Circulating fluidized-bed (CFB) risers with Geldart group B particles have found significant application in combustion reactions. The present work attempts to study the solids flow dynamics in a CFB riser that is operated with group B particles, using computational fluid dynamics (CFD) techniques. The key feature in the present study is that the various closure schemes in the CFD model have been evaluated against data from non-invasive experimental techniques:  computer automated radioactive particle tracking (CARPT) for solids velocity field and computed tomography (CT) for solids holdup. Since solids flow in a riser is multiscale in character, in addition to the measured averaged solids velocity profiles and solids fraction profiles in the experimental section, mean granular temperature profiles have also been compared. Two flow regimes (viz., fast fluidization and dilute phase transport) have been considered in this study.

No abstract

No abstract

Selective partial oxidations represent an important class of reactions in the process industry. Of particular interest is the partial oxidation of n-butane to maleic anhydride (MAN), which is arguably the largest commercialized alkane partial oxidation process. Partial oxidation of n-butane, which uses vanadium phosphorous oxide (VPO) as a heterogeneous catalyst, is believed to operate through a unique mechanism in which lattice oxygen oxidizes n-butane selectively to MAN. Past work has shown that performing partial oxidation reactions in gas-solids riser configuration is realizable and commercially viable, which has lead to commercialization of this technology in the last decade. Though the riser configuration allows optimal and independent control of the oxidation and reduction steps, the riser unit suffers from solid backmixing at walls, which in turn result into lower conversion, nonoptimal selectivity and diminished overall yield of desired product. In recent years, there has been growing interest in downers involving cocurrent downflow of both solids and gas phases, hence offering relatively uniform flow characteristics. In this contribution, we explore through modeling the implications of effecting partial oxidation reactions in a downer (gas-solids cocurrent downflow) compared to that in a conventional riser reactor (gas-solids cocurrent up flow) operated under equivalent operating conditions. Further, we explore the operational space of downers for these reactions, suggesting ways for improving the productivity of downer for partial oxidation applications. V

No abstract