Abstract:
A fired heater is adapted for increasing the output of a plant where the furnace capacity is considerably improved without corresponding increase in the pressure drop. The described technique utilizes a parallel thermal path in contrast to the conventional series thermal path for heating a hydrocarbon fluid. The fluid is divided into at least two paths where the fluid in the first path is heated primarily by radiation heat transfer mechanism and the fluid in the second path is heated primarily by convection heat transfer mechanism. The at least two fluid streams may then be combined to continue with other desired processing of the fluid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     Statements Regarding Federally Sponsored Research or Development 
     Not Applicable. 
     Reference to a Microfiche Appendix 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to fired heaters, also known as process furnaces, and more specifically to fired heaters used in processing hydrocarbons. 
     2. Description of the Related Art 
     Typical fired heaters are designed to heat hydrocarbons. Numerous processes on hydrocarbons are carried out in furnaces commonly known as fired heaters, or process furnaces, or fired heater furnaces, pipe stills. 
     Fired heaters are equipment in which fluid is heated to high temperatures by burning fuel gas or fuel oil in a combustion chamber. The tubes carrying the fluid are located in the center or on sides in the combustion chamber. The combustion chamber is lined with refractory material. The hot flue gases in the vicinity of the burners transmit heat to the fluid feed primarily by radiant heat transfer mechanism. This part of the heater is known as the radiant section or firebox section. The flue gases leaving the radiant section are typically at 1400–1800° F. and more heat can be recovered from these gases. Additional heat is recovered in the convection section where the flue gases are cooled by exchanging the heat with the fluid. In heaters, fluid generally enters the convection section first and then flows through the radiant section to maximize the heat recovery. In some heaters, process fluid enters through the radiant section and leaves through the radiant section. In these heaters, heat in the convection section is recovered by generating steam or preheating other hydrocarbon services. Flue gases are disposed off to the atmosphere through a stack. 
     Most refineries possess catalytic reforming units. In these catalytic reforming units, a hydrocarbon, for example, light petroleum distillate (naphtha) is contacted with platinum catalyst at elevated temperature and pressure. This process produces high-octane liquid product that is rich in aromatic compounds. The process upgrades low octane number straight run naphtha to high-octane motor fuels. In a typical unit, the feed to the unit is mixed with recycle hydrogen gas and it is heated first in heat exchangers and then in a fired heater. The feed is then sent to a reactor. Most reactions that occur in the reactor are endothermic reactions and occur in stages. The reactors are separated into several stages. Inter stage heaters may be installed between the reaction stages to maintain the desired temperature of the hydrocarbon feed. 
     Refineries have been de-bottlenecking their units to improve the fired heater capacity and improve thermal efficiency of the system.  FIG. 1  illustrates the commonly practiced concept of the technique (prior art) used for heating the feed. A typical existing unit  100  comprises a convection section  120  and a radiant section  150 . The feed is first sent to the convection section  120  through a plurality of fluid passes  122 ,  124 ,  126 ,  128 , and  130 , comprising fluid oath  135  for example. The preheated fluid then enters the radiant section  150  where it is heated further and the fluid exits through a fluid exit path  140 . The fluid exiting the fluid path  140  may then be further passed through a series of concatenated fired heaters similar to the fired heater system  100 . 
     Alternatively, the fluid may be introduced directly into the radiant section or in the convection section. Typically, when the fluid is directly introduced in the radiant section, a significant a mount of heat energy remains in the flue gases. A portion of this remaining energy may be recovered in the convection section by generating steam, preheating combustion air, or preheating other streams. Often times the refiners do not need the steam and they do not have other attractive choices. 
     In such fired u nits, the feed consists of hydrocarbon vapors and recycle hydrogen gas. The feed in vapor form has a very large volume and pressure drop across the heater is very important. Low-pressure drop minimizes recycle gas compressor differential pressure and the necessary compressor horsepower. The result is lower utility consumption. Low-pressure drop also permits operation at lowest reactor pressure. As a result, the heaters are designed as all radiant heaters with large manifolds at the inlets and outlets. Convection sections are typically used for steam generation or other waste heat recovery operations. Often times, the byproducts of waste heat recovery are not needed, and the heat is discharged in to the atmosphere. 
     BRIEF SUMMARY OF THE INVENTION 
     Exemplary techniques for heating hydrocarbon fluids in fired heaters are illustrated in which the fluid is divided into at least two fluid paths. The fluid in the first path is heated by predominantly one heat transfer mechanism and the fluid in the second path is heated by predominantly a second heat transfer mechanism. Thus, effectively, the technique provides for parallel heat transfer paths. 
     A fired heater furnace is adapted for processing hydrocarbons fluids such that the fluid path is divided into a plurality of paths. The fluid in each path is heated by predominantly different heat transfer mechanisms. After heating the fluids in different heating paths, the fluids are combined. The combined fluid may again be heated in a furnace coupled to the first furnace. Alternately, the combined fluid may be processed in a reactor and then sent to another furnace for heating. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A better understanding of the present invention can be obtained when the following detailed description of some embodiments is considered in conjunction with the following drawings in which: 
         FIG. 1  is a conceptual block diagram of typical heating of hydrocarbon fluids in a furnace (prior art). 
         FIG. 2  is a conceptual block diagram of heating of hydrocarbon fluids in a furnace according to the invention. 
         FIG. 3  is a diagram depicting a typical example system of heating of hydrocarbon fluids in a furnace (prior art) of  FIG. 1 . 
         FIG. 4  is a diagram showing an exemplary embodiment according to the invention of  FIG. 2  showing division of the fluid flow and heating thereof. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As noted above, what the refiners and fired heater owners need is improved recovery of the thermal energy so that the waste energy can be used without being restricted to aforementioned choices. It would be preferable to utilize the waste thermal energy to increase production capacity of the unit rather than heat auxiliary products or discharge that energy to the atmosphere when heating the auxiliary products is not desired. Increasing production by improved utilization of the waste energy also contributes to the quality of environment in that efficient utilization of energy leads to reduced environmental energy discharge. Techniques and apparatus disclosed herein achieve that aim by increasing production capacity with significantly lower increase in capital cost and provide techniques of efficiently utilizing the energy produced in the fired heaters to increase the output. 
     With reference to  FIG. 2 , is a conceptual block diagram of heating of hydrocarbon fluids in a furnace  200  according to the invention. A fluid feed line  235  is divided into a first group of fluid passes  222 , and a second group of fluid passes  224 ,  226 ,  228 , and  230 , for example. The feed fluid through fluid passes  222 , and  224  is heated in the convection section  210 , and the feed fluid through fluid passes  224 ,  226 ,  228 , and  230  is heated in the radiation section  250 . The processed fluids are then recombined in the fluid outlet  240 . The feed fluid corning out from the fluid outlet  240  may then again be sent through a next fired furnace similar to the furnace  200 , and so on until desired products are obtained, or yet another process may need to be performed on the hydrocarbon fluid. Division of fluids into a number of flow paths is well within the skill of those practicing the art. 
     With reference to  FIG. 3  is a diagram depicting a typical example system of heating of hydrocarbon fluids in a furnace (prior art) of  FIG. 1 . The fired heater  300  as shown has a fired furnace  310  fluidically coupled to a similar fired furnace  310  where output of the furnace  310  is fed as input to the furnace  410 . In this manner, a plurality of furnaces may be cascaded to process the hydrocarbon fluids. There may be further processing of the fluid before it is sent from one furnace to the next furnace. The furnace  310  roughly comprises of two sections from the perspective of thermal energy delivery to the input fluids: a radiant (or radiation) section  315 , a convection section  320 , and a stack section  325  for exhaust of unusable waste energy. The furnace  310  has at least one or more burners  380 . The hydrocarbon fluid enters the furnace  310  through path  338 . The fluid pressure and temperature are monitored at nodes  340 ,  350 ,  370  and  375 . There may be an optional manifold valve  375  to control the flow of the hydrocarbon fluids. When the fluid enters through the node  340 , it is first heated in the convection section  320 , and then heated in the radiant section  315 . The fluid heated in the radiant section then exits through the nodes  370  and  365  for further processing as desired. 
     Following TABLE I shows the pressure and temperature at the input node  340  and output node  365  of an example furnace of the conventional design, with a flow rate of 333,890 Lb/Hr. The TABLE I is further discussed below in the context of the invention to demonstrate the effect of implementing the invention. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                 Process 
                   
                 Node 
                 Node 
               
               
                 S.N. 
                 Conditions 
                 Units 
                 340 
                 365 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Flow rate 
                 Lb/hr 
                 333,890 
                 333,890 
               
               
                 2 
                 Opr. Temp. 
                 ° F. 
                 785 
                 985 
               
               
                 3 
                 Opr. Pres. 
                 Psi 
                 178.2 
                 174.0 
               
               
                   
               
             
          
         
       
     
     Now referring to  FIG. 4 , a diagram of an exemplary embodiment of fired heaters  500  according to the invention of  FIG. 2  is shown. The fired heater  500  as shown has a fired furnace  510  fluidically coupled to a similar fired furnace  610  where output of the furnace  510  is fed as input to the furnace  610 . In this manner, a plurality of furnaces may be cascaded to process the hydrocarbon fluids. The fluid heated in the furnace  510  may be processed in a reactor  567  to perform chemical reactions or other desired processing and then sent to furnace  610  for further heating. Since furnaces  510  and  610  are substantially alike, it would suffice to illustrate the technique and apparatus of the invention with reference to furnace  510 . 
     Again, referring to  FIG. 4 , the furnace  510  roughly comprises of two sections from the perspective of thermal energy delivery to the input fluids: a radiant (or radiation) section  515 , a convection section  520 , and a stack section  525  for exhaust of unusable waste energy. The furnace  510  has at least one or more burners  580 . An input hydrocarbon fluid path  538  is divided into at least two fluid paths, a first fluid path  530 , and a second fluid path  535 . In general, each of the fluid paths comprises a plurality of fluid passes. The fluid going into the first fluid path  530  is heated predominantly by radiation heat transfer mechanism. Since the fluid in the first fluid path  530  traverses in fluid passes which are in closer proximity of the burners  580 , the heat transfer mechanism is predominantly by radiation and to a secondary extent, the heat transfer mechanism is convection. It is estimated that various fluid passes in the radiation section  515  receive heat energy somewhere between 80–85% through the radiation heat transfer mechanism and remaining energy by the convection heat transfer mechanism. Likewise, it is estimated that various fluid passes in the convection section  520  receive heat energy somewhere between 80–85% through convection heat transfer mechanism and the remaining energy through the radiation heat transfer mechanism. Thus, the nomenclature of naming the sections of the furnaces should be understood to mean as involving the dominant heat transfer mechanism in those sections resulting from the proximity to the burners  580 . The estimated percentages may vary significantly in installation to installation due to their geometry and construction materials employed therein. 
     Referring to  FIG. 4  again, a certain fluid pressure at an input node  540  is maintained. The input fluid is divided by means of a divider  555  such that a reasonable fluid pressure differential between a node  550  and a node  560  is maintained. The fluid flow divider  555  may be a manifold, a fixed size orifice, or a manually controllable valve, or an automatically controllable valve that can maintain or control a pressure differential between the node  550  and the node  560 . Those skilled in the art may employ numerous other alternatives to maintain such pressure differential. As may be noted the fluid passing through the first path  530  is heated in the radiation section  515 , and the fluid passing through the second path  535  is heated in the convection section  520 . The fluids after being heated in the radiation section  515  and the convection section  520  are again combined in a manifold  575  for further heat treating and may be sent to another furnace  610  coupled to the furnace  510 . The process may be carried out in as many stages as required according to the need of the chemical reaction or the desired product. Table II shows the pressure levels at nodes  540 ,  545 ,  550  (input side nodes) and nodes  560 ,  565 ,  570  (output side nodes) of an exemplary implementation of the technique utilized in the apparatus illustrated herein. 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                   
                 Process 
                   
                 Node 
                 Node 
                 Node 
                 Node 
                 Node 
                 Node 
               
               
                 S.N. 
                 conditions 
                 Units 
                 540 
                 550 
                 570 
                 545 
                 560 
                 565 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Flow rate 
                 Lb/hr 
                 333,890 
                 258,970 
                 258,970 
                 74,920 
                 74,920 
                 333,890 
               
               
                 2 
                 Opr. Temp. 
                 ° F. 
                 785 
                 785 
                 985 
                 785 
                 985 
                 985 
               
               
                 3 
                 Opr. Pres. 
                 Psi 
                 178.1 
                 178.1 
                 175.7 
                 178.1 
                 175.7 
                 175.7 
               
               
                   
               
             
          
         
       
     
     Note that the pressure differential between the input side nodes ( 540 ,  545 , and  550 ) and the output side nodes ( 560 ,  565 , and  570 ) in the exemplary system is merely 2.4 psi. This low-pressure differential attained through the illustrated technique reduces power consumption used in the compressors and thus the size of the compressors may be accordingly reduced to maintain the same fluid flow. Lower pressure differential also permits the reactor operation at lower pressure. The advantageous lower pressure operation may also be utilized in designing relative sizes of the radiant section and the convection section to further optimize performance of a fired heater. 
     Now referring to Table I and Table II, it can be seen that the fluid pressure drop from the input node  340  to the output node  365  for the conventional fired heater system  300  is 4.2 psi. The corresponding pressure drop from the input node  540  to the output node  565  for the fired heater system of the exemplary illustrated system is mere 2.4 psi, i.e., input to output side pressure drop of the conventional system in this example is about 75% higher than the exemplary system. 
     Note that the higher pressure drop of the conventional design limits the performance of pumps and compressors and consumes substantial amount of energy. The performance of heaters illustrated in both cases is determined by performing simulations using a widely used computer program known as “DIRECT FIRED HEATERS FNRC-5” developed by PFR Engineering Systems, Inc. of Los Angeles, Calif. 
     Another major advantage of the technique and the apparatus illustrated herein is the reduction in initial cost resulting due to savings in the required external piping. In the conventional design, the full size inlet manifold and piping needs to be relocated to the convection section. In the illustrated technique, the apparatus, and the system, the size of manifold and piping is substantially reduced. 
     The techniques and the illustrated apparatus may be used to heat any kind of hydrocarbons fluid with proper adjustment of the size of the apparatus whether for production or development in the laboratories. Such adjustments in the size and routine fabrication details are within the skills of those practicing the art. 
     The foregoing disclosure and description of the preferred embodiments are illustrative and explanatory thereof, and various changes in the components, the fired heater configurations, and configurations of the techniques, as well as in the details of the illustrated apparatus and techniques of operation may be made without departing from the spirit and scope of the invention as claimed in the appended claims.