From circular to square-based section spouted beds: Scale-up and design overview of a multistage thermal unit

This work summarizes the results of several research projects to demonstrate the possibility of scaling-up spouted beds by assembling a number of square-based jet units. Cylindrical vessels with a bottom cone and parallelepiped ones with a frustum base do not differ to a large extent in terms of hydrodynamic features. The existing correlations can be used to predict these square-based spouted beds. Spouted beds can be very reasonably described as well-mixed reactors. By assembling several units, it is possible to approach a plug flow state for solids residence time distribution (RTD) in continuous processes. Spouting stability is fully reached when the modules do not interfere each other: this goal is obtained by positioning the downstream units at a certain lower level and having the solids move by overflow; placing internal vertical baffles between contiguous units is the solution. Two hydrodynamical models are proposed to describe single and multiple spouted beds; their predictions help in choosing the best geometrical configuration for the assemblage. A possible application of a multiple spouted bed reactor was envisaged for gasifying pelletized textile residues to generate syngas which can be directly used in situ as a co-fuel. Two units were built: a single 0.2 m side square-based spouted bed furnace and the consequent dual stage scale-up apparatus suitable for an auto thermal process. In the bottom stage, four units work independently to run combustion/gasification of low-quality solid residues, while the upper stage had a cascade of four modules to run pyrolysis/gasification of selected biomass and obtain valuable secondary products.