Abstract
Fluidized bed applications where the bed material plays an active role in chemical reactions, e.g. chemical looping combustion, have seen an increase in interest over the past decade. When these processes are to be scaled up to industrial or utility scale mass transfer between the gas and solids phases can become a limitation for conversion. Confined fluidized beds were conceptualized for other purposes in the 1960’s but are yet to be applied to these recent technologies. Here it is investigated if they can prove useful to increase mass transfer but also if they are feasible from other perspectives such as pressure drop increase and solids throughflow. Four spherical packing solids, 6.35–25.4 mm in diameter at two different densities, were tested. For mass transfer experiments the fluidizing air was humidified and the water adsorption rate onto silica gel particles acting as fluidizing solids was measured. Olivine sand was used in further experiments measuring segregation of solids and packing, and maximum vertical crossflow of solids. It was found that mass transfer increased by a factor of 1.9–3.8 with packing solids as compared to a non-packed reference. With high-density packing, fluidizing solids voidage inside the packing was found to be up to 58% higher than in a conventional fluidized bed. Low density packing material favoured its flotsam segregation and with it higher fluidization velocities yield better mixing between packing and fluidizing solids. Maximum vertical cross-flow was found to be significantly higher with low density packing that fluidized, than with stationary high-density packing. Conclusively, the prospect of using confined fluidized beds for improving mass transfer looks promising from both performance and practical standpoints.
Highlights
Reacting an active bed material solid with a gaseous component is a common application of fluidized beds, e.g., chemical looping combustion, combustion, gasification, fluid catalyst cracking, and chemical looping reforming
Having identified gaps in the existing body of literature, this paper aims to investigate the possible application, benefits and limitations, of confined fluidized beds in applications where mass transfer of gas between the emulsion and bubble phase is of importance to the process
An improvement in mass transfer by a factor 1.9–3.8 was found under the conditions tested here, improvement in mass transfer by a factor 1.9–3.8 was found under the conditions tested here, see see Figure 5b
Summary
Reacting an active bed material solid with a gaseous component is a common application of fluidized beds, e.g., chemical looping combustion, combustion, gasification, fluid catalyst cracking, and chemical looping reforming. In many of these processes, mass transfer between the reacting gas and the active particle can prove a limiting step towards conversion, as demonstrated by e.g., Mattisson et al [1]. The challenge of limited mass transfer between the bubble and emulsion phases can be addressed by means of using confined fluidized beds, i.e., inserting inert spheres of a size. Sci. 2019, 9, 634; doi:10.3390/app9040634 www.mdpi.com/journal/applsci Appl.
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