New one-dimensional Hydrodynamics of Circulating Fluidized bed Risers

Significance Statement

Circulating fluidized bed CFB are gas–solids reactors containing very fine particles that are fluidized at a rather high gas velocity and are blown out of the bed reactor. The CFB reactors have a wide range of industrial applications, like fluidized catalytic cracking, combustion and gasification, metallurgical processes.

Fast fluidization can only be achieved by continuously feeding of solid into the reactor to maintain the required solids holdup in a vertical riser. The flow structure in CFB risers are found to be complex and highly heterogeneous both in radial and axial directions. To solve the problem of riser that is gas–solids flow structure complexities, the author reviews that one-dimensional models was introduced, where flow properties are cross-sectionally averaged and change from various flow regimes along the risers.

In the recent work of Dr. Francisco Collado from University of Zaragoza in Spain and published in the journal Granular Matter, the author proposed to improve the 1-D hydrodynamics knowledge of CFB risers and use first principle to clarify questions raised under CFB hydrodynamics.

According to Dr. Collado the one-dimensional model was unsatisfactory and has it failure in providing answers to certain flow structure like how the solids volumetric fraction is defined at the dense bed, the bed height, and how the pressure drop evolves along the different regions.

The study used a research riser with many pressure drop taps to measure the axial pressure profile. He also based his studies on Richardson-Zaki (R-Z) equation for 1-D hydrodynamics vertical riser flow. The local solids concentrations were measured by two different techniques, namely γ-ray absorption and fiber-optical reflection probes, the data obtained was subjected to MATLAB for analysis.

The deviation between the sum of the individual differential pressure measurements and the measured overall pressure drop was found to be less than 5 %. The researcher found out that the height of bed can be approximated for both tests as the location where the characteristic particle velocity reaches its representative terminal value, that is ut = 0.83 m/s. The researcher pointed out that for one dimensional model after reaching its terminal value, the characteristic particle velocity does not vary until the riser outlet. The solids observed to accelerate from a zero vertical velocity at the bottom of the riser to their terminal velocity at the dense bed surface.

This study demonstrated in details the dense bed, the bed height, and how the pressure drop evolves along the different regions, the results obtained shows an improvement to the previous work (Kunii-Levenspiel models ) to clarify the flow structure.

Reference

Francisco J. Collado, New one-dimensional Hydrodynamics of Circulating Fluidized bed Risers, Granular Matter (2016) 18:78.

Mechanical Engineering Department, Universidad de Zaragoza, Zaragoza, Spain.

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