Abstract:
A coke drum includes a coke drum vessel having a substantially cylindrical wall and a conical bottom portion; a skirt downwardly depending from a transition of the cylindrical wall into the conical bottom portion, an inner annular space being defined between the skirt and the conical bottom portion; and a fluid injection system communicated with the inner annular space for injecting fluid at a desired temperature to heat or cool the inner annular space. The fluid injection system allows the inner annular space and junction of the skirt with the coke drum vessel to be heated or cooled to minimize temperature difference between these areas and a batch of coke to be introduced into the drum.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to coke drums and more particularly to reducing stresses in the coke drum during heating and cooling of the coke drum. 
         [0002]    Coke drums and delayed coke drums are vertical thin-walled pressure vessels that operate under severe conditions by cyclic heating and quenching operations. In use, a drum is filled with a hot coker feed and, after a period of time, the drum is emptied and prepared for the next filling. During filling with hot coker feed, the drum is exposed to the very high temperature of the hot coker feed, which can be as high as 900° F. or more. When the drum is to be emptied, in one step a quench water can be introduced into the drum, and at this stage, the drum can be much hotter than the quench water. 
         [0003]    Because of these extreme temperature cycles, coke drums are one of the pressure vessels that register most failure frequency in refineries. According to an API survey, skirt cracking was reported by 73% of companies surveyed who were using coke drums. In order to repair cracks in the skirt, operational windows are required, which require interruption of processes and, as a consequence, loss of opportunities and higher operational costs. 
         [0004]    In addition, incidents have been reported associated with hydrocarbon leaks and incipient fires, creating certain operational safety conditions. Studies have recognized that the cracks in coke drums result from low cycle fatigue induced by cyclic thermal stress. In order to attempt to address this issue, coke drums have been provided with a zone called a “hot box” which closes off a space between the skirt and the conical bottom portion of the coke drum. This helps to increase the skirt temperature during a heating stage in order to decrease the thermal effect. This is not enough, however, to avoid problems with skirt cracking. 
         [0005]    The need remains for a solution to the problem of skirt cracking. 
       SUMMARY OF THE INVENTION 
       [0006]    According to the invention, a solution to skirt cracking is provided. Specifically, a fluid injection system is provided to inject fluid of an appropriate temperature into the “hot box” and thereby avoid the severe thermal stresses normally encountered with coke drum operation. 
         [0007]    According to the invention, a coke drum is provided which comprises a coke drum vessel comprising a substantially cylindrical wall and a conical bottom portion; a skirt downwardly depending from a transition of the cylindrical wall into the conical bottom portion, an inner annular space being defined between the skirt and the conical bottom portion; and a fluid injection system communicated with the inner annular space for injecting fluid at a desired temperature to heat or cool the inner annular space. 
         [0008]    The fluid injection system can be provided as a toroid tube in the inner annular space and communicated with a source of the fluid. 
         [0009]    The fluid injection system can have a fluid inlet and a fluid outlet, and at least one baffle in the inner annular space between the fluid inlet and the fluid outlet. 
         [0010]    The coke drum can have a bottom wall section connected between the skirt and the conical bottom portion and closing off the inner annular space to define the so-called “hot box”. 
         [0011]    The fluid injection system can advantageously have a first toroid tube defining a fluid inlet, a second toroid tube defining a fluid outlet, and a series of baffles in the inner annular space between the first toroid tube and the second toroid tube. 
         [0012]    The series of baffles can be arranged so that they lead the fluid from the first or inlet toroid tube toward the second or outlet toroid tube, and positioned to provide that the fluid is always in contact with the skirt and conical surfaces of the drum. In one configuration, the baffles can be positioned between the skirt and conical surfaces with a gap between the baffle and these surfaces such that flow of fluid is guided along the surface of the skirt or conical section. 
         [0013]    The fluid injection system can be provided so as to inject fluid around an entire circumference of the inner annular space. 
         [0014]    A process for operating the system according to the invention, and also a method for retrofitting or adapting an existing coke drum to include the present invention, are also provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A detailed description of preferred embodiments of the invention follows, with reference to the attached drawings, wherein: 
           [0016]      FIG. 1  is a schematic representation of a coke drum; 
           [0017]      FIG. 2  shows an enlarged portion of  FIG. 1  showing a “hot box” portion of the coke drum with a fluid injection system according to the invention; 
           [0018]      FIG. 3  shows a further enlarged portion of  FIG. 2  including baffles according to a further embodiment of the invention; and 
           [0019]      FIG. 4  shows skirt and cone temperature over time for a coke drum without the fluid injection system of the present invention (solid lines) as compared to one with the fluid injection system of the present invention (dashed lines). 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The invention relates to a coke drum having a fluid injection system for reducing thermal stress and increasing lifetime of a coke drum and, particularly, increasing lifetime of a junction between the cone and skirt of a coke drum. 
         [0021]      FIG. 1  illustrates a coke drum  10  which typically has a substantially vertically oriented cylindrical portion  12  closed off at a top portion  14  and having a substantially conical bottom portion  16 . Further, a skirt  18  is typically attached to coke drum  10  at a transition point  20  from cylindrical portion  12  to conical bottom portion  16 . Skirt  18  can be attached to coke drum  10 , most typically by welding. 
         [0022]    Skirt  18  is typically used to support a coke drum during use. In use, coke drum  10  is filled with a hot coker feed and, after a period of time, the drum is emptied and prepared for the next filling. At two different stages in the use of a coke drum, the drum, including the junction of skirt and transition point, are normally subjected to extreme changes in temperature. As will be described herein, these changes in temperature are moderated and controlled so as to reduce thermal stresses through the drum and, particularly, at the junction of skirt and cone, and thereby extend the life of the drum. 
         [0023]      FIG. 2  is an enlarged view of the portion indicated in  FIG. 1  at A, and shows a portion of a substantially cylindrical wall  22  which extends around axis X to define a coke drum vessel. This wall thereby defines a substantially closed inner portion into which a coker feed is introduced for cooling and subsequent treatment. 
         [0024]    As shown, wall  22  transitions at the transition point  20  into an inwardly tapered wall  24  which defines conical bottom portion  16 . 
         [0025]    Skirt  18  is shown connected to the cylindrical drum and downwardly depending from transition point  20 . This connection of skirt  18  and wall  22 / 24  is therefore a junction of these components. At this junction, the components are typically welded, and it is the protection of this weld that is the focus of the present invention. 
         [0026]    It should be appreciated that skirt  18  and wall  24  define between them an enclosed inner annular space  26 . A radial wall  28  can also be included, for example extending between skirt  18  and wall  24  to close off a portion of inner annular space  26 . This portion is referred to as the “hot box”  30 . 
         [0027]      FIG. 3  is an enlarged portion of  FIG. 2  and shows tubes  32 ,  34  which define fluid inlet and outlet, respectively, for use in introducing a fluid to the hot box  30 . Tubes  32 ,  34  can, for example, be provided as toroid tubes and are preferably disposed around a circumference of conical bottom portion  16  in the hot box  30  or inner annular space  26  defined between wall  24  and skirt  18 . Tubes  32 ,  34  are communicated with a source of fluid at a variable temperature such that fluid can be introduced through tubes  32 ,  34  to either heat or cool the hot box  30  as desired. 
         [0028]      FIG. 3  shows a further enlarged portion of  FIG. 2 , particularly the portion indicated at B. As shown, one tube  32  is configured as an inlet and the other tube  34  is configured as an outlet for the fluid flow to be introduced into hot box  30 . Further, as shown in this illustration a series of baffles  36  can advantageously be positioned in hot box  30  between inlet tube  32  and outlet tube  34  such that fluid introduced through inlet tube  32  must travel a circuitous route to reach outlet tube  34 . This enhances the heat transfer from such fluid through the baffles  36  and into skirt  18  and wall  24  as well as the weld between these components. As shown, baffles  36  are preferably positioned to extend between the walls without reaching them, and this configuration serves to guide flow of fluid along the surfaces of the conical section and skirt as desired, for example as shown by the arrows in  FIG. 3 . Also as shown in  FIG. 3 , the toroid tubes  32 ,  34  and baffles  36  define a fluid flow area which extends roughly the entire height h of hot box  30 , and also which advantageously can extend around an entire circumference c of the hot box area. 
         [0029]    As set forth above, coke drums in general and the weld at the junction of skirt  18  and wall  22 ,  24  are subjected to extreme changes in temperature in two cycles of typical use of a coke drum. The first is when a drum is filled with a hot coker feed, which can have a temperature as high as 900° F. or more. According to the invention, fluid can be injected into hot box  30  through tube  32  so as to heat the temperature in hot box  30  and adjoining wall sections and weld to a temperature close to the expected temperature of incoming hot coker feed. In this way, when the hot coker feed is introduced into the drum, a AT between components of the drum and the hot coker feed can be minimized. Still further, ΔT between the skirt and the cone can be minimized as well, and this most advantageously reduces thermal stresses in the weld connection at the junction between these components. 
         [0030]    Thus, in advance of filling a coke drum with hot coker feed, the expected temperature of the hot coker feed can be determined using various means well known to the person of ordinary skill in the art, and hot fluid can then be introduced through tube  32  into hot box  30  at a temperature sufficient to elevate temperature of wall  24  and skirt  18  to substantially the same temperature as the expected incoming hot coker feed. Once the hot coker feed has rested in the coke drum and it is to be removed, a quench step is typically conducted where quench water is introduced into the drum. In advance of this step, fluid can be introduced through inlet tube  32  which is at a temperature suitable to reduce temperature of the hot box  30  and walls defining same to be substantially the same as the temperature of the expected quench water. In this way, during quenching, AT between skirt and cone can also be minimized. 
         [0031]    The fluid to be used in the present invention can be any type of fluid which would be suitable for delivering the desired amount of heating and/or cooling to hot box  30 . According to one embodiment of the invention, this fluid is advantageously air. 
         [0032]    In the typical environment of use of the apparatus of the present invention, air is readily available from a number of different sources and at a number of different temperatures, and this therefore advantageously facilitates deployment of systems in accordance with the present invention. Of course, if desirable for other reasons different fluids could be used. 
         [0033]    It should also be appreciated that all components defining the hot box, including tubes  32 ,  34  and baffles  36 , should be fabricated from materials which are well resistant to any detrimental effects of being exposed to hot air. 
         [0034]    In order to demonstrate the advantageous results of use of the present invention, tests were taken using a coker drum without a fluid injection system according to the present invention, and a coker drum with the fluid injection system of the present invention. 
         [0035]    In the course of these tests, temperature of the skirt section and the cone section were monitored during a full cycle of use of a coke drum. Results are shown in  FIG. 4 . In  FIG. 4  a solid line referred to at reference numeral  38  shows cone temperature over time during the test, for a coke drum without the fluid injection system of the present invention. As shown, from a starting point of approximately 65° C., the temperature of the cone upon filling rapidly climbed, over the period of approximately 6.5 hours to a temperature of approximately 390° C. Still referring to  FIG. 4 , the solid line indicated by reference numeral  40  is the corresponding skirt temperature over time, and  FIG. 4  shows that once the drum is filled, there is a ΔT between cone  38  and skirt  40  of 30° C. or more. Further, these large ΔT values are throughout the process, including times of steep temperature changes, for example during introduction of a hot coker feed, or at the time of quenching. 
         [0036]    Still referring to  FIG. 4 , the dashed line indicated at  42  shows cone temperature for a coke drum including a fluid injection system in accordance with the present invention. Further, the skirt temperature of this device is shown in  FIG. 4  at reference numeral  44 . As shown, the spacing between dashed lines  42 ,  44  or ΔT when the fluid injection system of the present invention is used, is significantly smaller than the ΔT in the drum without a fluid injection system. This advantageously reduces thermal stresses to which the junction between the skirt and cone is subjected, and thereby increases the useful life of the coke drum and, particularly, the welds at this junction. 
         [0037]    It should therefore be appreciated that the system in accordance with the present invention is advantageously used in a method wherein temperature of a coker feed to be delivered to the drum is determined, fluid of a desired temperature is then introduced through the fluid injection system and inlet tube  32 , to bring the temperature of hot box  30  including both cone and skirt, to a substantially similar temperature in advance of introduction of the coker feed into the drum. 
         [0038]    Once the coker feed in the drum has rested a sufficient period of time, and the drum is to be emptied and cleaned, the temperature of a quenching water can be determined and additional fluid injected to the hot box to cool the hot box to substantially the same temperature as the quenching water. In this way, the ΔT during quenching can also be minimized in accordance with the present invention. 
         [0039]    It should also readily be appreciated that the fluid injection system of the present invention can be easily adapted to incorporate into an existing coke drum without such a fluid injection system. This could be done, for example, by introducing tubes  32 ,  34  into the inner annular space  26  defined by skirt  18  and wall  24  and further by adding baffles  36  and a radial wall  28  if necessary, such that an existing coke drum could then be protected with the system of the present invention. 
         [0040]    It is to be understood that the invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are susceptible of modification of form, size, arrangement of parts and details of operation. The invention rather is intended to encompass all such modifications which are within its spirit and scope as defined by the claims.