(508e) Multi-Scale Modeling of Selective Oxidation in a Circulating Fluidized Bed Downer Reactor | AIChE

(508e) Multi-Scale Modeling of Selective Oxidation in a Circulating Fluidized Bed Downer Reactor

Authors 

Suryawanshi, V. - Presenter, Indian Institute of Technology - Delhi


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. This is an important part of commercial processes for manufacture of tetrahydrofuran (THF), such as the one licensed by DuPont (Contractor et al., 1987; Stadig, 1992). For commercializing this technology, the catalyst (vanadium phosphorous oxide (VPO)) was developed to have attrition-resistant properties, so that it became possible to select a circulating fluidized bed (a system with favorable reaction engineering attributes but suffering from catalyst attrition) for effecting this reaction with acceptable selectivity and reactor yields.

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. While there seems to be yet disagreement on the exact nature of the lattice oxygen diffusion and its role in determining the rate mechanism for partial oxidation, reports in literature (e.g. Centi et al. (1985), Mills et al. (1999), Huang et al. (2002)) have shed light on this mechanism and have cited arguments in favor of a circulating fluidized bed configuration. In general, consensus seems to be that a reaction zone in which the well-regenerated solid VPO catalyst gets optimal residence time (both mean residence time and variance of solids RTD) for the reaction (with reference to the lattice oxygen diffusion time and the other time-scales of relevance to the kinetic mechanism), results in a desirable contactor for this process. How the residence time distribution in solids interplays with the kinetic time constants in a riser has already been explored (Roy et al., (1999, 2000)).

In this contribution, we explore the implications of effecting partial oxidation of n-butane in a downer (gas-solids cocurrent downflow) mode.

First, a detailed two-dimensional axisymmetric, transient model of gas-solids downer flow is developed, using computational fluid dynamic (CFD) tools. A two-fluid ensemble averaged approach is employed for modeling the phases. The forces considered in the model are: drag force between solids and gas, solids collisional forces using granular phase kinetic theory, and gas-phase turbulence. The transient velocity and solids volume fraction profiles are time and azimuthally averaged and compared against data reported in the open literature, such as that of Zhang et al. (1999) and Zhang and Zhu (2000).

Further, the hydrodynamic CFD model is coupled through a scalar transport equation to calculate the solids and gas-phase residence time distribution (RTD). This solids phase RTD is coupled to the kinetics of Centi et al. (1985), which employs triangular series?parallel network, and the kinetics of Mills et al. (1999), which involves a parallel network of three reactions and takes into account solid phase oxygen diffusion effect. The CFD-RTD model coupled with these kinetic schemes is to make predictions of reactor performance of the downer reactor. The model predictions are presented for the conversion of n-butane, yield of MAN, and selectivity to MAN as a function of various operating variables. Sensitivity studies are presented highlighting the impact of various hydrodynamic and design parameters on the reactor performance. The work also makes recommendations for a more detailed experimental and computational study to follow.

References:

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Contractor, R. M., H. E. Bergna, H. S. Horowitz, C. M. Blackstone, U. Chowdhary and A. W. Sleight (1987), ?Butane Oxidation to Maleic Anhydride in a Recirculating Solids Reactor?, Catalysis, 645.

Mills, P. L., H. T. Randall, J. S. McCracken (1999), ?Redox Kinetics of VOPO4 with Butane and Oxygen using the TAP Reactor System?, Chem. Engng. Sci. 54(13-14), 3709.

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