Large bubbles in Geldart B bubbling fluidized bed reactors can cause poor contacting of gas and solids, back-mixing of gas, and solids entrainment. To improve the performance of industrial-scale fluidized beds, internal baffles may be used to suppress bubble growth, and to break apart large bubbles. Baffles also help to reduce back-mixing of gas, thereby promoting high conversion of gaseous reactants.
Two baffles, consisting of a horizontal perforated plate, were positioned above the distributor, and the effect on bubble break-up was simulated. The simulated baffles were made of a flat surface with numerous 2-cm holes, through which gas and solids can flow. The number of holes in each baffle is set by keeping the fraction of area open to flow fixed at 30%.
In this study, a two-dimensional fluidized bed of 3.24 m tall and 0.38 m width is simulated with and without internal baffles. The bed is made up of 300 µm Geldart B particles, and the bed operates in a vigorous bubbling regime. The interpenetrating two-fluid model using Eulerian-Eulerian flow field method as implemented in MFIX is used to simulate the conditions of the fluidized bed. Discretization was second order (SuperBee algorithm), and the grid was Cartesian, with uniform grid spacing of 5 mm. Additional grid resolution was generated by use of the filtered two-fluid model proposed by Milioli et al. (2013). A variable time-stepping scheme with a maximum time step of 10-4 s and minimum 10-7 s is used to assist with convergence. The baffles were modeled as impermeable surfaces where gas and solids can flow through 2-cm openings from the holes.
We have developed an algorithm to track bubbles discretely, as they rise through a fluidized bed. It will be shown that the simulated baffles promote the bubble break-up significantly, as expected, producing good mixing of gas and solids. The bubble break-up is quantifiable. The effect of baffles on bed height expansion has also been explained by comparing the simulated results of fluidized bed with and without baffles.
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