Thermally polarized H-1 NMR microimaging studies of liquid and gas flow inmonolithic catalysts

The feasibility of gas flow imaging in moderately high magnetic fields empl oying thermally polarized gases at atmospheric pressures is demonstrated ex perimentally. Two-dimensional spatial maps of flow velocity distributions f or acetylene, propane, and butane flowing along the transport channels of s haped monolithic alumina catalysts were obtained at 7 T by H-1 NMR, with tr ue in-plane resolution of 400 mum and reasonable detection times. The resol ution is shown to be limited by the echo attenuation due to rapid molecular diffusion in the imaging gradients of magnetic field. All gas flow images exhibit flow patterns that are not fully developed, in agreement with the r ange of Reynolds numbers (190-570) and the length of the sample used in gas flow experiments. The flow maps reveal the highly nonuniform spatial distr ibution of shear rates within the monolith channels of square cross-section , the kind of information essential for evaluation and improvement of the e fficiency of mass transfer in shaped catalysts. The water flow images were obtained at lower Re numbers for comparison. These images demonstrate the t ransformation of a transient flow pattern observed closer to the inflow edg e of a monolith into a fully developed one further downstream. (C) 2000 Aca demic Press.