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Design of a magnetic chromatography system for particle separation
Magnetic Chromatography (MC) systems are devices that use strong magnetic field gradients for separating particles of different
magnetic susceptibility in a colloidal mixture. The nonuniformity in the magnetic field is created by introducing thin ferromagnetic wires in a uniform magnetic field; the thinner the wire, the greater
the magnetic field nonuniformity. One proposed application of MC systems is for nuclear fuel reprocessing for separating nuclear fuel elements from the process water.
A typical MC system
includes a channel in which fluid is flowing in the axial direction. The channel is subjected to a magnetic field (by using superconducting magnets) in a transverse direction. Pulses of a colloidal
mixture (particle slug) are introduced into the fluid stream at the inlet of the channel. As the particle pulse is transported by fluid convection in the axial direction, magnetic forces transport the
particles in the cross-stream direction at a rate dependent on the magnetic susceptibility of the particles. The particles with smaller magnetic susceptibility remain near the center of the channel and
those with larger susceptibility are drawn towards the walls, where the magnetic field gradients are stronger. For a laminar flow, because the fluid velocity is higher near the center of the channel, the
weak magnetic particles exit the channel earlier than other particles, creating axial classification or separation of particles.
The performance of an MC system depends on the relative competition
between axial particle diffusion and fluid convection; and cross-stream particle diffusion and magnetic forces. A clear understanding of these interacting processes is necessary for assessing the
feasibility of an MC process and for system design and optimization. The objective in this project was to develop a comprehensive mathematical model for an MC system.
This mathematical model uses
a finite-volume method for numerical solution of the governing equations for fluid flow, particle transport, and magnetic field. Suitable simplifications are introduced to reduce the complexity of the
equations and the computational effort. A novel technique that uses an expanding grid has been incorporated for efficient solution of the particle transport equation in a channel with a large aspect
ratio. The model has been extensively verified and has been used to study the effect of operating conditions and system configuration on the system performance as well as to assess the feasibility of new
processes.
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