Abstract:
In one aspect, the present invention relates to a furnace having a heated portion arranged adjacent to an unheated portion. A plurality of straight tubes are formed of a first material and are at least partially disposed in the heated portion. A plurality of return bends are operatively coupled to the plurality of straight tubes. The plurality of return bends are formed of a second material and are at least partially disposed in the unheated portion. The first material exhibits a maximum temperature greater than the second material thereby facilitating increased run time of the furnace. The second material exhibits wear-resistance properties greater than the first material thereby facilitating wear-resistance of the furnace.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to, and incorporates by reference for any purpose the entire disclosure of, U.S. Provisional Patent Application No. 61/774,421, filed Mar. 7, 2013. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to an apparatus for refining operations, and more particularly, but not by way of limitation, to delayed coking operations utilizing a heater coil having straight tubes constructed of a first material and return bends constructed of a second material wherein the first material and the second material exhibit differing thermal properties, in particular, but not by way of limitation, design-maximum tube-metal temperatures. 
         [0004]    2. History of the Related Art 
         [0005]    Delayed coking refers to a refining process that includes heating a residual oil feed, made up of heavy, long-chain hydrocarbon molecules, to a cracking temperature in a furnace. Typically, furnaces used in the delayed coking process include a plurality of tubes arranged in a multiple-pass configuration. Heating of the residual oil feed cracks the heavy, long-chain hydrocarbon molecules producing gas, lightweight products, and solid coke. The gas and lightweight products are further refined into various liquid fuels and gas fuels. The solid coke is subsequently crushed and sold as a fuel source. 
         [0006]    During the delayed coking process, solid coke forms on an inside surface of the plurality of tubes. This phenomenon is known as “fouling.” Solid coke is an insulator and causes a temperature of a material forming the plurality of tubes (referred to herein as a “tube-metal temperature”) to incrementally increase during operation. For example, a clean tube may require a tube-metal temperature of, for example, 945° F. to heat the residual oil feed to 900° F. In contrast, a fouled tube might require a substantially higher tube-metal temperature to heat the residual oil feed to 900° F. Over a period of use, the plurality of tubes eventually reach a design-maximum tube-metal temperature. As used herein, the term “design-maximum tube metal temperature” refers to a maximum safe operating temperature of the plurality of tubes. Above the design-maximum tube metal temperature, thermal stresses can contribute to wear and fatigue of the plurality of tubes thereby rendering the furnace unsafe for operation. Upon reaching the design-maximum tube-metal temperature, the plurality of tubes must be cleaned to remove the solid coke. Cleaning brings the plurality of tubes back to the tube-metal temperature conditions associated with a clean tube. 
         [0007]    Cleaning the plurality of tubes typically involves at least one of mechanical cleaning, steam-air decoking, pigging, or online spalling. Online spalling involves removing a fouled pass including a plurality of tubes from service and thermally shocking the plurality of tubes. The plurality of tubes are rapidly heated (expanded) and cooled (contracted) over a set period of time. During cooling, the fouled tube contracts causing a portion of the solid coke accumulated therein to break free. The solid coke is flushed out of the fouled tube and processed in a coke drum. The advantage of online spalling is that only one pass is spalled at a time allowing remaining passes to operate normally. However, the efficacy of online spalling may decrease each time it is performed. 
         [0008]    Pigging involves passing a foam or plastic “pig” having metal studs and grit through the tube. As the pig passes through the fouled tube, the pig rotates and scrapes the solid coke from an inside surface of the fouled tube. Steam-air decoking involves circulating a steam-air mixture through the plurality of tubes at elevated temperatures. Air from the steam-air mixture is used to burn the solid coke from the inside surface of the plurality of tubes while steam from the steam-air mixture ensures that the burning temperatures do not exceed the design-maximum tube-metal temperature. 
         [0009]    In most cases, during cleaning, at least a pass of the plurality of tubes must be removed from the residual oil feed. In some cases, the entire furnace must be taken out of service. This results in a reduction of productivity and a loss of profits. Thus, it is of great importance to design the furnace to maximize a period of time between cleanings. 
         [0010]    U.S. Pat. No. 7,670,462, assigned to Great Southern Independent L.L.C., relates to a system and method for on-line cleaning of black oil heater tubes and delayed coker heater tubes. A high-pressure water charge is injected through the heater tubes during normal process operations to prevent heater tube fouling and downtime. The water charge undergoes intense boiling and evaporation. The intense boiling induces a scrubbing action within the heater tubes. 
         [0011]    Furthermore, a shocking action is induced by expansion and contraction of the heater tubes resulting from the water charge flowing through the heater tubes followed by a hotter process fluid flowing through the heater tubes. 
         [0012]    U.S. Patent Application Publication No. 2007/0158240, assigned to D-COK, LP relates to a system and method for on-line spalling of a coker. An off-line heater pipe is added to on-line coker heater pipes. When an on-line pipe is to be spalled, flow is diverted to the off-line pipe thus allowing for full operation of the coker heater. 
       SUMMARY 
       [0013]    The present invention relates generally to refining operations. In one aspect, the present invention relates to a furnace having a heated portion arranged adjacent to an unheated portion. A plurality of straight tubes are formed of a first material and are at least partially disposed in the heated portion. A plurality of return bends are operatively coupled to the plurality of straight tubes. The plurality of return bends are formed of a second material and are at least partially disposed in the unheated portion. The first material exhibits a design-maximum tube-metal temperature greater than the second material thereby facilitating increased run time of the furnace. The second material exhibits wear-resistance properties greater than the first material thereby facilitating wear-resistance of the furnace. 
         [0014]    In another aspect, the present invention relates to a method of manufacturing a heater process coil. The method includes forming a plurality of straight tubes from a first material and forming a plurality of return bends from a second material. The plurality of straight tubes are joined to the plurality of return bends. The plurality of straight tubes and the plurality of return bends are oriented within a furnace such that the plurality of straight tubes are at least partially disposed within a heated portion and the plurality of plug headers are at least partially disposed within an unheated portion. The first material exhibits a design-maximum tube-metal temperature greater than the second material thereby facilitating increased run time of the furnace. The second material exhibits wear-resistance properties greater than the first material thereby facilitating wear-resistance of the furnace. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A more complete understanding of the method and system of the present invention may be obtained by reference to the following Detailed Description when taken in conjunction with the accompanying drawings wherein: 
           [0016]      FIG. 1  is a schematic diagram of a refining system according to an exemplary embodiment; 
           [0017]      FIG. 2A  is a plan view of a furnace according to an exemplary embodiment; 
           [0018]      FIG. 2B  is a cross-sectional view of a furnace tube showing an accumulation of solid coke therein; and 
           [0019]      FIG. 3  is a flow diagram of a process for manufacturing a heater coil according to an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Various embodiments of the present invention will now be described more fully with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, the embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
         [0021]      FIG. 1  is a schematic diagram of a refining system according to an exemplary embodiment. A refining system  100  includes an atmospheric-distillation unit  102 , a vacuum-distillation unit  104 , and a delayed-coking unit  106 . In a typical embodiment, the atmospheric-distillation unit  102  receives a crude oil feedstock  120 . Water and other contaminants are typically removed from the crude oil feedstock  120  before the crude oil feedstock  120  enters the atmospheric distillation unit  102 . The crude oil feedstock  120  is heated under atmospheric pressure to a temperature range of, for example, between approximately 650° F. and approximately 700° F. Lightweight materials  122  that boil below approximately 650° F.-700° F. are captured and processed elsewhere to produce, for example, fuel gas, naptha, gasoline, jet fuel, and diesel fuel. Heavier materials  123  that boil above approximately 650° F.-700° F. (sometimes referred to as “atmospheric residuum”) are removed from a bottom of the atmospheric-distillation unit  102  and are conveyed to the vacuum-distillation unit  104 . 
         [0022]    Still referring to  FIG. 1 , the heavier materials  123  enter the vacuum-distillation unit  104  and are heated at very low pressure to a temperature range of, for example, between approximately 700° F. and approximately 800° F. Light components  125  that boil below approximately 700° F.-800° F. are captured and processed elsewhere to produce, for example, gasoline and asphalt. A residual oil feed  126  that boils above approximately 700° F.-800° F. (sometimes referred to as “vacuum residuum”) is removed from a bottom of the vacuum- distillation unit  104  and is conveyed to the delayed-coking unit  106 . 
         [0023]    Still referring to  FIG. 1 , according to exemplary embodiments, the delayed-coking unit  106  includes a furnace  108  and a coke drum  110 . The residual oil feed  126  is preheated and fed to the furnace  108  where the residual oil feed  126  is heated to a temperature range of, for example, between approximately 900° F. and approximately 940° F. After heating, the residual oil feed  126  is fed into the coke drum  110 . The residual oil feed  126  is maintained at a pressure range of, for example, between approximately 25 psi and approximately 75 psi for a predetermined cycle time until the residual oil feed  126  separates into hydrocarbon vapors and solid coke  128 . In a typical embodiment, the predetermined cycle time is approximately 10 hours to approximately 24 hours. Separation of the residual oil feed  126  is known as “cracking.” The solid coke  128  accumulates starting at a bottom  130  of the coke drum  110 . 
         [0024]    Still referring to  FIG. 1 , according to exemplary embodiments, after the solid coke  128  reaches a predetermined level in the coke drum  110 , the solid coke  128  must be removed from the coke drum  110  through, for example, mechanical or hydraulic methods. Removal of the solid coke  128  from the coke drum  110  is known as, for example, “cutting,” “coke cutting,” or “decoking.” Flow of the residual oil feed  126  is diverted from the coke drum  110  to at least one second coke drum  112 . The coke drum  110  is then steamed to strip out remaining uncracked hydrocarbons. After the coke drum  110  is cooled by, for example, water injection, the solid coke  128  is removed via, for example, mechanical or hydraulic methods. The solid coke  128  falls through the bottom  130  of the coke drum  110  and is recovered in a coke pit  114 . The solid coke  128  is then shipped from the refinery to supply the coke market. In various embodiments, flow of the residual oil feed  126  may be diverted to the at least one second coke drum  112  during decoking of the coke drum  110  thereby maintaining continuous operation of the refining system  100 . 
         [0025]    While cracking of the residual oil feed  126  primarily takes place within the coke drum  110 , premature cracking often occurs within portions of the furnace  108 . Premature cracking leads to fouling of the furnace  108  thereby necessitating periodic cleaning of the furnace  108 . Increased feed rates commonly associated with many refining operations present the potential for rapid fouling of the furnace  108 . In many cases, any increase in productivity of the furnace  108  results in increased production throughout the refining system  100 . 
         [0026]    To this end, efforts have been made to construct the furnace  108  from materials having higher design-maximum tube-metal temperatures. For example, austenitic materials such as, for example, TP347H have a design-maximum tube-metal temperature approximately 200° F. higher than commonly-used ferritic materials such as, for example, 9Cr-1Mo; however, austenitic materials are considerably softer than ferritic materials and often experience excessive wear and erosion. Such wear and erosion can lead to premature failure of the furnace  108  resulting in loss of production and costly repairs. Thus a design of the furnace  108  is needed that utilizes materials of sufficient strength to prevent premature wear of the furnace  108  but allows for a longer operation time between successive cleanings. 
         [0027]      FIG. 2A  is a plan view of a furnace according to an exemplary embodiment.  FIG. 2B  is a cross-sectional view of a furnace tube showing an accumulation of solid coke therein. Referring to  FIGS. 2A and 2B , a furnace  200  includes a heater process coil  202  arranged in a plurality of flow passes  204 . In various embodiments, the furnace  200  may be, for example, a delayed coker heater, a crude heater, a vacuum heater, a visc breaker heater, or any other appropriate device for heating fluid in a refining operation. The plurality of flow passes  204  includes a plurality of straight tubes  206  connected to a plurality of return bends  208  and a plurality of plug headers  209 . In a typical embodiment, the plurality of return bends  208  are wrought or cast 180° bends with a heavy back wall that connect, at one end, two straight tubes of the plurality of straight tubes  206 . In some embodiments, furnaces utilizing principles of the invention may include return bends at both ends of the straight tubes  206 . The plurality of plug headers  209  are cast and are disposed at an opposite end of the plurality of straight tubes  206  and connect two straight tubes of the plurality of straight tubes  209 . The plurality of return bends  208  and the plurality of plug headers  209  are disposed outside of a heated portion  210  of the furnace  200 . Thus, in a typical embodiment, the tube-metal temperature of the plurality of return bends  208  and the plurality of plug headers  209  will not exceed a temperature of a fluid  212  contained therein. The plurality of straight tubes  206  are located within the heated portion  210  of the furnace  200 . Thus, a tube-metal temperature of the plurality of straight tubes  206  will be higher than the temperature of the fluid  212  contained therein due to an insulating effect of the solid coke  128  accumulated therein. In a typical embodiment, a maximum tube-metal temperature of a clean straight tube  206  is approximately 1030° F. 
         [0028]    Still referring to  FIGS. 2A and 2B , during operation of the furnace  200 , the tube-metal temperature of the plurality of straight tubes  206  rises at a rate of approximately 1.5° F. per day due to accumulation of solid coke therein. For straight tubes  206  constructed of ferritic material such as, for example, 9Cr-1Mo, an online spalling process begins when the tube-metal temperature of the plurality of straight tubes  206  reaches, for example, approximately 1250° F. or more. As previously discussed, online spalling requires removing at least one flow pass of the plurality of flow passes  204  from operation. Use of austenitic materials such as, for example, TP347H in the plurality of straight tubes  206  allows for an additional 200° F. of temperature rise. This additional temperature rise equates to approximately an additional 130 days of operation between cleanings thereby increasing productivity and profit. However, due to the relative softness of austenitic material, the plurality of return bends  208  and the plurality of plug headers  209  are particularly vulnerable to excessive wear and erosion during spalling. This results in premature failure of the plurality of return bends  208  and the plurality of plug headers  209 . 
         [0029]    Still referring to  FIGS. 2A and 2B , in a typical embodiment, the heater process coil  202  includes the plurality of straight tubes  206  constructed of an austenitic material such as, for example, TP347H and the plurality of return bends  208  and the plurality of plug headers  209  constructed of a ferritic material such as, for example, 9Cr-1Mo. The plurality of return bends  208  and the plurality of plug headers  209  are connected to the plurality of straight tubes  206  through a connection process such as, for example, welding. As previously mentioned, the plurality of straight tubes  206 , constructed of the austenitic material, are located within the heated portion  210  of the furnace  200  and the plurality of return bends  208  and the plurality of plug headers  209 , constructed of the ferritic material, are located outside of the heated portion  210  of the furnace  200 . By placing the plurality of return bends  208  and the plurality of plug headers  209  outside of the heated portion  210 , it becomes less likely that the plurality of return bends  208  and the plurality of plug headers  209  will reach the design-maximum tube-metal temperature associated with the ferritic material. Because the ferritic material is harder than the austenitic material, such a configuration allows the benefit of longer run times without problems associated with premature failure of the plurality of return bends  208  and the plurality of plug headers  209 . 
         [0030]    The advantages of such an arrangement will be apparent to one skilled in the art. For example, by constructing the plurality of straight tubes  206  of the austenitic material, the furnace  200  can operate for approximately an additional  130  days between cleanings thereby increasing productivity and profit. In addition, constructing the plurality of return bends  208  and the plurality of plug headers  209  from the ferritic material reduces wear and erosion of the plurality of return bends  208  and the plurality of plug headers  209 . However, by placing the plurality of return bends  208  and the plurality of plug headers  209  outside of the heated portion  210 , an operation of the furnace  200  is not limited by the lower design-maximum tube-metal temperature associated with the ferritic material. 
         [0031]      FIG. 3  is a flow diagram of a process for manufacturing a heater process coil according to an exemplary embodiment. A process  300  begins at step  302 . At step  304 , a plurality of straight tubes are formed of an austenitic material. At step  306 , a plurality of return bends and a plurality of plug headers are formed of a ferritic material. At step  308 , the plurality of straight tubes, the plurality of return bends, and the plurality of plug headers are joined together end-to-end through a connection process such as, for example, welding. According to an exemplary embodiment, care must be taken to utilize a welding material that is compatible with both the ferritic material, the austenitic material, and any fluid that may be disposed therein. That is, the welding material must not induce corrosion of either the ferritic material or the austenitic material. Furthermore, the welding material must accommodate a thermal expansion differential between the ferritic material and the austenitic material. 
         [0032]    Still referring to  FIG. 3 , at step  310 , the process heater coil is secured in a furnace such that the plurality of straight tubes are secured within a heated portion of the furnace and the plurality of return bends and the plurality of plug headers are disposed outside of the heated portion. The process  300  ends at step  312 . Such an arrangement allows greater operation time of the heater coil between successive cleanings while, at the same time, guards the plurality of return bends against premature wear or failure. 
         [0033]    Although various embodiments of the method and system of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth herein.