Stoffübergang in flüssigkeitsgetragenen Fluidbetten magnetischer Partikel

Abstract

Mass Transfer in Liquid Fluidized Beds of Magnetic Particles The work described here focused on fundamental studies regarding fluidized beds of magnetic particles passed by a liquid flow. Various combinations of external magnetic fields and particle types were compared in terms of mass transfer characteristics. Weakly acid ion exchanger and inert particles with inclusions of maghemite as well as alginate particles with magnetite inclusions were applied. In detail, the following operation modes were investigated: 1. Conventional fluidized bed (FB) 2. Fluidized bed of magnetic agglomerates (MFB) 3. Magnetically stabilized fluidized bed (field-first MSFB and flow-first MSFB) 4. Magnetically stirred reactor (MSR) Compared to conventional fluidized beds, orientation of particles in the form of chains during the MSFB modes leads to smaller resistance forces of the fluid acting on the particles and, thus, allows for much higher flow rates without the risk of particle discharge. Moreover, it is demonstrated by the studies that the influence of the magnetic field may lead to a significant reduction of the mass transfer coefficients of liquid-borne fluidized beds. This particularly applies to relatively small particles. It is possibly due to an inefficient flow through the bed and the tendency of MSFBs to form flow channels. Within the framework of the work performed, this relationship could be described quantitatively for the first time. The mass transfer behavior of magnetically stirred reactors, where magnetic particles are passed by a flow under the influence of an alternating magnetic field, was studied as a function of the applied frequency and flow rate. The alternating magnetic field generates a torque that acts on the particles and, thus, initiates a rotation of the particles in phase with field frequency. This effect causes an increased relative velocity between the exchange surface and fluid flow and consequently results in a decreasing thickness of the diffusive boundary layer. The diffusion path is reduced and higher mass transfer values are reached. Within the framework of the work performed, it was succeeded for the first time to quantitatively determine mass transfer coefficients in a MSR. The mass transfer rates in the MSR were found to exceed those of conventional fluidized beds by a factor of up to four.