The analysis of hot spots in large scale fluidized bed reactors

Abstract

The influence of gas velocity and bed diameter on temperature and hot spot profile in gas–solid fluidized beds is studied based on a 2D pseudo homogeneous phase model. A dimensionless number, the fluidized Prater number β_f, is introduced into fluidized bed reactors to estimate the temperature gradient and hot spot profile based on various operating parameters and a quantitative relationship is established. Contrary to fixed beds, in fluidized beds, with low gas velocity and small bed diameter, there tend to be large temperature gradients and hot spots. With the increase of gas velocity in the turbulent regime and decrease in the bed diameter, β_f, the hot spot gradually disappears and the temperature profile tends to be more uniform. The hot spot in highly exothermic reactions in fluidized beds can be effectively minimized by increasing gas velocity in the turbulent regime and enlarging the bed diameter. An operating map for the industrial hydrogenation of nitrobenzene reaction is provided to estimate the possible axial temperature gradient from operating parameters. This method can be applied to other similar reactions for selecting operating parameters and methods to eliminate hot spots and produce a uniform temperature gradient.