Fast fluidization solids backmixing

Fig. 32. Influence of solids circulation rate on gas backmixing at a velocity higher than the incipient fast fluidization velocity (after Li and Wu, 1991).

Bader et al. (1988) used common salt as a solid tracer, which was injected into a flowing catalyst bed. Solids samples, withdrawn downstream of tracer injection, were leached with water and the salt concentration determined by electrical conductivity of the solution. Their results indicated substantial solids backmixing. Li et al. (1991) observed solids mixing in a fast fluidized bed combustor by using raw coal as a tracer, which was injected into the ash bed. Their results also showed that near-perfect mixing prevailed. Similar experiments was also conducted by Chesonis et al. (1991) in a cold model. 

Owing to the rapid formation and dissolution of particle clusters which contribute to high slip velocities and solid backmixing but preserve a limited extent of gas backmixing, the fast fluidized bed regenerator exhibits unique axial and radial profiles for voidage, temperature and carbon concentration (see Figs. 9 and 11 and Table VIII). 

This class of reactions, carried out in fluidized beds, involves parallel and series reactions, with reaction intermediates being the desired products. Industrial examples include partial oxidation of n-butane to maleic anhydride and o-xylene to phthalic anhydride. The vigorous solid mixing of fluidized beds is valuable for these reactions because they are highly exothermic. However, gas backmixing must be minimized to avoid extended gas residence times that lead to the formation of products of total combustion (i.e., CO2 and H2O). For this reason, fluidized bed catalytic partial oxidation reactors are operated in the higher velocity regimes of turbulent and fast-fluidization. 

Gas plus catalyst soUd Usually BFB. For fast reactions, gas film diffusion may control and catalyst pore diffusion mass transfer may control if catalyst diameter >1.5 mm. Heat transfer heat transfer coefficient wall to fluidized bed is 20-40 X gas-wall at the same superficial velocity, h = 0.15-0.3 kW/m K. Nu = 0.5-2. Heat transfer from the bed to the walls U = 0.45 to 1.1 kW/m °C. from bed to immersed tubes U = 0.2 to 0.4 kW/m °C from solids to gas in the bed U = 0.017 to 0.055 kW/m °C. Fluidized bed usually expands 10-25 %. Backmix type reactor which increases the volume of the reactor and usually gives a loss in selectivity. Usually characterized as backmix operation or more realistically as a series of CSTR if the height/diameter > 2 Usually 1 CSTR for each H/D= 1. If the reactor operates in the bubble region, then much of the gas short circuits the catalyst so the overall apparent rate constant is lower by a factor of 10. 

---