  We present a novel NMR technique that provides a non-invasive, direct
measurement of gas exchange in a three-dimensional gas-fluidized bed of solid
particles. The NMR spectrum of hyperpolarized 129Xe gas in an Al2O3 particle
bed displays three resolved peaks corresponding to xenon in bubbles, the
interstitial spaces (emulsion), and adsorbed on particles. Modified NMR
exchange and saturation-recovery sequences, together with data analysis based
on an exchange-coupled set of Bloch equations, yield gas exchange rate
constants between the emulsion and adsorbed phases, and between the bubble and
emulsion phases. The results are in approximate agreement previously unverified
predictions from well-known models of fluidized bed behavior. Incorporation of
NMR imaging methodologies would straightforwardly allow similar measurements on
a spatially-resolved basis.
