Cross verified independent measurements of correlated bubble property distributions as part of the scale-up of a catalytic bubbling fluidized bed reactor

Methanation can be applied as part of a Power-to-Gas (PtG) concept to store renewable electricity. Using a catalytic bubbling fluidized bed (BFB) methanation reactor allows a flexible and economic operation. Properties of bubbles rising through the fluidized bed significantly influence the performance of such reactors. In this work, the hydrodynamics in a BFB reactor with vertical heat exchanger tubes are investigated. The aim is to develop and verify the experimental methods and to establish a hydrodynamic data base for the development of a reactor model, which shall facilitate the scale-up of the process towards an industrial scale. Independent experimental methods, namely X-ray tomography, X-ray radiography and optical probe measurements, have been applied to measure bubble property distributions and their correlations, and to cross verify the results. The X-ray measurements were conducted in a cold-flow model with and without vertical internals that represent the heat exchanger tubes. The cold-flow model matches the geometry of a pilot plant, referred to as GanyMeth, in which the optical probe measurements were carried out, to ensure the comparability of the results. X-ray tomography was used to reconstruct individual bubbles in a BFB. It could be shown that the sizes, rise velocities and shapes of the bubbles are broadly distributed and correlated. When vertical internals are present, the mean sizes and rise velocities of the bubbles decrease, their shapes elongate on average and the mean cross-sectional bubble hold-up is reduced. Systematic tomography simulations were performed for the case without internals, to verify the applicability and to determine the limitations of the method. Accordingly, the results of all measurements conducted between the heights of 16 and 56 cm for fluidization numbers in the range of 1.5 – 6 are credible. Radiographic X-ray imaging was used to independently verify the bubble size distributions in BFBs that were measured by means of X-ray tomography for a chosen measurement height and two superficial gas velocities without vertical internals. Through the measurement of injected single bubbles, the correlations between the size, rise velocity and shape of bubbles were also confirmed, again only for the case without internals. The evaluation of measurements with internals proved to be challenging: While a reduction of the bubble sizes and rise velocities could be confirmed, the shape of injected bubbles were observed to be flattened rather than elongated as expected from the X-ray tomography results. X-ray radiography was also used to verify the optical probe methodology through simultaneous measurements of injected bubbles, and to assess the dynamic evolution of bubbles in BFBs qualitatively.Comprehensive hydrodynamic experiments were conducted in the GanyMeth pilot plant. Chord lengths and rise velocities of bubbles were measured for two types of bed material, at seven pressures between 1 and 11 bar, at six fluidization numbers between 2 to 10, at six heights of up to 137 cm and six evenly spaced radial positions. With increasing pressures, the chord lengths shortened, the bubble hold-up increased and an earlier onset of slugging fluidization occurred. The results at ambient pressure matched those obtained via X-ray tomography at comparable settings. The feasibility to retrieve the volume equivalent bubble diameters from the measured chord lengths and rise velocities could be demonstrated.