95 Example 4 Fcc Regenerator

Fluidized catalytic cracking (FCC), which converts heavy oil to value added low boiling point products is an important process in refineries around the world. During cracking reactions, catalyst is deactivated rapidly owing to coke deposition. In industrial FCC units, the deactivated catalyst is continuously regenerated by employing a regenerator connected to the cracking reactor. Besides regenerating the catalyst (by contacting it with air), the FCC regenerator also provides the heat required for the endothermic cracking reactions.

A schematic diagram of a typical industrial FCC regenerator is shown in Fig. 9.23. The spent catalyst particles are circulated through the regenerator. The orientation, size and location of the spent catalyst distributor are important parameters controlling solids mixing. The regenerated catalyst is withdrawn from the outlet located at the

FIGURE 9.21 Typical predicted results for the loop reactor (excluding vapor space). (a) Vector plots (liquid phase), (b) contours of gas volume fraction (legend not shown due to confidentiality constraints). Reproduced in colour plate section between pages 210 and 211.

bottom conical portion. Air is introduced in the regenerator through a distributor located just above the bottom conical part of the reactor. The regenerator is operated in a dense bed (or turbulent bed) regime (superficial gas velocity is much higher than the minimum fluidization velocity). The extent of regeneration of catalyst particles depends on effective contacting between supplied air and catalyst particles. Most of the supplied air passes through the regenerator in the form of large gas bubbles (voids). These voids interact with each other and may coalesce or break up within the dense bed. As these voids rise through the dense bed, a macroscopic circulation of catalyst particles is set-up within the dense bed. When these voids break up at the top surface of the dense bed, solid particles are thrown into the free board region (dilute bed). Reaction of coke on these solid particles with un-reacted oxygen in the dilute bed region may cause excessive temperature excursions (called after-burning, which has a detrimental effect on throughput as well as catalyst and equipment life). Some of

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