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Novel Routes to Fluidized Bed Process Intensification Using a Centrifugal Field

Juray De Wilde, Materials and Process Engineering Dept. (IMAP), Universite catholique de Louvain (UCL), Reaumur, Place Sainte Barbe, 2, Louvain-la-Neuve, B - 1348, Belgium

The operating conditions in conventional, that is, gravitational fluidized bed reactors are determined and limited by the use of earth gravity. Internal mass and heat transfer limitations can be encountered by the impossibility to fluidize smaller size particles, the van der Waals forces becoming too important compared to the weight of the particle bed in the earth gravity field and the counteracting drag force. External mass and heat transfer limitations can be encountered by the impossibility to work at higher gas-solid slip velocities. With heterogeneous (catalytic) reactions, as well internal as external mass or heat transfer limitations may impose limitations on reaction rates and, for example, the activity of the catalyst.

Fluidization in a centrifugal field may allow overcoming the above mentioned limitations and may, as such, allow fluidized bed process intensification. Two novel routes to fluidization in a centrifugal field are presented and their advantages discussed and illustrated.

In rotating fluidized beds in a static geometry, the centrifugal field is generated by injecting the fluidization gas tangentially in the fluidization chamber via multiple gas inlet slots in its outer cylindrical wall. A combined tangential-radial fluidization of the particle bed is obtained by forcing the fluidization gas to leave the fluidization chamber via a centrally positioned chimney. The fluidization gas flow rate influencing both the centrifugal force and the counteracting radial gas-solid drag force in a similar way renders rotating fluidized beds in a static geometry extremely flexible with respect to the fluidization gas flow rate and the gas-solid contact time. In particular, dense operation at high fluidization gas velocities is possible, allowing fast and highly endothermic or exothermic reactions to be carried out in a rotating fluidized bed in a static geometry of much smaller volume than when using conventional fluidized bed technology.

In rotating fluidized beds around a rotating chimney, the centrifugal field is generated by a rotating chimney consisting of multiple blades and positioned centrally in the fluidization chamber. The fluidization gas being forced to leave the fluidization chamber via the chimney, a radial gas-solid drag force counteracting the centrifugal force is generated. Furthermore, both the fluidization gas and the particles obtain a tangential velocity by the action of the rotating chimney. This results in a combined tangential-radial fluidization of the particle bed.

The tangential injection of the fluidization gas in the fluidization chamber via multiple gas inlet slots in its outer cylindrical wall and the centrally positioned rotating chimney can eventually be combined to obtain extremely flexible fluidization technology. In such case, the rotating chimney allows to increase locally, that is, in the vicinity of the chimney, the centrifugal force. Solids losses via the chimney can, as such, be drastically reduced. This is shown to be particularly advantageous when fluidizing small, micro- or nano-scale particles.

Experimental and numerical data on the fluidization behavior with different types of particles, the pressure drop over the fluidization chamber and the rotating chimney, the gas-solid mass and heat transfer characteristics in the fluidization chamber and the solids losses via the chimney are presented and discussed. The advantages of fluidization in a centrifugal field using the novel technologies presented are illustrated with potential applications.