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Draft tubes have emerged as promising internal devices to overcome the scale up limitations of the spouted bed. Although only a few parameters define draft tubes, the design of these devices is complex and remains a challenging task, as knowledge concerning the gas–solid flow pattern in the spouted bed is required for a proper implementation. Therefore, this study aims to propose a new criterion for the design of draft tubes (nonporous and open-sided) based on the average spout diameter measured by a borescopic technique in conical spouted beds without draft tube. Based on the radial and axial particle velocity profiles, hydrodynamic analysis, and cycle time distributions, the new configurations are compared with those without tube and with conventional draft tubes. The new open-sided tube provides maximum particle velocities of 480.90 ± 14.60 mm/s in the spout and 90.10 ± 4.40 mm/s in the annulus. These values are lower and higher than the particle velocities obtained without draft tube in the spout and annulus regions, respectively, which means it modifies particle residence time in these regions. The new open-sided tube provides a more efficient gas–solid contact than any conventional one, with spouting behaviour being very similar to the configuration without tube (differences in the minimum spouting velocity being in the 2–6% range), which is a key aspect for scale up purposes.

Spouted bed stability and operation is greatly affected by particle features. Accordingly, the hydrodynamic behaviour of conical spouted beds has been studied for fine particles differing in size and density in a wide range of inlet air flow rates. This knowledge is essential for a successful scaling up and industrial implementation of the spouted bed. Therefore, the effect air velocity and solid properties (density and size) have on local solid velocity has been ascertained in a fountain confined conical spouted bed using a borescope technique (Particle Tracking Velocimetry, P T V ) applied to several bed configurations. The results show a close relationship between the inlet air velocity and the local solid velocity, with the gas-solid contact being especially vigorous in the configurations without draft tube and with the open-sided draft tube. The solid circulation flow rate is lowest when a nonporous draft tube is used due to the low solid vertical velocities in the annulus, even at high air flow rates. Nevertheless, vertical velocities in the annular zone increase when particle size and density are increased, although these velocities are lower in the spout and fountain regions due to the higher momentum exchange required for their acceleration.

A novel borescopic technique together with the monitoring of pressure fluctuation signals (power spectral distribution, PSD) has been used to track fine particles and characterize solid flow dynamics in fountain confined conical spouted beds. Radial and axial particle velocity profiles have been obtained for different configurations, and spout-annulus and fountain core-periphery interfaces have been delineated. The downward particle velocities in the annulus peak at intermediate positions in this zone, whereas the upward velocities in the dilute zones (spout and fountain core) peak at the axis or close to this position. Among the different configurations analysed in this work, the system without draft tube shows the greatest vertical particle velocities in almost all the different radial and axial positions. The evolution of the spout size along the bed depends on the configuration used, but all of them differ from those commonly reported in the literature. Thus, the spout expands from the bed bottom to the surface, without any neck at an intermediate bed level. Furthermore, the cross-sectional spout shape has been delineated in the systems with open-sided draft tube, and significant spout expansion is observed due to air percolation from the spout into the annulus through the opened faces. Finally, the average spout diameters of the systems without draft tube and with open-sided draft tubes have been compared with those predicted by literature correlations. Those proposed by San Jose et al. and Volpicelli et al. provide the best fit for the configurations without draft tube and with open-sided draft tube, with their relative errors being 9.83% and 8.88%, respectively.

This study examined elutriation, attrition, and segregation in a conical spouted bed with a fountain confiner and incorporating an open sided draft tube. Fine silica sand with a wide particle size distribution was employed as a model material, operating in both the batch and continuous modes. The use of a fountain confiner is crucial when operating with fine particles, because otherwise the bed rapidly exhibits significant entrainment. The extent of attrition was quantified using a tracing technique based on differently-coloured sand fractions as well as monitoring size distributions by sieving. Particle breakage was found to be the primary attrition mechanism, and the fountain confiner was determined to limit the elutriation of fine particles resulting from breakage. Consequently, only a small fraction of the finest particles were entrained from the bed. The incorporation of a confiner increased operational stability while reducing segregation, especially in the upper half of the bed where the majority of segregation typically occurs. Thus, the bed was perfectly mixed apart from very minimal segregation close to the wall and at the bottom of the contactor. These results provide a basis for the design and operation of larger scale equipment for the continuous drying of materials.

Bed symmetry has been analysed in fountain confined conical spouted beds operating with fine particles. Thus, vertical particle velocities and the spout shape have been determined in a wide range of spouting air velocities and bed configurations. Bed symmetry is widely accepted in the spouted beds without draft tubes or with nonporous ones, but this is not the case when open-sided draft tubes are used. Thus, radial particle velocity profiles differ depending on the cross-sectional angular plane considered in the open-sided draft tubes. The spout expands preferentially along the tube opened surface, and is wider as spouting air velocity and aperture ratio are increased. The literature correlations proposed for the average spout diameter have been analysed and no one is valid for estimating this parameter in the whole range of spouting air velocities and configurations analysed in these systems operating with fine particles. This work has received funding from Spain's Ministry of Science and Innovation (PID2019-107357RB-I00 (AEI/FEDER, UE)), the Basque Gov- ernment (IT1218–19 and KK-2020/00107) and the European Commis- sion (HORIZON H2020-MSCA RISE-2018. Contract No.: 823745). M. Tellabide thanks Spain's Ministry of Education, Culture and Sport for his Ph.D. grant (FPU14/05814). I. Estiati thanks the University of the Basque Country for her postgraduate grant (ESPDOC18/14).

A methodology based on the borescopic technique has been developed and validated for tracking solid movement within a conical spouted bed. The procedure developed and fine-tuned allows monitoring the descending and ascending optical flow of both spherical and irregular particles in the annulus (dense zone) and spout (dilute zone) in the bed. A high speed camera (up to 16,000 fps) fitted with a fiber optic continuous light source was used for recording, and an image treatment algorithm modified and adapted to account for differences in particle shape and bed density (dilute or dense) was applied to the registered data. The procedure allows gathering information on the velocity of particles in a wide range of sizes, from a few millimeters ( d p ¯ = 7 mm) to micrometers ( d p ¯ = 93 μm).

Many applications of the conical spouted beds involve operations using coarse and fine particle mixtures, which require high gas velocities due to the coarse particles, thereby leading to fine particle entrainment. In order to avoid entrainment, a new device has been proposed to confine the fountain. Thus, a study has been conducted on the influence the geometry and configuration of both confiner and draft tube have on particle entrainment, operating pressure drop, operating air flow rate and maximum cycle time. The results show that the fountain confiner greatly stabilises the operation and significantly reduces (above 50%) particle entrainment, but its influence on the other parameters is of rather low significance. Furthermore, the distance between the lower end of the device and the bed surface has a great impact on the performance of the spouted bed regime. Entrainment monitoring throughout time showed that its rate depends on the configuration, but remains constant with time.

A device and methodology have been developed for measuring the circulation rate of fine particles in conical spouted beds. A comparison with particle tracking shows the method is appropriate. The influence of the spouted bed variables have been investigated on the average cycle time, maximum cycle time and circulation rate.

The design factors of fountain confined conical spouted beds have been assessed in order to determine their best combination for scaling up and enhancing fine particle retention. The results evidence that the length of the confiner and the height of the draft tube entrainment zone are the key design factors, causing the highest entrainment reduction. Thus, the optimum confiner length is within the range of 2-3 times the bed surface diameter and the desirable height of the entrainment zone is approximately 1-2 times the tube diameter. This study also concludes that the best system for attaining high stability with minimum elutriation is the one equipped with an open-sided draft tube with an aperture ratio of around 20%.