| OCR Text |
Show complete length of the tube. ThIs requires that energy flux be highest at the tube inlet where the reaction rate is high and the process fluid temperature is low. At the exit, the chemical reaction rate has slowed as the ratio of reactants to products has decreased and the process fluid has reached a peak temperature of 1500° to 1600°F. In this region, the flux supplied to the process must be lower than at the furnace inlet to avoid overheating of process tubes. Modeling constraints used in the ARCS reformer design are described below, and firebox performance specifications are presented in Table 2. The geometric model consisted of 22 process tubes, each 32 feet long, with an OD of 5 inches and a one-half inch tube wall thickness. The fuel gas was a mixture of 85 percent process gas and 15 percent natural gas TABLE 2. REFORMER FURNACE PERFORMANCE PARAMETERS Fired Duty 35.30 MMBtu/hr (LHV) Absorbed Duty 18.75 MMBtu/hr Steam Recovery in Convective Section 10.57 MMBtu/hr Heat Recovery in Air Preheater 2.51 MMBtu/hr Efficiency of Firebox 53.2% (LHV) Overall Efficiency 90.4% (LHV) with a lower heating value (LHV) of 255 Btu/scf (4100 Btu/lb). The representative gas used for modeling purposes had the following composition: CH4 - 14% C02 - 41% CO - 9% H2 - 36% The reformer fireqox was divided into 4 unequal zones in the vertical (flue gas flow) direction. The first 2 zones had 2 burner segments each, the third zone had 1 burner segment, and the fourth zone had no burners. The process tubes were placed in 2 staggered rows containing 11 tubes per row. For this design study the burners were fired on one side only. This configuration represents a 5 million scfd reformer unit, or due to the modular nature of reformer design, a 5 million scfd "module" of a larger top fired unit. Unit capacity and overall thermal efficiency of the conventional top-fired unit and the ARCS unit were chosen to be equivalent. This allowed facility physical dimensions to be varied to meet system requirements. 9 |
