Reformer process with variable heat flux side-fired burner system

An apparatus for heating a fluid in a process having a process heat requirement includes a shell, at least one tube having a design temperature inside the shell, a plurality of burners adjacent the inner wall of the shell, and transfer means whereby flue gas flows from a first interior region to a second interior region of the shell. A first portion of the tube is in the first interior region and a second portion of the tube is in the second interior region. The first and second portions of the tube are adapted to contain a flow of the fluid. The burners produce the flue gas in the first interior region and a variable heat flux approximating the process heat requirement and simultaneously maximizing the heat flux to the first portion of the tube while maintaining substantially all of the first portion of the tube at the design temperature.

DETAILED DESCRIPTION OF THE INVENTION The invention is an apparatus and a method for an advanced reforming process using a variable heat flux side-fired burner system. The invention is not, however, limited to reforming applications. Persons skilled in the art will recognize that the apparatus, method, and burner system may be used in many other fired process heating applications, including but not limited to fluid heating and hydrocarbon cracking (e.g., ethylene cracking). The process uses a reformer (or furnace) that has the following features: an integrated side-fired burner that produces a prescribed variable heat flux profile tailored to the process to achieve maximum capacity and maximum thermal efficiency, and avoids coking at the tube inlet; firing on both sides of tubes in the lower region (adjacent the tube outlet) to maximize the heat flux without overheating the tube (in the fired region) and no firing in the upper region, with optimal radiant heat transfer between the process gas and the flue gas due to counter-current flow; radially-directed firing and process tube alignment for optimal heat transfer around the tube circumference; unique burner system design to provide continuous and linear firing with variable heat flux; and optional refractory walls that extend radially from the inner wall of the furnace toward the center of the furnace, wherein the burners fire on both sides of each refractory wall, which helps to achieve the optimal heat flux distribution to the tubes. FIGS. 3 and 4 show a schematic of the reformer 20 of the present invention (without optional refractory walls), and FIGS. 5 and 6 show a schematic of the reformer with optional refractory walls 22 . Referring to FIGS. 3 - 6 , the reformer 20 of the present invention includes a refractory lined shell 24 . In a preferred embodiment, the shell is cylindrical. However, persons skilled in the art will recognize that the cross-sectional area of the shell may have alternate shapes, such as a polygon (e.g., a triangle, square, rectangle, pentagon, hexagon, octagon, etc.), an ellipse, or other shapes. Multiple variable heat flux burners 26 are located adjacent the inner wall of the shell 24 near the lower end of the shell. In a preferred embodiment, the burners are recessed in the refractory lining of the inner wall. As shown in FIGS. 3 and 5 , envelopes of the flames 28 are sheet-like. At the upper end of the shell (opposite the burner end), there are one or more openings 30 that allow the flue gas (containing combustion products) to flow from the shell. Conventional reformer tubes 32 containing catalyst are positioned within the interior of the shell to utilize high intensive radiant heat directly from the flames of the burners. In the preferred embodiment, the burners are equally spaced apart peripherally around the inner wall of the reformer, with neighboring burners being equidistant from one or more reformer tubes positioned in a vertical plane midway between the neighboring burners. Persons skilled in the art will recognize that the burners may be arranged differently in other embodiments, one of which is illustrated in FIGS. 7 - 10 . As shown in FIG. 7 , one or more burners 26 may be arranged within each of the (four) sectors of the reformer 20 to achieve the desired variable heat flux profile to the tubes 32 in the lower region of the reformer. As shown in FIGS. 8, 9 and 10 , multiple burners are fired in each pie-shaped sector (“quadrant”) at different elevations on both sides of tubes arranged in a given ray of tubes. FIG. 8 shows three burners firing within each quadrant and within a given slice of the reformer as a means of increasing the heat release rate within that slice of the furnace. FIG. 9 shows two burners firing within each quadrant and within a given slice of the reformer as a means of increasing the heat release rate within that slice of the furnace. And FIG. 10 shows one burner firing within each quadrant and within a given slice of the reformer as a means of increasing the heat release rate within that slice of the furnace. The burners in FIGS. 7 - 10 are arranged as shown to ensure that the amount of heat that a given segment of each tube receives is approximately equal. (Persons skilled in the art will recognize that many burner arrangements other than that shown in FIGS. 7 - 10 are feasible. For example, one or more additional burners could be added in each sector at each elevation in FIGS. 8 - 10 .) In the preferred embodiment, the tubes 32 are arranged in four sectors of the reformer 20 and one variable heat flux burner 26 is located at the inner wall in each sector, as shown in FIGS. 4 and 6 . (Persons skilled in the art will recognize that the reformer may be evenly divided into any number of equally-sized sectors with a ray of tubes in each sector.) Each burner produces a substantially continuous flame along the height of the burner. The flame front only extends a short distance from the burner toward the centerline of the reformer. Heat is radiated directly from the flame and the burner tile to the tubes and is emitted and reflected from the inner wall. Mixed-feed enters the inlet header 34 and is distributed to the upper end of each reformer tube 32 . Product synthesis gas exits at the lower end of each reformer tube and is removed from the reformer at the outlet header 36 , and the flue gas exits from openings 30 at the top of the reformer. FIGS. 5 and 6 show a reformer that incorporates four vertical refractory walls 22 , one in each sector. The hot flue gases (containing combustion products) flow radially across both sides of each refractory wall, which is thereby heated and radiates heat to the tubes. The refractory walls 22 are perpendicular to the inner wall of the shell 24 . These refractory walls may be made of a composite of conventional refractory materials, such as high temperature fired bricks, or a solid casting of a refractory, such as alumina. Fuel is fired in the lower region of the reformer 20 . The fuel is fired via the variable heat flux burners 26 , which produce a prescribed heat flux profile along the tube length—one that is tailored to the specific requirements of the process (reforming, or cracking, or other) taking place inside the tube. This prescribed heat flux profile simultaneously addresses constraints such as design tube metal temperature and coking, and maximizes average heat flux (capacity) and thermal efficiency. The heat flux profile produced by the variable heat flux burner 26 is designed to radiate the maximum amount of heat to each segment of the tube 32 in the fired zone without exceeding the tube design temperature limits. The objective is to provide the maximum possible heat flux through each tube segment that is located in the vicinity of the process outlet. (For reforming applications, the maximum heat flux is tied to the reforming reaction process that occurs as the gas flows through the tube.) As an intentional result, the tube wall temperature is maintained at a uniform value over most of the entire fired length of the tube. For a cylindrical reformer design, each variable heat flux burner 26 is located along the inner wall of the reformer 20 mid-way between two rays of tubes 32 (as illustrated in FIGS. 4 and 6 ), and an optional refractory wall 22 extends in the radial direction along the center line of each burner (as illustrated in FIG. 6 ). The optional refractory walls are incorporated along the height of the fired region, but not in the unfired region. The refractory walls are used to ensure that the optimal heat flux distribution is attained in the fired region. In the preferred embodiment, each variable heat flux burner 26 fires fuel substantially continuously over the height of the reformer 20 occupied by the burner. The heat release intensity per unit length of the burner is very low and varies smoothly along the height of the burner. The firing rate per burner 26 can be varied for startup and operation at reduced production rates. An objective is to achieve the same relative heat release profile (% of total heat release versus burner height) over the burner capacity range. (Persons skilled in the art will recognize that an adjustable burner could be designed to vary the relative heat release profile.) In this way, the variable heat flux profile produced by the burner satisfies all of the constraints (e.g., tube temperature, coking problem) over the turndown range. At turndown conditions, the tube wall temperature will again be similar to design conditions, only cooler. For the configurations shown in FIGS. 3 and 5 , combustion products flow upward. The combustion products produced by the lower sections of the variable heat flux burners 26 combine with the combustion products produced by the upper sections of the burners and flow upward. The upper region of the reformer 20 is not fired. The flue gas (containing combustion products) flows counter-currently with the incoming process gas and exits at the top of the reformer. Counter-current flow helps to maximize the heat flux to the tubes in the upper region of the furnace by maximizing the temperature driving force. This arrangement helps to maximize throughput (capacity) while simultaneously achieving maximum possible radiant efficiency. The improvement in radiant efficiency at reduced rates will be even better than any improvement realized with co-current reforming technology. The burner and tile design, as well as the design layout of the reforming tubes 32 , achieve a uniform heat flux at each elevation in the reformer 20 , both around the circumference of individual tubes and from tube to tube. As a result, the ratio of the peak heat flux to the worst tube (tube with the maximum local heat flux at a given elevation) and the average heat flux to all the tubes at the same elevation is held as close to unity as possible. The optional refractory walls 22 further ensure that this challenging objective is met. FIG. 11 plots the process duty-process temperature curve, also known as an S curve, for typical reformer tube design conditions (space velocity, catalyst size and shape, end-of-run catalyst activity). This plot is obtained from a steam methane reformer simulation program. At a given distance from the inlet of the tube, an amount of heat is transferred to the process gas that causes both the sensible and chemical enthalpy of the process gas to increase. The S curve plots the fraction of heat absorbed (from the inlet to a given point inside the tube) versus the process gas temperature at that point. The reformer tube is designed for a given operating life corresponding with specified limits on tube wall temperature and process pressure. Once the local conditions inside of the tube are known, it is possible to calculate the maximum local heat flux that the tube can sustain. FIG. 12 plots the maximum heat flux as a function of the process temperature for the conditions corresponding to FIG. 11 . From the above discussion, it is clear that limits exist on the magnitude of heat flux to the tube and these limits depend on the process temperature and extent of the reforming reaction. This information is of key importance in specifying the design requirements of the variable heat flux burner for the side-fired application. To maximize the reformer efficiency with downward process flow, it is desirable to maximize the firing in the bottom region of the reformer and to allow counter-current heat exchange between the combustion products and incoming process gas. FIG. 13 illustrates the shape of the optimal firing profile for reforming natural gas with steam to produce hydrogen. This profile is for one specified design tube metal temperature and specific fixed process conditions. In other words, the reformer tubes are designed for one temperature and the process conditions of temperature and pressure are fixed, as is the capacity (i.e., fixed amounts of natural gas and steam are fed to the reformer). The amount of fuel fired per unit increment of reformer height tends to increase from a low value at the bottom of the reformer. The amount of fuel fired to the upper-most section of the burner tends to be less than the amount set by tube temperature constraints (reach target on total firing duty). FIG. 14 shows the corresponding tube heat flux profile. In the fired region, the heat flux reaches the maximum heat flux limits as shown in FIG. 12 . In the unfired region, the heat fluxes are below these limits. FIG. 15 plots the corresponding temperature profiles for flue gas, tube wall and process gas. In the fired region, the tube wall temperature is held at the design limits. With counter-current flow, the difference between the flue gas and tube wall temperatures is maximized and the temperature profiles do not pinch at the flue gas exit. The advantages of the novel burner and arrangement of the present invention include: the variable heat flux burner produces the desired heat flux profile along the tube to maintain relatively constant reformer tube metal temperature in the fired region; fewer burners, piping, valves and instrumentation are required, thereby minimizing capital cost for the reformer, or heater, or ethylene cracker; and the furnace (or reformer) is capable of producing higher capacity and higher radiant efficiency. Also, the unique placement of the variable heat flux burner solves the coking problem that occurs at the tube inlet (in conventional reformers) and extends run time between catalyst changes. In a typical commercial reformer, such as that illustrated schematically in FIG. 1 , process gas flow is upward and the reformer 10 is up-fired from three burners 12 located near the center of the furnace floor. The reformer contains tubes in a cylindrical arrangement. 1 TABLE 1 Comparison of physical configuration Present Reformer Design Prior Art Invention Number of burners 3 4 Firing direction Upward Side Process flow direction Upward Downward In comparing a typical commercial reformer with a reformer according to the present invention, the following parameters were kept at the same values for both reformers for the comparison: inside furnace diameter; inside furnace height; reformer tube design temperature and pressure; catalyst activity; catalyst size and shape (single hole); air preheat temperature; and steam-to-carbon (S/C) ratio. The following benefits of the reformer according to the present invention were expected in making this comparison: 30% increase in H 2 production capacity (with same size furnace); 12% less firing per unit H 2 production; convection section is smaller because of 12% reduction in flue gas flow; 38% less catalyst per unit H 2 production; 40% less tube material per unit H 2 capacity; and 40% increase in average heat flux based on inside tube surface area. Additional work also was done to examine the heat flux distributions within this novel reformer which utilizes variable heat flux burners. The results indicate (for a furnace with 4 sectors) that the heat flux around the tubes and from tube to tube is expected to be uniform at each elevation. FIGS. 16 - 23 show several arrangements of the variable heat flux burner 26 of the present invention. (Persons skilled in the art will recognize that other burner arrangements are possible.) An important feature of the burner used in the preferred embodiment of the present invention is that it is a relatively long burner, preferably oriented in the vertical direction on the inner wall of the reformer 20 . To be flexible, the burner may be divided into multiple sections, as shown in FIGS. 16 - 18 . But all sections preferably share a common air supply 38 and a common fuel supply 40 . This way, the piping, valves, and controls are simplified. Each section has a predetermined firing pattern. When joined together, the multi-section burner produces a heat release profile that optimally matches for the process conditions. As shown in FIGS. 16 - 18 , each section has the same way of introducing fuel and air that produces a compact flame, similar to a conventional wall radiant burner. But it is stressed that FIGS. 16 - 18 , show a single burner 26 with multiple sections, rather than multiple burners, because all sections share a common air supply 38 , a common fuel supply 40 , and a common burner control system (not shown). FIG. 16 is a side view of one design of a variable heat flux burner 26 used in the present invention. Air 38 enters a conduit 42 at the bottom or top of the burner and air flow to each burner unit or section is regulated by a damper 44 . A flame deflector 46 may be associated with each burner unit or section to shape the flame. FIG. 17 provides a view of the burner without deflectors as viewed from inside the reformer 20 . FIG. 18 provides a view of the burner from outside of the reformer 20 showing the supply of fuel 40 to the individual burner units or sections via a distribution system including a manifold 48 , piping 50 and control valves 52 . Different types of burner tips may be used with the burner 26 . FIG. 19 is a schematic of one such burner tip 54 , and FIG. 20 is a schematic of another such burner tip 56 . As indicated, the flame 28 in the preferred embodiment has a sheet-like shape shown in FIG. 3 and also in FIG. 21 . The sheet-like shape may be generated from a variable heat flux burner 26 such as that illustrated in FIGS. 16 - 18 . However, other burner arrangements and flame shapes, such as those shown in FIGS. 22 and 23 , also may be used to generate the variable heat release pattern in the present invention. For the arrangement illustrated in FIG. 22 , the burner units are equally spaced but each unit is designed to fire at a different firing rate to generate a variable heat release pattern. For the arrangement illustrated in FIG. 23 , all burner units are designed to fire at the same firing rate and the spacing between burner units is varied to provide a variable heat release pattern. A variable burner unit spacing can also be combined with burner units designed to fire at different rates to achieve the prescribed heat flux profile. Although each burner unit in the preferred embodiment of the invention burns the same fuel, different fuels may be used in alternate embodiments. For example, selected burner units may be designed to fire a liquid fuel (such as naphtha) and other burner units may be designed to burn a gaseous fuel (such as the offgas produced from pressure swing adsorption). It also is feasible for selected burner units to be designed to fire a mixture of fuels (such as a mixture of natural gas and offgas from pressure swing adsorption). Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.