10

FIGURE 12.23 Influence of gas distributor on particle and gas temperatures (simulations of ethylene polymerization, 14000 particles, u = 3umf, t = 6.25, from Kaneko et al., 1999).

0.5 0.0 0.5 Radial distance, r/R
0.5 0.0 0.5 Radial distance, r/R

FIGURE 12.24 Comparison of simulation results with the experimental data of Bader et al., 1988 (from Gao et al., 1999).

FIGURE 12.24 Comparison of simulation results with the experimental data of Bader et al., 1988 (from Gao et al., 1999).

is strongly affected by high velocity jets, leading to lower gasoline yields. Their computational model could correctly capture the performance of two commercial riser reactors. As observed in many commercial risers, the yield of gasoline attains a maximum at a certain height within the riser and then declines due to overcracking. Several operational modifications are proposed to avoid such overcracking. Gao et al. (1999) demonstrated application of their computational model to evaluate a reaction-terminating technique to eliminate overcracking. In this technique, water is injected into a riser reactor at a suitable height. The injected water reduces the temperature and thus, reduces overcracking. Various alternatives were evaluated by Gao et al. (1999) and the performance of their simulation is shown in Fig. 12.25. It can be seen that the detailed computational flow model coupled with an appropriate reaction model was able to identify the most promising operational strategy to enhance gasoline yield. Theologos et al. (1999) also included feed atomization effects in their CFD model of a FCC riser reactor. This model was able to simulate feed vaporization and the influence of feed droplet sizes on the vaporization zone. They, however, used a rather simplified kinetic scheme (3-lump model) to represent cracking reactions.

Even if the detailed chemistry is not incorporated in the flow model, computational flow models can be used to resolve such hardware related-issues as configuration of feed nozzles, reactor internals, erosion due to solids particles and so on. Gustavs-son and Almstedt (2000) applied a two-fluid model to understand the erosion of heat exchanger tubes immersed in bubbling fluidized beds. The simulation results provide detailed information about how bubble passage, wake impact and wake passage affect local fluid dynamics around cooling tubes and thereby affect erosion. Predicted results qualitatively agree with the experimental evidence. Such CFD-based models allow extrapolation of bench- and pilot-scale data to larger scales and cold flow simulation data to actual operating conditions (high temperatures, high pressures). In general, the computational flow models can be used (1) to understand the basic phenomena and (2) to simulate the influence of complex reactor hardware (feed nozzles, distributor, internals and so on) on the performance of industrial reactors. Invariably, it will be

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