| OCR Text |
Show In each of the 4 thermal zones, the equations describing radiant and convective energy transfer between the burner, gas and tubes were formulated and solved to yield the burner fired duty needed to match tube side flux and temperature values. The flue gas temperature in each zone was also determined. Direct radiation from burners, gas phase radiation, and convection to tubes were modeled. Reradiation from burners and refractory were also calculated. Flue gas flow was driven by draft fans, so natural convection was not considered. Modeling results are presented in Figure 3. Figure 3a presents the desired tube metal temperature profile, and the resulting gas temperature profiles necessary to achieve these tube temperatures. The tube metal profile is fairly flat with a temperature variation of about 75°F along the length of a process tube. This flux profile is achieved through a significant variation in burner fired duty as a function of heater elevation as shown in Figure 3b. Figure 3c shows the absorbed flux of the process tubes, which represents the absorbed heat of reaction from the chemical conversion process as well as sensible heating of the process gas. The absorbed flux at the tube wall varies by a factor of greater than 6 along the length of the process tubes, from 30 MBtu/hr-ff at the entrance of the furnace to less than 5 MBtu/hr-ft2 at the exit. This variation in absorbed duty is accomplished by varying burner heat flux as a function of burner elevation, and this variation is necessary to achieve the tube temperature profile presented in Figure 3a. Table 3 provides a breakdown of the contributions of the different heat transfer mechanisms to the overall absorbed flux in the 4 furnace zones. TABLE 3. CON1RIBUTIONS TO ABSORBED FLUX (%) Gas Radiation Zone 1 Zone 2 Zone 3 Zone 4 Surface 77.51 78.80 75.05 26.82 Radiation Gas Radiation 19.46 17.18 19.83 58.07 Convection 3.03 4.02 5.12 15.11 A cost comparison was made between the conventional reformer and the radiant burner system at the conclusion of the reformer design task, and a detailed cost breakdown is presented in Reference 6. The ARCS design resulted in a reduced radiant box volume, and as a result of this the furnace cost, excluding burners and downstream N Ox removal equipment, was $932K for the conventional system and $879K for the radiant burner system. The conventional burner cost was estimated by the furnace designer to be $32K, resulting in a break even cost for the radiant burner system of $85K or $2.4K per MMBtu/hr of fired duty for the radiant burners. This breakeven cost is within the range of current radiant burner product costs. The ARCS provides process control benefits that cannot be provided 10 |
