OCR Text |
Show mass transfer rates in the tubular reactor. The particle Reynolds number was estimated from d G/u, where G is the superficial mass flux, and u is the gas viscosity evaluated at the bulk gas temperature.* The Schmidt number was assumed to be constant and equal to 0.74. Devolatilization and Combustion Experiments Table 2 gives the nominal experimental conditions which were investigated. The experiments were actually conducted in two phases. In phase I (series A-S), the primary objective was to establish the relative importance of external mass transfer and intrinsic chemical reaction. Data for pure devolatilization (i.e., heating in an atmosphere of pure N?) and particle temperature data were also obtained to enable more precise analysis of the char combustion kinetics. As is discussed in one of the following sections, these experiments demonstrated that external mass transfer was dominant for particles >0.6 cm in diameter but was of minor importance for particles <0.1 cm in diameter. Because most of the data from Phase I were dominated by external mass transfer, it was necessary to run additional experiments with particles <0.1 cm in diameter to characterize the intrinsic chemical rate expression. This was accomplished in Phase II (series U-Y). Phase I - Temperature and weight-loss profiles Figures 6 and 7 illustrate particle temperature and weight loss curves typical of those observed in Phase I. Figure 6 refers to pure N9 operating conditions, where initial heating and devolatilization occurred but lack of oxygen prevented combustion. Note that after the initial heating period [usually about 10 s at 1090 K(1500°F)] Based on the recommendation of Smith (15), the particle diameters in Sh and Re were assumed to be of the diameter of spheres with equivalent surface areas. -Some references evaluate the gas properties at some mean film temperature between the particle and bulk gas temperatures. With the degree of uncertainty involved, we did not feel such a distinction was warranted. 18 |