Reshaping the Structure of Fluidized Beds
Multiphase catalytic reactors are widely used throughout the chemical process industries. Several types of reactors are available for multiphase reactions that employ solid catalyst, such as the packed bed, fl uidized bed, and slurry bubble column; many variations on these three archetypes exist. Despite their frequent application, each design suffers from drawbacks. In packed beds, relatively large (millimeter scale) particles are used to keep the pressure drop (and, thus, the energy consumption) acceptably low. This makes diffusion lengths long and leads to poor mass transfer. Moreover, packed beds are sensitive to fl ow maldistribution. This can lead to problems, such as hot spot formation and runaway. Fluidized beds and slurry reactors couple short intraparticle diffusion lengths with good heat transfer. However, they can suffer from backmixing, catalyst attrition and particle fl uid separation problems, and they are diffi cult to scale up. Increasing the gas fl owrate in a fl uidized bed typically increases the bubble size and reduces the rate of mass transfer between the bubbles and the solids, which is often the rate limiting step in a fl uidized bed (Figure 1) One way to overcome the disadvantages of multiphase reactors is to structure the reaction environment. This introduces additional degrees of freedom and allows decoupling of confl icting design objectives, such as high mass transfer vs. low pressure drop in a packed bed, or high gas fl owrate vs. small bubble size in a fl uidized bed. Although structuring is more straightforward in reactors with a fi xed catalyst, for instance, by installing structured packings such as those commonly used in distillation columns, it is also possible to change the hydrodynamic structure of reactors containing a mobile catalyst (or no catalyst) such as fl uidized beds, bubble columns, and slurry bubble columns. An example of this is the extension of the homogeneous regime in bubble columns to high velocities by homogeneously injecting the gas at the bottom of the column (1) There are several reasons to impose structure on fl uidized beds: It helps to limit bubble size, which leads to better mass transfer and, subsequently, higher conversion and better selectivity. A smaller bubble size reduces erosion, attrition, and elutriation. Structuring helps to fl uidize powders that are diffi cult to fl uidize because they are wet or the particles are very small. A more homogeneous gas pattern prevents channeling. Adding structure to a gas solid fl uidized bed reactor changes the system's hydrodynamics and can improve performance. J. Ruud van Ommen John Nijenhuis Delft Univ. of Technology Marc Olivier Coppens Rensselaer Polytechnic Institute Delft Univ. of Technology Reshaping the Structure of Fluidized Beds