Investigation of the hydrodynamics of packed-fluidized beds: characterization of minimum fluidization velocity and solids flux

Paper in proceeding, 2022

Fluidized bed reactors have many industrial applications. Recently, fluidized beds have extended its range of applications to new areas, such as chemical-looping-combustion (CLC) for inherent CO2 capture. Advantages of fluidized beds includes good mixing of solids and gases, temperature uniformity and excellent mass and heat transfer. However, at elevated gas velocities, gas-solid mass transfer becomes hampered in deep, bubbling fluidized-bed reactors, due to bubble coalescence and bubble growth. Other unwanted phenomena that can take place at these conditions are slugging and channeling. These phenomena put certain restrictions on the design of reactors, as well as their operating conditions. As a result, there are limitations on the length-to-diameter-ratio and gas velocity. Several methods have been proposed to overcome these issues and improve the quality of gas-solid fluidization and gas-solid mass transfer. This includes breaking down bubbles by applying helical rotating screws, helical large mesh screens, vertical and horizontal tray type of internals. 1 However, the use of fixed mechanical constructs involves problems such as erosion and maintenance.

Packed-fluidized beds represents another possible solution. In this concept, random packings such as for example glass beads, ceramic balls and raschig rings are applied to a bubbling bed. Fluidization of fine particulate matter takes place in the void between the larger immobile packing elements. This concept can physically prevent bubble growth.2 Only a few studies have been conducted on this concept, and they have focused on spherical packings with low void factor. Such packings can be expected to have hindering effects on solids flux in the reactor vessel. This becomes critical to applications such as CLC, where high flow of solid bed material between the two reactors is required. Thus, the impact of packings on solid flux it is an important subject to investigate.

Recently, Aronsson et al. 3 successfully applied spherical packings in CLC experiments and found improved reaction rates compared to a conventional bubbling fluidized bed. Nemati et al.2,4 have investigated RMSR metal ring saddles (void factor > 0.95) and compared them to spherical packings and others. It was found that the evolved RMSR packing have limited (sometimes positive) effect on particle inventory, pressure drop and heat transfer, while it greatly improves fuel conversion for CLC.

The present work investigates the use of different types of random packing materials, including RMSR metal saddle rings, Raschig rings (RR), Aluminum Silicate Balls (ASB) and Expanded Clay Aggregates (ECA), and their effect on the fundamental hydrodynamics of a bubbling fluidized bed. Experiments were conducted in a cylindrical reactor with an inner diameter of 12 cm and a height of 1 m. The bed material was silica sand, with mean particle sizes of 119,181, and 303 mm. Different properties of such as minimum fluidization velocity, pressure drop and maximum solids flux were investigated. Results showed that the RMSR had the smallest effect on these parameters, while ASB had the largest. It is concluded that both packings are of high interest, but for different applications.