Segregation of large and lighter particles in a bubbling fluidized bed with solids crossflow

Paper in proceeding, 2025

Segregation is a natural phenomenon in the fluidization of binary fluidized beds. Under bubbling conditions, lighter particles tend to float while denser solids tend to sink, with the solids size amplifying this effect. Although this behaviour is often a problem as it prevents well-mixed operations, it can be exploited as a method to separate the desired solid product from the bed material. In order to precisely design and operate such a process, the impact of operational variables on the resulting separation effectiveness requires investigation. The present study revolves around the intention of achieving a continuous segregation and separation of the lean solid phase (typically large and lighter particles) from a bubbling fluidized bed with a crossflow of solids.
This work aims to study the effectiveness of segregation and separation of the lean solids phase in a bubbling fluidized bed of Geldart type B solids operated with a net crossflow of solids. The separation of biochar is used as reference case and the experiments are carried out in a fluid-dynamically downscaled cold flow model that resembles the conditions of a hot bubbling bed of sand fluidized by flue gases at 700 °C mixed with biochar particles. Magnetic solid tracing is used to measure the concentration of the lean solids phase in two different ways: i) by measuring the local concentration profiles inside the bed to assess the segregation, and ii) by acquiring concentrations at the inlet and outlet of the unit to evaluate the separation. Fluidization velocity, the rate of the solids crossflow and the total concentrations of lean solids phase were varied. The results indicate that segregation is more pronounced at lower fluidization velocities, particularly near the minimum fluidization condition. However, separation was most effective at fluidization numbers u/umf =<2, as the splashing is not vigorous enough to push the particles into the separation pipe. These finding highlights that maximized segregation alone does not necessarily lead to optimized separation. Instead, separation is influenced by multiple factors, including operating conditions and the configuration of the unit. A comprehensive understanding of these factors is essential for optimizing fluidized bed systems for continuous solid product recovery.