Particle mixing in bubbling fluidized bed reactors with continuous particle exchange

Abstract

This paper reports on experiments conducted with a cold flow model utilized for the investigation of the particle residence time distribution and mixing characteristics in a bubbling fluidized bed with continuous solids exchange. The investigated system is of a rectangular cross section (0.4 × 0.2 m) with a bed height of 0.17 m. A measurement device based on an alternating current bridge circuitry coupled with lock-in amplifier technology was built in the scope of quantifying the solids residence time distribution, whereby a pulse-injected ferromagnetic tracer creates the input signal. The implementation of a profound mathematical routine ensures the reproducible calculation of the particles mean residence time and characteristic values describing particle mixing phenomena. Therefore, the E-curve was modeled by mathematical convolution of the exit age distributions available for an ideally mixed continuous stirred tank reactor and a plug flow reactor with axial dispersion. It is shown that the in-bed mixing is highly dependent on the fluidization rate as well as on the solids circulation rate. Albeit the lowest superficial gas velocity equals a fluidization number of 4.7, the formation of dead spaces and short-circuit flows was observed under these conditions. Axial dispersion coefficients in the range of 5·10-3 to 7·10-1 m2 s−1 were obtained.