Gas‐solids flow behavior: CFB riser vs. downer

Abstract

Comparisons are made in a circulating fluidized-bed riserrdowner system between a 15.1 m high, 0.10 m ID riser and a 9.3 m high, 0.10 m ID downer, based on the measurements of the radial distributions of the local solids holdups and local particle ®elocities along the two columns. Although the core-annulus flow structures exist in both the riser and downer, the radial flow structure in the downer differs largely from that in the riser. The radial distributions of solids holdup and particle ®elocity in the downer are much more uniform than those in the riser, thus ensuring the low back mixing and the narrow particle residence time distribution in the downer. The axial flow structure in the downer is also more uniform than that in the riser. Due to the high particle acceleration and the high particle ®elocity in the downer, the o®erall solids holdup is significantly lower than that in the riser. The microflow structure in the downer, characterized by the low intermittency indices, is also more uniform than that in the riser. These key properties of the downer make it a ®ery promising candidate for industrial applications where short reaction times and high product selecti®ity are required.

A Comparison of Flow Dynamics and Flow Structure in a Riser and a Downer

“…Riser reactors have many industrial applications including fluid catalytic cracking, calcinations, Fischer-Tropsch synthesis and combustion. CFB risers have many advantages over conventional bubbling and turbulent fluidized bed reactors, such as high gas-solids contact efficiency and reduced axial dispersion [3]. However, CFB riser reactors still have some shortcomings, such as severe solids backmixing and significant clustering and nonuniform axial and radial solids distributions, thus reducing the gas-solids contact efficiency and reaction selectivity.…”

“…A comparison of the hydrodynamics in the riser and in the downer has been carried out to investigate their differences and similarities [3,6]. However, only statistical analysis was used in the study of Zhang et al [3] and very limited operating conditions of low solids flux (G s < 50 kg/m 2 /s) were performed in the study of [6]. Transient flow behavior over a large range of operating conditions must be compared to fully understand the flow dynamics and flow structures in the riser and the downer 1) .…”

“…Hydrodynamics in downer reactors have been studied with many research papers published as summarized by Zhang [5]. A comparison of the hydrodynamics in the riser and in the downer has been carried out to investigate their differences and similarities [3,6]. However, only statistical analysis was used in the study of Zhang et al [3] and very limited operating conditions of low solids flux (G s < 50 kg/m 2 /s) were performed in the study of [6].…”

Modeling of partial oxidation in gas–solids downer reactors

“…As in the literature 16,38 as well as in the present work, the downer is proposed as an alternative to gas-solids contacting in risers, it is prudent to investigate its ''comparative'' potential. Most works reported in the literature on downer hydrodynamics relate to fluid catalytic cracking application 38,39 ; the present contribution is arguably one of the first considerations of a synthesis reaction for making a chemical intermediate in a downer.…”

“…14 Roy et al 12 studied the effect of catalyst deactivation, n-butane feed concentration and solids backmixing, and in summary concluded that the highly backmixed solids phase and corresponding exposure to longer residence times for a fraction of the catalyst inventory results in reduced selectivity of the process to the desired product (MAN). In last two decades, a new ''quick'' reactor concept, the so called ''downer'' reactor 15,16 has been proposed to overcome the limitations of riser reactor. Downer is a counterpart of riser in which both gas and solids phase are moving cocurrently downwards at high velocities (in the range of 3-15 m/s).…”