Fluidized bed chemical reactors – Old and new applications
May 22-27, 2016
Application of fluidized beds as chemical reactors is dominated by their use for heterogeneously catalyzed gas-phase (cGP) and gas-solid reactions (GS). In recent decades much attention was given to chemical looping (CL) for energy generation. The aim of this review is to discuss potentials of fluidized-bed technology for other reaction classes. The material used in the paper is mainly based on various examples studied in the Technology Development organization of the Bayer Company. Nonetheless, the examples illustrate a broad spectrum of reactions that can benefit from the fluidization technique but also related challenges. For example, hydrochlorination of silicon represents a very interesting class of heterogeneously catalyzed Solid-to-Gas reactions (cS2G). In cS2G reactions, a particulate catalyst has to be anchored on the surface of the fluidized particles and should not be lost during the progressing conversion of the substrate. Gas-to-Solid (G2S) is another reaction class that represents a broad spectrum of applications. It can be performed catalytically like gas-phase polymerization reactions or by means of catalytic chemical vapor deposition. Examples of such systems are respectively polymerization to butadiene rubber (BR) and synthesis of carbon nanotubes. Gas-phase polymerization allows the synthesis of polymers without a solvent. Fluidized-bed technology is therefore an enabler for green chemistry. Especially fluidized bed synthesis of sticky BR represents a big challenge with respect to stable operation. In both mentioned applications the catalyst is consumed. Therefore, it is important to run the reaction under conditions that prevent catalyst deactivation in order to achieve high catalyst utilization and, in turn, a high product quality. The CNT synthesis technology is a nice example of the multiscale design since not only a high yield but also desired product properties have to be achieved. The G2S reactions can be also performed without a catalyst. A good example is pyrolysis of silane to high purity silicon. The main challenge in this reaction type is the prevention of gas-phase reactions that yield dust. Furthermore, control of purity and morphology of the product is of primary importance. Uncatalyzed chemical vapor deposition is applied not only to generate growth of solid particles but it is also used for decoration of the surfaces. The classical application is the deposition of metal nanocrystals on the oxidic surfaces, e.g. for catalytic applications. It is especially challenging when these nanoparticles have to be deposited in pores of fluidized particles. Sometimes these active metal nanoparticles have to be deposited of the non-fluidizable support, e.g. carbon black. In this case fluidization has to be supported, e.g. mechanically. Examples of catalyst manufacturing will be presented. Fluidized-bed reactor is also a good apparatus for so called tandem reactions. Catalytic pyrolysis of biomass or waste are good examples of this class, in particular, they represent Solid-to-Gas-to-Gas reactions (S2G2G). In the first step solid particles are thermally decomposed to gas and solid residue. Gas released during pyrolysis is catalytically converted to value added products. The binary mixture of particles with different morphology has to be fluidized in a stable manner. Furthermore, the catalyst has to be separated from the produced char, regenerated and recycled. The looping concept has been already mentioned for energy applications. However, several attempts of using CL in chemical syntheses have been reported. The purpose of CL was either to avoid gas-phase reactions or to overcome thermodynamic limitations. The interesting applications of CL are oxidation of HCl and oxidative coupling of methane. Usually, catalytic fluidized-bed reactors are used for highly exothermic reactions like selective oxidations. Catalytic aromatization of methane is an example of an application of the FB for highly endothermic reactions exhibiting fast catalyst deactivation. Even if there are some similarities to the commercialized technology for paraffin dehydrogenation, this reaction is much more challenging.
Leslaw Mleczko, "Fluidized bed chemical reactors – Old and new applications" in "Fluidization XV", Jamal Chaouki, Ecole Polytechnique de Montreal, Canada Franco Berruti, Wewstern University, Canada Xiaotao Bi, UBC, Canada Ray Cocco, PSRI Inc. USA Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/fluidization_xv/90