CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 57, 2017 

A publication of 

 
The Italian Association 

of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors: Sauro Pierucci, Jiří Jaromír Klemeš, Laura Piazza, Serafim Bakalis
Copyright © 2017, AIDIC Servizi S.r.l. 
ISBN 978-88-95608- 48-8; ISSN 2283-9216 

Hydrodynamics of the Nonporous Draft Tube Conical Spouted 
Bed Provided with a Device for Retaining Solids 

Mikel Tellabide, Alvaro Casado, Idoia Estiati, Haritz Altzibar, Martin Olazar 
Deparment of Chemical Engineering , University of the Basque Country, P.O. Box 644, 48080 Bilbao, Spain 
mikel.tellabide@ehu.eus 

One of the main limitations of the conical spouted bed lies in its scaling up and a key parameter to solve this 
problem is the ratio between the inlet diameter and particle diameter. Therefore, operation with fine particles 
(smaller than 1 mm particle diameter) requires the use of draft tubes, but the fountain height obtained is very 
high, which causes severe solid entrainment. A new solid retaining device has been developed to avoid bed 
loosing which allows operating with higher air velocities. Thus, runs have been carried out in conical spouted 
beds of different geometry to ascertain the effect of the device on the minimum spouting velocity. Different 
contactor angles, static bed heights, device diameters and distances from the bed surface to the lower end of 
the device have been used. It can be concluded that the hydrodynamics of conical spouted beds with 
nonporous draft tube and solid retaining device is influenced by the static bed height and contactor angle, but 
not greatly by the device geometry. 

1. Introduction 

There are many applications that require a high contact between two phases and frequently they involve a 
fluid and a solid. The most common technique used for this purpose is a fluidized bed, but although there is a 
wide range of fluidized bed regimes, they perform poorly whenever particles are coarse, sticky or have a wide 
size distribution. Thus, the spouted bed regime is an alternative contact method to fixed and fluidized beds. 
Different modifications of the original spouted bed have been proposed in the literature with the aim of 
improving its performance (Epstein and Grace, 2011), with conical spouted beds being suitable for operating 
with solids that are difficult to handle due to their irregular texture or because they are sticky. Nowadays, many 
researchers work with this technology because of their great advantages, i. e., lower pressure drop and a 
better contact between the two phases than fluidized beds, but the main difference lies in its cyclic movement 
of the solid. Furthermore, this technique has been applied in many processes, such as granulation (Borini et 
al., 2009), coating (da Rosa et al., 2010), drying (Correa et al., 2012), combustion (Wang et al., 2013) and 
pyrolysis (Park et al., 2016). 
Spouted beds are gas-particle contactors in which the gas is introduced through a single nozzle at the centre 
of a conical or flat base. The air penetrates the bed of particles creating a central spout zone, a fountain above 
the spout and an annulus surrounding the spout. The gas passes upward through the spout, fountain and 
annulus, while the particles are conveyed up the fountain core and down the fountain periphery and annulus. 
The recirculation of particles between the spout and annulus is one of the more important characteristics of 
spouted beds.  
However, there are situations in which the gas-solid contact is not fully satisfactory due to the instability of the 
bed, especially at large scale. The main limitation of this technology lies in its scaling up and a key parameter 
to solve this problem is the ratio between the inlet diameter and particle diameter. Olazar et al. (1992) 
observed that the inlet diameter should be smaller than 20-30 times the particle diameter in order to avoid a 
slugging regime and achieve spouting status. Therefore, operation with fine particles (smaller than 1 mm 
particle diameter) requires the use of draft tubes.  
Nonetheless, draft tubes modify the hydrodynamics and solid circulation flowrate of the system, changing 
minimum spouting velocity, operating pressure drop, solid circulation patterns, gas distribution and particle 
cycle times (time required for a solid to complete a full cycle, crossing all the zones in the spouted bed). The 

                               
 
 

 

 
   

                                                  
DOI: 10.3303/CET1757137

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Tellabide M., Casado A., Estiati I., Altzibar H., Olazar M., 2017, Hydrodynamics of the nonporous draft tube conical 
spouted bed provided with a device for retaining solids, Chemical Engineering Transactions, 57, 817-822  DOI: 10.3303/CET1757137 

817



performance of the lower conical section of the contactor is different when the draft tube is used, and largely 
depends on the bed geometry, draft tube diameter, length of the entrainment zone, and operating conditions 
(Nagashima et al., 2009). 
Different draft tube configurations are reported in the literature: conventional nonporous draft tubes, porous 
draft tubes, and open-sided draft tubes. The latter have been developed by our research group (Olazar et al., 
2012) and they are especially suitable for vigorous contact. Moreover, Nagashima et al. (2009) have 
developed porous and nonporous draft tubes with a conical-cylindrical geometry. 
The most used draft tubes are nonporous draft tubes due to the great advantages they present (Ishikura et al., 
2003): decrease the minimum spouting velocity and the peak and operational pressure drops, better control of 
solid circulation, narrower residence time distribution, the maximum spoutable bed height is avoided, and 
higher bed stability is attained. Moreover, this type of draft tube enables the operation with solids of any size 
or nature, such as biomass (Saldarriaga et al., 2015). Nevertheless, the use of nonporous draft tubes limits 
the access of the air to the bed (poor contact between gas and solids) decreasing heat and mass transfer 
rates, narrows the distribution of solids cycle times, decreases the degree of mixing, increases risk of tube 
blockage and makes the design more complex. 
Accordingly, the use of nonporous draft tube is the unique way to get a stable spouting status with fine 
particles, but this leads to very high fountains and a huge amount of solids is entrained from the bed. This 
situation limits the use to a small range of gas flowrate and a poor solid circulation is obtained. In order to 
avoid this problem, a new internal device has been proposed. This device is placed above the bed and traps 
the particles in the fountain, avoiding their entraining. It is a cylindrical tube (made of polymethyl methacrylate) 
with the upper outlet closed. Therefore, it is possible to operate at high values of gas flowrate without bed 
loosing and improving solid circulation rate.  
Thus, the main aim of this work is to analyze the effect of a solid retaining device on the minimum spouting 
velocity using nonporous draft tubes with fine particles (0.25 mm of Sauter mean particle diameter). This work 
is a starting point in the hydrodynamic study of draft tube conical spouted beds equipped with a solid retaining 
device developed by our research group. 

2. Experimental 

Different runs have been carried out in a pilot plant shown in Figure 1. 
 

 
 

Figure 1. Diagrammatic representation of the pilot plant. 

818



This plant is composed by a blower, flowmeter, pressure drop gauge, contactor, filter and cyclone. The blower 
has a power of 5.5 kW and supplies air to the contactor. The gas flow is measured by means of a flowmeter, 
which is used in the range of 0-600 Nm3/h. The blower supplies a constant flowrate, and the flow that enters in 
the contactor is controlled by acting on a motor valve that reroutes the remaining air to the outside or changing 
the frequency of the motor of the blower if strict control is necessary. The pressure measurements are carried 
out by means of two pressure taps, which are connected to the input and output of the contactor. In order to 
separate the solids dragged by the air, a filter and a cyclone are connected parallel. The filter traps fine 
particles meanwhile the cyclone separates coarser particles from the air. 
Moreover, the unit alows operating with contactors of different geometry. These contactors are made of 
polymethyl methacrylate and have a conical geometry. Figure 2a shows the geometric factors of these 
contactors. The column diameter (Dc, 0.36 m) and the base diameter (Di, 0.068 m) are the same for the 
different angles used (γ) of 28, 36 and 45º. The gas inlet diameter used is 0.04 m. 
Furthermore, a nonporous draft tube has been used and Figure 2b presents a scheme of this device. The 
dimensions of the nonporous draft tube are: length of the tube LT= 0.20 and 0.27 m; height of the entrainment 
zone (distance between the gas inlet nozzle and bottom of the draft tube) LH= 0.07 m; diameter of the tube 
DT= 0.04 m. 
Furthermore, a new device has been used to avoid solid entrinment from the bed and is shown in Figure 2c. 
This solids retaining device is a cylindrical tube made of polymethyl methacrylate with the upper outlet closed. 
The dimensions of this device are as follows: length of the device LRS= 0.5 m; diameter of the device DRS= 0.2 
and 0.15 m. A stainless steel cone is coupled at the top of the device to avoid the deposition of the solid on 
the device, i.e., the solids deposited on the cone slip and fall down onto the bed. The device is placed above 
the bed (Figure 2d) and traps the particles in the fountain. Thus, it is possible to modify the distance from the 
bed surface to the lower end of the device. Different values of this distance have been tested to ascertain the 
effect it has on the minimum spouting velocity; HRS= 0.02, 0.04, 0.06, 0.08 and 0.10 m.  
The material used for operation is fine sand, which belongs to group B of Geldart classification. The density of 
the sand is 2390 kg/m3 and its Sauter mean diameter 0.25 mm. The static bed heights used are 0.20 and 0.27 
m. 

 
 
Figure 2. a) Geometric factors of conical spouted bed contactors, b) nonporous draft tube configuration, c) 
device for retainig solids and d) its location in a conical spouted bed.  

3. Results 

3.1 Design of experiments 

In order to study the effect of the insertion of a device for retaining solids, the device configuration and the 
different factors of the contactor on the minimum spouting velocity, runs have been carried out by combining 
all these factors corresponding to the contactors (γ), solids retaining device (DRS, HRS) and operating 

819



conditions (H0) according to a experimental design. To avoid instability problems, all runs (with nonporous 
draft tube) have been performed with the gas inlet diameter equal to that of the draft tube (0.04 m). 
Approximately 60 experimental runs have been carried combining all mentioned factors. 
From each system analyzed, the minimum spouting velocity is determined. The first step for determination of 
this property is to loosen the bed. Thus, as air velocity is increased, pressure drop increases to a maximum 
value, and a subsequently increase in air velocity leads to fountain formation and a sharp decrease in 
pressure drop. The value of the peak pressure drop is similar for all the runs, so it has not been taken into 
consideration.  
Nevertheless, the fountain created is very high due to the high air velocity required to break the bed and 
obtain the fountain. Moreover, the fountain created is very dilute because the solids can only enter the spout 
from the annulus through the lower end of the draft tube. Therefore, only a small fraction of solids access to 
the spout. 
Once the fountain is obtained, the air velocity is decreased until the minimum spouting velocity is precisey 
obtained. This velocity is the minimum one when the fountain remains constant and stable at the lowest 
possible velocity. Our polymethyl methacrylate contactor allows a clear visualization of the contactor inside 
and ensure determining the minimum air velocity for a stable fountain. 

3.2 Experimental results 

Figure 3 shows as an example typical trends of the evolution of the minimum spouting velocity in the 
experimental runs carried out by changing differing operating parameters. 
As observed, the effect of the static bed height (Figure 3a) is the most significant one because it causes the 
biggest differences in the minimum spouting velocity. Thus, the values of minimum spouting velocity are much 
higher as the static bed height is increased for beds made up of fine particles. This trend is similar to that 
reported in the literature for conical spouted bed with coarse particles and 0.64 mm Sauter mean diameter fine 
particles (Altzibar et al., 2013). Moreover, despite the fact that only one system has been studied, similar 
trends of minimum spouting velocity with static bed height have been obtained using open-said draft tubes 
and without tubes (Olazar et al., 1992; Kmiec, 1983; Altzibar et al., 2013; San José et al., 2007). A higher 
static bed height means a higher amount of solids in the bed, and therefore a higher air velocity is required to 
maintain the fountain stable. 
Figure 3b shows the evolution of minimum spouting velocity as contactor angle is changed. The values show 
a minimum for 45º, and narrower angles (28º and 36º) lead to higher values of this velocity. Although the wider 
angle contains the greater amount of solids in the bed, it leads to the lowest minimum spouting velocity due to 
the high support of the bed on the walls. Furthermore, the narrower angle means a more vertical contactor 
walls, and therefore a higher air velocity is required to maintain a spouting status (Altzibar et al, 2013). 
Figure 3c shows that as the device diameter is greater, the minimum spouting velocity is slightly lower. In this 
case, a stable fountain is created with a rather low air velocity due the presence of the draft tube. In fact, 
stability is provided by the draft tube and not by the fountain device. The latter is essential to hold all the solids 
in the bed and avoid their entrainment. 
Finally, Figure 3d shows the influence of the distance from the bed surface to the lower end of the device on 
the minimum spouting velocity. In this case, five levels have been used for the distance, but there is no 
general trend on the influence of this parameter. As observed, the lowest distances lead to the highest 
minimum spouting velocities and, as the distance from the upper surface of the bed is longer, this velocity 
decreases. Nevertheless, when this distance is too long, it affects air trajectory and a big crater is formed on 
the bed surface. 
 
 
 

820



  

  
 
Figure 3. Influence of the static bed height a), contactor angle b), solids retaining device diameter c) and 
distance from the bed surface to the lower end of the device d) on the minimum spouting velocity. 
Experimental conditions: a) γ, 36º; DRS, 0.15 m; HRS, 0.04 m; b) H0, 0.27 m; DRS, 0.15 m; HRS, 0.04 m; c) γ, 45º; 
H0, 0.20 m; HRS, 0.1 m; d) γ, 45º; H0, 0.27 m; DRS, 0.2 m. 

4. Conclusions 

A hydrodynamic study of conical spouted bed equipped with nonporous draft tube and a device for retaining 
solids has been carried out for 0.25 mm Sauter mean diameter fine particles. Minimum spouting velocities 
have been measured in draft tube conical spouted beds for different geometric factors of the contactor (γ), 
solid retaining device (DRS, HRS) and different static bed heights (H0). 
The results obtained based on an experimental design show that the minimum spouting velocity is influenced 
by the static bed height and contactor angle, but the solid retaining device geometry hardly affects. 
The minimum spouting velocity increases as the static bed height is increased due to the higher amount of 
solids in the bed. In addition, the minimum spouting velocity has the lowest values for an angle of 36º, and a 
wider one (45º) and a narrower one (28º) lead to higher values of this velocity. 
Furthermore, different factors of solid retaining device (DRS, HRS) hardly affect the minimum spouting velocity, 
and the main role of the device lies in avoiding solid entrainment and stabilizing the bed. 
 

3

3,5

4

4,5

5

5,5

6

15 20 25 30

u
m

s
(m

/s
)

H0 (cm)

a

3

3,5

4

4,5

5

5,5

6

23 33 43

u
m

s
(m

/s
)

γ (º)

b

3

3,5

4

4,5

5

5,5

6

10 15 20 25

u
m

s
(m

/s
)

DRS (cm)

c

3

3,5

4

4,5

5

5,5

6

0 5 10

u
m

s
(m

/s
)

HRS (cm)

d

821



Acknowledgments  

This work has been carried out with the financial support of the Ministry of Economy and Competitiveness of 
the Spanish Government (Project CTQ2013-45105-R). M. Tellabide is grateful for the Ph.D. grant from the 
Ministry of Education, Culture and Sport (FPU14/05814). I. Estiati is grateful for the Ph.D. grant from the 
Department of Education, University and Research of the Basque Country (BFI-2012-234).  

Reference  

Altzibar H., Lopez G., Bilbao J., Olazar M., 2013. Minimum spouting velocity of conical spouted beds equipped 
with draft tubes of different configuration. Industrial and Engineering Chemistry Research, 52, 2995-3006. 

Park H.C., Lee B. –K., Yoo H.S., Choi H.S., 2016. [TC2015] fast pyrolysis characteristics of biomass in a 
conical spouted bed reactor. Environmental Progress and Sustainable Energy,  

Borini G.B., Andrade T.C., Freitas L.A.P., 2009. Hot melt granulation of coarse pharmaceutical powders in a 
spouted bed. Powder Technol, 189, 520-527. 

Correa N.A., Freire F.B., Correa R.G., 2012. Control of the drying process in spouted bed. Transport 
Phenomena in Particulate Systems, 124-147. 

Da Rosa G.S., Rocha S.C.D., 2010. Effect of process conditions on particle growth for spouted bed coating 
urea. Chemical Engineering and Processing: Process Intensification, 47, 1252-1257. 

Epstein N., Grace J.R., 2011. Spouted and spout-fluid beds. Fundamental and Applications; Cambridge 
University Press: Cambridge, U.K.. 

Ishikura T., Nagashima H., Ide M., 2003. Hydrodynamics of a spouted bed with a porous draft tube containing 
a small amount of finer particle. Powder Technology, 131, 56-65. 

Nagahsima H., Ishikura T., Ide M., 2009. Effect of the tube shape on gas and particle flow in spouted beds 
with a porous draft tube. Canadian Journal of Chemical Engineering, 87, 228-236. 

Kmiec A., 1983. The minimum spouting velocity. The Canadian Journal of Chemical Engineering, 61, 274-280. 
Olazar M., San José M.J., Aguayo A.T., Arandes J.M., Bilbao J., 1992. Stable operation for gas-solid contact 

regimes in conical spouted beds, Industrial and Engineering Chemistry Research, 31, 1784-1792. 
Olazar M., Lopez G., Altzibar H., Amutio M., Bilbao J., 2012. Drying of biomass in a conical spouted bed with 

different types of internal devices. Drying Technology, 30, 207-216. 
Saldarriaga J.F., Atxutegi A., Aguado R., Altzibar H., Bilbao J., Olazar M., 2015. Influence of contactor 

geometry and draft tube configuration on the cycle time distribution in sawdust conical spouted beds. 
Chemical Engineering Research and Design, IO2, 80-89. 

San José M.J., Alvarez S., de Salazar A.O., Olazar M., Bilbao J., 2007. Operating conditions of conical 
spouted beds with a draft tube. Effect of the diameter of the draft tube and of the height of entrainment 
zone. Industrial and Engineering Chemistry Research, 46, 2877-2884. 

Wang Q., Kong X., Liu H., Xiao G., 2013. Combustion experimental study on spouted bed for daqing oil shale 
semi-coke. Energy Sources, 614-615, 95-98. 

822