Solids lateral mixing and compartmentalization in dynamically structured gas–solid fluidized beds

An adequate use of gas pulsation can create an ordered, dynamically structured bubble flow in a bed of Geldart B particles. A structured bed is more homogeneous, responds to external control and is scalable. While earlier studies have focused on describing the self-organization of the gas bubbles, the solid mixing and gas–solid contact patterns have remained unclear. In this work, the solids circulation and mixing behavior in structured and unstructured beds at various pulsation frequencies are compared with a traditional fluidized bed operation. The degree of lateral mixing is hereby quantified through an effective lateral dispersion coefficient extracted from CFD-DEM (discrete element modelling) simulations in a thin fluidized bed system. Mixing shows major quantitative and qualitative differences amongst the investigated cases. The coordinated motion of the gas bubbles wraps the solid flow into a series of compartments with minimal interaction, whereby effective lateral dispersion coefficients are an order of magnitude lower than in an unstructured operation. More importantly, unlike a traditional bed, dispersion in a structured bed is driven by advection and is no longer a diffusive process. Compartmentalization decouples the time scales of micro- and macromixing. Every pulse, the compartments rearrange dynamically, causing a level of local axial mixing that is scale-independent. While further work is necessary to fully understand the compartmentalization at a larger scale, the circulation described here indicates that a dynamically structured bed can provide a tight control of mixing at low gas velocities and a narrower distribution of stresses in the solid phase compared to traditional devices

Evaluation of high-emissivity coatings in steam cracking furnaces using a non-grey gas radiation model

The efficiency of the application of high-emissivity coatings on the furnace walls in steam cracking technology can only be evaluated on the basis of a description of radiative heat transfer distinguishing between the frequency bands. To this end, a non-grey gas radiation model based on the exponential wide band model (EWBM) has been developed and applied in the context of three-dimensional CFD simulations of an industrial naphtha cracking furnace with side-wall radiation burners. Applying a high-emissivity coating on the furnace wall decreases the net outgoing radiation from the furnace wall in the absorption bands and increases the net outgoing radiation from the furnace wall in the clear windows. Since radiation that is emitted by the furnace wall and travels through the flue gas in the clear windows can reach the reactor tubes without partially being absorbed by the flue gas, contrary to radiation that is emitted by the furnace wall and travels through the flue gas in the absorption bands, the thermal efficiency of the furnace increases. It was found that application of a high-emissivity coating on the furnace walls improves the thermal efficiency of the furnace (∼1%), the naphtha conversion (∼1%) and the ethylene yield (∼0.5%). These differences are small but, considering the industrial importance and scale of the steam cracking process, significant. © 2007 Elsevier B.V. All rights reserved.status: publishe