Klaiber, M. / Franzreb, M. / Lahann, J. (2023)

Hochschulschrift KIT, KITopen-ID: 1000160870

Abstract

Fluidized bed electrodes are suitable for a wide range of electrochemical reactions due to their particularly high surface-to-volume ratio. In addition, these fluidized electrodes exhibit tolerance to solid contaminations and gas bubbles in the reaction chamber. Compared to other reactor types, such as fixed-bed reactors, fluidized bed reactors exhibit exceptionally high heat and mass transfer and have an axial pressure drop that is virtually
independent of flow velocity. Furthermore, fluidized bed reactors are characterized by low operating costs, low susceptibility to operating errors and homogeneous mixing of the reaction chamber. Particularly complex in the use of fluidized bed electrodes is the unification of the three points I) fluidizability of the particles, II) transport of the electric charge within the particle electrode, and III) inertness of the electrode material toward secondary reactions. In particular, sufficient contact between the particles of the fluidized bed forms a challenge. Magnetic stabilization of the fluidized bed offers a possible solution, but this requires the use of magnetic electrode materials. Nevertheless, the synthesis of electrode particles with high magnetic susceptibility is still a challenge. Therefore, the aim of this dissertation was to develop a fluidized bed electrode that I) can be fluidized, II) can be magnetized to improve particle contact, and III) is inert to side reactions. In addition, a manufacturing process should be developed that would allow the particles to be fabricated in sufficient quantities for a pilot plant. None of the prerequisite material properties for forming a fluidized bed electrode were met by a single material, so a composite material was synthesized in this work. This composite material consists of a magnetic core and an electrically conductive shell that is inert to secondary reactions. Iron oxide was selected as the magnetizable component for the particle core and dispersed together with graphite in a poly(methyl methacrylate) solution to create a magnetic suspension. Graphite has a proven track record as an elecrode material in a variety of electrochemical processes and for this reason was selected to encase the magnetic suspension. To produce the particles, processes were developed that could spray magnetic suspension droplets with the aid of an applied electrical high voltage. Accelerated towards the coating material, the suspension droplets were captured by the
graphite powder. The spreading and subsequent drying of the polymethyl methacrylate solution in the coating powder ensured the binding of the particle shell. For uniform distribution of the suspension droplets in the coating material, the powder was manually moved through the area of the spray cone. The use of a conveyor belt enabled automated transport of the coating material through the spray cone, which ensured continuous syn-
thesis of the particles and ultimately led to optimization of this manufacturing process. Extensive characterization of the electrode particles confirmed the core-shell structure using energy dispersive X-ray spectroscopy, Raman spectroscopy, bright field microscopy and electron microscopy. The electrical conductivity, a basic requirement of the particles for use as particle electrodes, was determined to be 28 S/m for the dry bulk. Characterization of the magnetic properties revealed a saturation magnetization of 11.1 Am2/kg, sufficiently high for magnetic stabilization, and a low remanence of 1.3 Am2/kg. Analysis of the particle size distribution revealed a Sauter diameter of 172 μm, allowing calculation of a minimum fluidization velocity of the electrode particles of 2.26·10-4 m/s. An experimental analysis of the expansion behavior of the fluidized bed electrode showed that the particles, due to the size distribution, were already fluidized below the calculated minimum fluidization velocity at 1 ml/min. In the fluidized bed reactor, this fluidization rate corresponded to a flow rate of 2.39 ml/min. In addition, scanning electron microscopy revealed a rough particle surface, which increased the surface area compared to smooth particles, and thus increased the total surface area of the electrode. Analysis of the particle shape revealed a sphericity of 0.91. This round particle shape is a prerequisite for homogeneous fluidization of the fluidized bed and was illustrated by anaglyph images, for three-dimensional viewing of the synthesized particles. Magnetic stabilization of the fluidized bed increased particle contact when the particle electrode was used in
a fluidized bed reactor. This increased the space-time yield of conversion for the redox system potassium ferricyanide / potassium ferrocyanide by 2.7 times. Compared with fluidized bed electrodes known from literature, the core-shell particles showed higher dry particle bed conductivity. In addition, the core-shell particles exhibited a rounder particle shape, higher density and minimum fluidization velocity, significantly larger effective electrode surface area, higher magnetic remanence, and lower saturation magnetization than the literature-known particles. These particle properties provided 2-fold higher current densities in reactor experiments with the fluidized bed electrode synthesized in this work during the conversion of the redox system than using the particle electrode already known from the literature. These results underline the potential of the particles developed in
this work for use as a fluidized bed electrode in electrochemical reactions.

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