(68f) Real-Time Magnetic Resonance Velocimetry and Thermometry of Gas-Solid Systems

Particle systems are challenging to study experimentally. While optical techniques are limited to image the surface of most three-dimensional (3D) systems, intrusive probes only provide measurements at a single spatial location and tend to interfere with the flow field. Hence, a variety of sophisticated tomographic modalities have been developed to provide insights into 3D particle systems. These include x-ray tomography [1], electrical capacitance tomography [2], position emission particle tracking [3] and magnetic resonance imaging (MRI). While the former three modalities can only detect particle positions, MRI additionally allows to measure particle velocities, diffusion, temperature, and chemical reactions [4]. Due to this unparalleled variety of achievable imaging contrasts, MRI is a promising tool for chemical engineering research. However, owing to the sequential nature of data acquisition in MRI, the temporal resolution has been notoriously low [5].

We have recently developed a real-time MRI methodology [6], that increases the temporal resolution of particle velocity measurements by more than four orders of magnitude compared to previous measurements. The methodology relies on a combination of scan acceleration techniques as well as MRI hardware and signal source engineering (Figure 1, left).

In this work, we extend the capabilities of real-time MRI to include temperature measurements of the particle phase (Figure 1, bottom right). Spatially resolved temperature-sensitivity was generated by exploiting the temperature-dependent proton frequency (PRF) shift of water molecules [7,8] combined with MR imaging gradients.

A cylindrical fixed bed reactor model of diameter and height of 80 mm was filled with core-shell thermal tracers. The tracers were manufactured from hollow polypropylene spheres of diameter 10 mm filled with an aqueous solution of paramagnetic rare earth salts. Hot air (60 °C) was injected through a central orifice at the bottom of the bed. 3D thermal maps of the bed were acquired during heating and cooling of the system from and to room temperature. These measurements were compared against optical reference thermometers placed inside the bed. The MR-based temperature measurements exhibited good agreement with these reference measurements.

With an acquisition time of a few seconds for the whole volume, the MR thermometry methodology presented here constitutes a powerful technique to characterize thermal dynamics in a large variety of gas-solid reactors. We are confident that the presented technique will contribute to a better understanding of mass and heat transfer inside gas-solid reactors. It can further be used to test numerical simulations and to aid reactor design.

References

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