Characterising gas behaviour during gas–liquid co-current up-flow in packed beds using magnetic resonance imaging

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Magnetic resonance (MR) imaging techniques have been used to study gas phase dynamics during co-current up-flow in a column of inner diameter 43mm, packed with spherical non-porous elements of diameters of 1.8, 3 and 5mm. MR measurements of gas hold-up, bubble-size distribution, and bubble-rise velocities were made as a function of flow rate and packing size. Gas and liquid flow rates were studied in the range of 20–250cm3s−1 and 0–200cm3min−1, respectively. The gas hold-up within the beds was found to increase with gas flow rate, while decreasing with increasing packing size and to a lesser extent with increasing liquid flow rate. The gas hold-up can be separated into a dynamic gas hold-up, only weakly dependent on packing size and associated with bubbles rising up the bed, and a ‘static’ hold-up which refers to locations within the bed associated with temporally-invariant gas hold-up, over the measurement times of 512s, associated either with gas trapped within the void structure of the bed or with gas channels within the bed. This ‘static’ gas hold-up is strongly dependent on packing size, showing an increase with decreasing packing size. The dynamic gas hold-up is comprised of small bubbles – of order of the packing size – which have rise velocities of 10–40mms−1 and which move between the packing elements within the bed, along with much larger bubbles, or agglomerates of bubbles, which move with higher rise velocities (100–300mms−1). These ‘larger’ bubbles, which may exist as streams of smaller bubbles or ‘amoeboid’ bubbles, behave as a single large bubble in terms of the observed high rise velocity. Elongation of the bubbles in the direction of flow was observed for all packings.