Process design and performance analysis of a narrow-tubes circulating fluidized bed with a double-heat exchanger riser

• A Narrow-Tube Circulating Fluidized Bed, NTCFB, is designed for exothermic reactions. • Riser column consists of two heat exchangers of sufficiently large heat transfer areas. • NTCFB can operate efficiently as circulating fluidized bed with smooth fluidization. • ∆P across the shell and tube riser, was than 0.02 atm even at high gas velocities. • Measurements of circulation rates assures promising design of NTCFB. Operating at high temperatures, highly exothermic catalytic reactors require high transfer rates and safer operating conditions to achieve high performance. Fixed beds, moving bed contractors, normal and even usual design assembly of circulating beds, cannot provide large areas suitable for extremely high heat transfer rates. This study presents a novel design of a circulating fluidized bed (CFB). The designed Narrow Tubes Circulating Fluidized Bed (NTCFB) was constructed and tested in cold-model experiments. The riser column of NTCFB consists of mainly two heat exchanges. A double concentric pipe heat exchanger in the bottom attached directly to the plenum and a shell-and -tube heat exchanger at the top that has a bundle of 69 narrow tubes with 6 mm inside diameter. The hydrodynamics behavior in the NTCFB was investigated, where experiments were carried out to examine the flow characteristics and hydrodynamic behavior of the NTCFB in terms of several operating parameters such as the pressure drop across the grid and the top shell-and-tube riser in the absence and the presence of bed particles. Very small pressure drops across the grid and top riser were noticed at high gas velocities as high as 8.0 m/s. Pressure drops were found to be <2.0 % of the atmospheric pressure for the two parts of the NTCFB. Besides, Gas-solid flow behavior was investigated in terms of solid circulation rates in the NTCFB. Fluidization runs smoothly without pressure fluctuations for large bed particles up to 2.5 kg loading, irrespective of gas velocity.