as reflux to the fractionator  22  via line  74 , and at least a portion of the remainder recovered as light naphtha through line  76 . A process gas overhead product  70  is also recovered from separation zone  68 .  
           [0012]    As is evident from the above description, conventional coking systems use substantial amounts of “recycle” in the coker feed, principally to reduce viscosity of the feed and minimize fouling of the coker furnace. However, recycle in the feed effectively reduces coking capacity and the production of valued light hydrocarbons; thus, there is a need in the industry to improve the coking process to enhance recovery of valued, light hydrocarbons. In this regard, the process taught in U.S. Pat. No. 4,394,250 includes adding a catalyst and hydrogen to the coker feed to facilitate production of high amounts of useful light products. However, &#39;250 also includes recycle of the bottoms from the fractionator to the coker feed.  
           [0013]    Similarly, U.S. Pat. No. 4,455,219 describes a conventional coking process in which feed to the coker includes fresh feed, heavy fractionator bottoms recycle and coker gas oil added as a diluent to minimize recycle of heavier fractions. As noted above, the heavy coker gas oil is a cracked product from the coking reactions in the coke drum. This gas oil can be processed elsewhere in the refinery for the production of useful liquid products; it does not require additional reaction in the coker. Thus, the addition of the heavy coker gas oil in the coker feed consumes coke drum capacity which is more economically utilized for the raw, heavy feed which must be coked.  
           [0014]    Whereas, the &#39;219 patent does not resolve the problem of heavy coker gas oil in the coker feed U.S. Pat. No. 4,518,487 directly addresses the problem and purports to eliminate fractionator bottom and heavy coker gas oil recycle. In &#39;487, feed to the coker furnace is not combined with fractionator bottoms; instead a higher boiling range distillate is combined with the fresh feed in amounts necessary to prevent fouling of the furnace tubes. This higher boiling range distillate is drawn off the coker product stream and, thus represents recycle, although not of heavy fractionator bottoms.  
           [0015]    The present invention further improves upon &#39;487 and provides for complete elimination of all coker product recycle and thereby increases throughput of feed to the delayed coker process resulting in greater and mores economic recovery of valued light hydrocarbon products. Coker furnace tube fouling is minimized and managed by non-recycle methods, such as by on-line steam spalling.  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention is directed to a process for coking a heavy oil feedstock with elimination of all of recycle. Conventional coking processes require recycle of lighter distillate product streams to reduce the viscosity of feed streams, to reduce carbon deposition in heating zones, and to facilitate recovery of liquids from high temperature streams exiting the coking vessel or within fractionation zones. However, adding recycle streams to the coking vessel effectively reduces the amount of heavy oil feed stocks that may be processed for coke production.  
           [0017]    The process of the present invention provides for a greater throughput of heavy oil feedstock in a coking process. Among other factors, the present invention is based on the discovery of methods for increasing recovery of products from a coking process rather than recycling the products back to the process, as in the prior art. In one preferred embodiment, the entire bottoms product from the coking fractionator is recovered as a product stream rather than being recycled to the coking process for additional coking. In another preferred embodiment, fresh feed is first passed through a surge drum then directly to the coking drums without passing through the coker fractionator. In another preferred embodiment, liquid products from the coking process are passed directly to the fractionator rather than being blended with the fresh coker feed. Thus, the most important and unique aspects of the present invention include: (1) directly passing heavy hydrocarbon feed to coking vessels; (2) combining coker overhead vapors; (3) passing the overhead vapor combination directly to a fractionator and (4) recovering fractionator bottoms as product for further processing in other refining systems.  
           [0018]    Accordingly, the present invention provides a method for coking a heavy hydrocarbon feed comprising:  
           [0019]    (a) directly feeding the heavy hydrocarbon feed through at least one pre-heater to a coking furnace;  
           [0020]    (b) heating the feed in the coking furnace to coking temperatures;  
           [0021]    (c) alternately feeding the heated feed directly to a first coke vessel and a second coke vessel to form a first hydrocarbon vapor product and a first coke accumulation in the first vessel and a second coke accumulation in the second vessel.  
           [0022]    Another aspect of the invention comprises combining coker overhead vapor and liquid products from a blowdown system and condenser in a quench step and feeding the combination to the fractionator for separation into coker products. In a preferred embodiment, the product stream recovered from the coker fractionator includes coker fractionator bottoms, heavy coker gas oil, light coker gas oil, a jet fuel cut, light naphtha, heavy naphtha, and process gas. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 illustrates the prior art coking process and depicts various points in the prior art process where product recycle takes place.  
         [0024]    [0024]FIG. 2 illustrates a specific embodiment of the present invention and depicts the elimination of coker product recycle. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    Suitable hydrocarbon feed stocks for delayed coking are described in the art. The feedstock may be derived from petroleum, shale, coal, tar and/or other hydrocarbon sources. It is typically heavy, low-grade oil such as heavy virgin crude, reduced crude, topped crude, residua from refining processes such as thermal or catalytic cracking processes or blends of such stocks. These feed stocks may be hydrotreated, if desired, before being fed to the coking process to remove sulfur, metals, and other contaminants.  
         [0026]    One embodiment of the present invention is illustrated in FIG. 2. In the present invention, a surge drum or tank  104  is for temporary in-line storage of the fresh coker feed  110  and serves the purpose of absorbing or minimizing sudden changes in the pressure or flow rate of feed into the coking system. Surge drum  104  has a capacity of between 0.1 and 100 minutes of feed throughput, based on the design feedrate of the coking process. The fresh feed  110  is fed to at least one heat exchanger  114 . Then, in contrast to the conventional process, the preheated feed is passed through the surge drum  104  and line  126  to furnace  128  and heated therein to coking temperatures. From the furnace  128 , the heated feed is passed via line  130 , through valve  190  and into either coke drum  136  through conduit  132  or into coke drum  138  through conduit  134 . In the preferred process of the present invention, the fresh feed is not passed to the fractionation column  122 , nor does it contain bottoms product from the fractionator  122 . More preferably, the heated coker feedstock  130  does not contain any fractionator bottoms product, heavy coker gas oil or any coker product recycle.  
         [0027]    Furnace  128 , typically a pipe still type furnace, heats the feedstock mixture to coking temperatures, normally in the range of about 850° F. to 1100° F., and preferably in the range of 900° F. to 975° F. The mixture exits the furnace  128  through line  130  and is alternately introduced into the bottom of either coke drum  136  or  138 , at substantially the above-indicated temperatures and at pressures usually ranging between about 20 to 200 psig, though higher pressures may be used if desired. The coke drum is insulated and may also be pre-heated, such as by introduction of hot gas and vapor from the sister vessel into the drum, so as to maintain the drum&#39;s contents at a temperature in the range of about 800° F. to about 1200° F., more usually 750° F. to 950° F. Inside the drum, the heavy hydrocarbon in the mixture thermally cracks to form cracked hydrocarbon vapors and coke.  
         [0028]    The vapors are continuously removed overhead from the active drum through either line  140  or  142 . The vapors that are taken overhead from the coke drum(s) are carried by line  150  to a fractionator  122 . Coke accumulates in the active drum until it reaches a predetermined level at which time the feed to the drum is shut off and switched to the second, sister coke drum  138 . The operation of drum  138  is identical to that of drum  136 . This switching permits drum  136  to be taken out of service, opened, and the accumulated coke removed therefrom using conventional techniques.  
         [0029]    After the coke drum  136  is filled with coke, and the feed  130  is redirected to the second coke drum  138 , steam is immediately introduced through  192  to remove any remaining hydrocarbon liquid. The steam-stripped liquid is passed to fractionator  122  through line  150  for a period of time and then the drum vapors are redirected to the blowdown system  148  via line  144 . Heavier oils stripped out of the drum to the blowdown system are condensed. While the prior art processes use coker gas oil recovered from the fractionator for quenching the vapors, the present process provides that the oil accumulation in the blowdown system  148  is injected into the coke drum overhead vapor  150  as quench, thus delivering this oil to the fractionator  122  for further distillation and product recovery. This arrangement provides the refiner the capability of increasing the quench  158  of the coke drum overhead vapor  150  and reducing the flash zone temperature in the fractionator  122  without increasing recycle to the coking zones. Increased quench of the overhead vapor will further reduce coking of the products streams in the fractionator, thus extending fractionator run time between turnarounds.  
         [0030]    After drum  136  is drilled and is empty of coke, the drum is preheated, using product vapor from drum  138 . This may be accomplished by diverting a portion of the vapor from drum  138  through line  196  into drum  136 . The resulting flow of condensed liquid and uncondensed vapor out the drum bottom through line  152  is routed to coke condensate drum  154 . Vapor leaves the coke condensate drum  154  through a balance line  156  and into fractionator  122 . Liquid  198  is routed from the coke condensate drum  154  into the blowdown system  148 , from where it is combined with other liquid products, e.g. stream  144 , for quenching the vapors in stream  150  through stream  158 .  
         [0031]    Likewise, after the Coke drum  138  is filled with coke, the feed  130  is redirected to the first coke drum  136 . Steam  194  is immediately introduced to drum  138 . The liquid produced during the steam stripping operation is passed to fractionator  122  through lines  142  and  150  for a period of time and then the drum vapors are redirected to the blowdown system  148  through line  146 . Preheating drum  138  using product vapor from drum  136 , with condensed liquid and uncondensed vapor out of the drum bottom of vessel  138  being routed to the coke condensate drum  154  is accomplished in the way described above for drum  136 .  
         [0032]    Products recovered from fractionator  122  may include a bottoms product  108 , a heavy coker gas oil product  118 , a light gas oil product  119 , a jet fuel distillate  120  heavy naphtha  121 , a light naphtha  176  and a process gas overhead product  170 . The heavy coker gas oil is typically returned to the fractionator  122  via line  116  to cool the fractionator bottoms product and reduce coke formation in the fractionator  122 . Boiling ranges of the various product fractions are broadly defined in Table I, below.  
                                                   Product   Boiling range                           Bottoms Product 108   &gt;650° F.           Heavy Coker Gas Oil 118    650° F.-1150° F.           Light Coker Gas Oil 119    350° F.-750° F.           Jet Fuel Distillate 120    250° F.-570° F.           Heavy Naphtha Product 121    180° F.-400° F.           Light Naphtha Product 176    50° F.-250° F.           Process Gas 170   &lt;100° F.                      
 
         [0033]    The above description of preferred embodiments of the invention is intended to be descriptive and not limiting as to the scope of the invention, which is defined by the following claims.