Effect of Gas Distributor Structure on Fluidization Characteristics in a Gas-Sol

Jang Kwangbyol; Feng Yali; Li Haoran

(1. School of Ciνil & Resource Engineering, Uniνersity of Science and Technology Beijing, Beijing 100083;2. National Key Laboratory of Biochemical Engineering, Institute of Process Engineering,Chinese Academy of Sciences, Beijing 100080)

Abstract: In the present paper, the experimental method and computational fluid dynamics (CFD) method were used to investigate the effect of gas distributors with different ori fice sizes and ori fice pitches on fluidization characteristics in a gas-solid fluidized bed. The Euler-Euler two fluid model (TFM) approach based on the kinetic theory of granular flow(KTGF) and the standard k-epsilon turbulence model was employed in the numerical simulation by using ANSYS Fluent 15.0. The results showed that the ori fice size and the ori fice pitch of gas distributor had a signi ficant in fluence on the flow characteristics in the gas-solid fluidized bed. With a decreasing orifice size and orifice pitch of gas distributor having the same opening area, the distributor pressure drop, the initial bubble size, and the height of dead zone just above the distributor were decreased, and the bed pressure drop was increased more than that of the larger ori fice size and ori fice pitch of distributors, the distribution of solid volume fraction was also more homogeneous for the smaller ori fice size.

Key words: fluidized bed; gas distributor; ori fice size; bubble; numerical simulation

1 Introduction

Nowadays the fluidized beds are widely used in many plants including chemical, petrochemical and mineral industry, owing to their favorable mass and heat transfer, uniform temperature, and harmonious gas-solid mixing. As an important component of the fluidized bed, gas distributor plays a key role in the fluidization and the stable operation of fluidized bed. Its structural characteristics can affect the mixing and heat transfer between gas and solid particles[1-2].

Many researchers have reported that the various gas distributor structures can affect the fluidization characteristics of solid particles in the fluidized bed.Dong, et al.[3]studied the gas-solid flow characteristics in a fluidized bed with various opening areas of perforated gas distributors by using experimental and numerical methods. It was found that distributors should have small opening area to improve the fluidization quality.Adoption of smaller ori fices instead of larger ones on the perforated distributor plate design not only could lead to a better gas distribution, but would also result in a better reaction efficiency[4]. In the fluidized drying process,when a speci fic ori fice size was adopted, it could increase the drying rate of bed material[5]. It was found that the ori fice pitch of gas distributor could affect the dead-zone formation of solid particles just above the distributor plate and the concentration of solid particles in the bed column[6-7].

Despite many previous investigations on the performance of gas distributors, the effect of ori fice size in distributor on the gas-solid flow in the fluidized bed has been neglected. Upon designing the gas distributor, it is necessary to consider that the blockage is caused by using small orifice distributor and that the uneven gas distribution is caused by using large orifice distributor.Both cases can affect the fluidization quality in the fluidized bed, however no literature report related to this problem has been found. In the present study,the laboratory scale experiments and CFD numerical simulation were carried out to study the fluidization characteristics in gas-solid fluidized bed with various ori fice sizes and ori fice pitches in gas distributor operating with the same orifice opening area. The simulation results were compared with the experimental results and literature values, which showed a most favorable agreement. Many factors were investigated such as the distributor pressure drop, the bubble formation, the particle dead zone on the distributor, the bed pressure drop, and the distribution of solid volume fraction.

2 CFD Numerical Simulation

2.1 Mathematical model

Numerical simulations were carried out by the Eulerian-Eulerian two fluid model based on the kinetic theory of granular flow (KTGF) and the standard k-epsilon model in turbulence modeling. Many researchers[3,8-9]have used the Eulerian-Eulerian approach to simulate the dense gas-solid systems coupled with the kinetic theory of granular flow (KTGF).

Mass conservation equations of gas (g) and solid (s)phases are given by:

The momentum conservation equations are as follows:

The solid phase transport equation for granular temperature can be expressed by:

The diffusion coefficient of granular energy (Kθs) and granular temperature (θs) are expressed as:

The collisional energy dissipation (γθs) can be obtained by:

The drag models represent the interaction between gas phase and solid phase, and it is well known that the accurate prediction by two-fluid model (TFM) strongly depends on the appropriate drag model[10]. In this paper,the Gidaspow drag model is chosen.

2.2 Numerical methods

Previous studies reported that the results of overall gas-solid flow behavior obtained by using 2D and 3D models are very similar, and the bubble evolution in 2D simulation is more distinct, while 2D models can provide adequate prediction abilities[3,9].

In the present study, ANSYS 15.0 ICEM CFD was used for modeling and meshing a two-dimensional geometry with a width of 0.19 m and a height of 1 m. The modeling equations were solved by using ANSYS15.0 FLUENT software. The 2D geometry was used to investigate the hydrodynamics of gas-solid fluidized bed with different gas distributors. The phase coupled SIMPLE method was used for pressure-velocity coupling. A time step of 1 ms and 50 iterations were used to ensure the accuracy and convergence of numerical simulations. All simulations were performed for 30 s and the average values of a duration of 20―30 s were used as the time-average values of simulation result.

2.3 Boundary condition

The CFD simulations employed air as the primary phase with a density of 1.225 kg/m3, a pressure of 1.01 ×105Pa,and a viscosity of 1.789 ×10-5kg/(m·s), and used spherical glass beads as the secondary phase with a density of 2 500 kg/m3and a diameter of 0.12 mm. Boundary conditions of inlet and outlet were velocity inlet and pressure outlet,respectively. Non-slip boundary condition (no slip) was applied for both gas and solid. The initial bed height was 165 mm, the solid volume fraction was 0.6 and the restitution coefficient is 0.9.

3 Experimental

The experimental system in this study mainly included a bed body, an air supply system, and the measuring devices in the laboratory scale. The scheme of experimental system is shown in Figure 1. The main body of the fluidized bed was made of plexiglass to observe the internal fluidization state, and the inner diameter and height of the fluidized bed were 190 mm and 1 500 mm, respectively. Experiments were carried out with four different gas distributors with an orifice size of 1 mm, 2 mm, 3 mm, and 5 mm, respectively,while all distributors had 190 mm in diameter and 8 mm in thickness. The orifices of the distributor plate were arranged as the equilateral triangle and were all vertical to the distributor surface. Figure 2 and Table 1 illustrate the distributor geometry which contains details of different ori fice sizes and ori fice pitches used in this work.

Glass beads were used as the bed material with a density of 2 500 kg/m3, a diameter of 0.12 mm, and a sphericity of 1 as B group of the Geldart classi fication of solids. The static bed height was 165 mm.

Figure 1 Scheme of experimental system with fluidized bed

Figure 2 Design of four gas distributor plates

Table 1 Structural parameters of gas distributor plates

The fluidizing gas was distributed evenly in the bed column through the perforated distributor. The pressure drop across the distributor was measured by manometers provided just above and below the distributor. The superficial gas velocity was varied gradually from zero to 0.9 m/s for gas-solid fluidized bed and the gas velocity was measured by a heat-sensitive anemometer. Bed pressure drop and distributor pressure drop were metered with four types of distributors.

4 Results and Discussion

4.1 Grid independence study

In the present study, three types of grid size were considered for determining the effect of the grid size on the simulation results, such as the coarse grid (5 mm ×5 mm), the medium grid (3 mm × 3 mm), and the fine grid (2 mm × 2 mm). Figure 3 shows the comparison of bed pressure drop simulated with three types of grid solutions. It is found that the simulation results were affected by the grid size selected thereby. The medium grid size was selected while considering the accuracy of simulated results and the simulation time. The grid size of gas distributor domain used in the simulation was 0.5 mm×0.5 mm and another size of the calculation domain grid was 3 mm×3 mm for all cases.

4.2 Effect on distributor pressure drop

The distributor pressure drop should ensure the favorable conditions for the entire fluidization, and for this reason, an adequate distributor pressure drop should be guaranteed.If the distributor pressure drop exceeds a certain value necessary for the performance of the fluidized bed, the fast gas flow will pass through the bed, which will lead to a reduced gas residence time in the bed and a decreased contact efficiency between gas and solid particles. On the contrary, if distributor pressure drop is too low, it will decrease the driving force of gas on solid particles.

Figure 3 Comparison of bed pressure drop with different grid sizes

Figure 4 shows the experimental and simulated results with different distributors under various superficial gas velocities. The simulation results were similar to those experimental data at low gas velocity, but there were a little difference for high gas velocity. One of the reasons is the void of solid particles filling in the initial bed column.

It can be seen from Figure 4 that the distributor pressure drop increases with the increase of superficial gas velocity, this variable trend is the same as in the experiment of Rahimpour, et al[11]. It also shows that the distributor pressure drop increases with the increase in orifice size and orifice pitch of the distributor. The difference of distributor pressure drop is obvious at the high gas velocity. The distributor pressure drop for case 4 and case 3 is larger than that for case 2 and case 1. The pressure drop across the distributor plate is the lowest for case 1 (with an ori fice size of 1 mm and an ori fice pitch of 6 mm), and the highest for case 4 (with an ori fice size of 5 mm and an orifice pitch of 26 mm). The pressure drop across the distributor was affected by many factors such as the ori fice geometry and the surface roughness[12].

It is conceivable that employing the small orifice size and ori fice pitch in the distributor having the same ori fice opening area can increase the frictional area, which would affect the gas flow through the distributor and increase the resistance to gas flow. And the gas flow through the large ori fice has a stronger driving force on the particles around an ori fice than one with the small ori fice size, which can produce a higher distributor pressure drop. Another reason is attributed to the blockage of solids surrounding an ori fice and the back flow of particles above the distributor.The experimental and simulated results indicated that distributor geometry can affect the gas flow and the suspension of solid particles just above the distributor,and the distributor pressure drop can be affected by the geometry of gas distributor.

4.3 Effect on the fluidization above distributor

The flow behavior of gas and solid on the distributor can affect significantly the whole fluidization quality in the fluidized bed. Therefore, it is of great significance to improve the even distribution of gas and the appropriate bubble size on the gas distributor. It is conceivable that the gas distribution on the distributor plate with a large number of ori fices and smaller ori fice pitch is more uniform than the gas distribution with a small number of orifices and larger orifice pitch. In the present study, it can be found that the orifice pitch in distributors for case 3 and case 4 are larger than that for case 1 and case 2, therefore, the gas distribution just above the distributor plate for case 1 and case 2 is more uniform than that for case 3 and case 4.

Figure 4 Experimental and simulated pressure drop pro files of four gas distributors

Figure 5 Formation of particle dead-zone and initial bubble just above distributor

The height of particle dead zone on the distributor is decreased with a reducing orifice pitch, which can lead to strengthened movement of solid particles and improved floating of solid particles on the distributor.Figure 5 shows the theoretical scheme on the process for the formation of the particle dead zone just above the distributor. The height of particle dead zone is estimated by Eqs. (14) and (15).

In Eqs. (14) and (15), it can be found that the height of particle dead zone on distributor with the smaller ori fice size and ori fice pitch (case 1 and case 2) is lower than that with the larger orifice size and orifice pitch (case 3 and case 4) that make up the same opening area.

In the gas-solid fluidized bed, bubbles evolution including formation, coalescence and collapse, can affect immediately the mixing of gas and solid particles and enhance the heat and mass transfer process. The investigation of bubble evolution is focused on the bubble size. Yuu, et al.[13]reported that there are many small vortices in the bubble, and the vorticity of the vortices surrounding the small bubbles is relatively high, so that it has a significant influence on various operation in fluidized bed. The flow of the large bubbles with fast velocity can make the gas stay for a short time in the bed column because of short-circuit of bubbles, and it can obstruct the fine mixing between gas and solid and decrease the reaction efficiency in fluidized bed. The bubble size usually depends on the initial bubbles formed just above the gas distributor plate. The bubble diameter of the multi-orifice distributor plate can be obtained according to the following Eq.[3,11,14].

The formation and movement of bubbles in a fluidized bed are quite complicated. It can be seen from Eqs. (16) and(17) that the bubble size is affected by some factors, such as the super ficial gas velocity, the bed width, and the initial bubble size. It can be seen from Figure 5 and the equations discussed above that the initial bubble size is affected by the ori fice size and the ori fice pitch of distributor.

Figure 6 shows the formation and growth of initial bubble simulated with four types of distributor plates (ε: solid volume fraction), and the initial bubble size is smaller upon employing the small orifice and orifice pitch of distributor having the same opening area, and eventually it is possible to obviously decrease the dead zone height of the solid particles just above the distributor plate.

Figure 7 shows the experimental results on the evolution of bubbles in the fluidized bed with four distributor plates. It is found that the initial bubble formed in case 4 is larger than that of other cases, resulting in moving the larger bubbles in the bed to hinder the even distribution of gas and solid. The initial bubble size for case 1 and case 2 is smaller because the distributor plates have the smaller ori fice size and ori fice pitch.

4.4 Effect on bed pressure drop

The bed pressure drop is an important parameter in a fluidized bed, which can directly or indirectly reflect the fluidization of solid particles and the gas-solid flow behavior. In a fluidized process, the bed pressure drop is increased with an increasing super ficial gas velocity, until the solid particles are entirely fluidized.

Figure 8 shows the experimental results of bed pressure drop, which increased with an increase in superficial gas velocity. The variation trend of bed pressure drop was basically same as that mentioned in the literature reports[15]. It is found that the bed pressure drop is obviously affected by the employed ori fice size and ori fice pitch of gas distributor. The bed pressure drop is higher upon employing the distributors with smaller ori fice size and ori fice pitch (as revealed by case 1 and case 2).

The high bed pressure drop in cases 1 and 2 indicates the improvement of solid particle fluidization, which can provide favorable conditions for developing fluidization quality.However, in cases 3 and 4, the bed pressure drop is lower, so it shows an uneven distribution of gas and solid particles.

Figure 6 Contours of bubble formation on different distributor plates

4.5 Effect on distribution of solid volume fraction

Figure 9 shows the contour of transient solid volume fraction simulated with four types of gas distributors. In Figure 9, the distribution of solid volume fraction for case 1 and case 2 are more homogeneous than other cases, and especially when case 4 and case 3 were adopted, large bubble flow patterns were detected in the bed, which could affect the even distribution of gas and solid.

Figure 10 shows the time-averaged radial direction distribution of the solid volume fraction for four types of distributors. The fluctuation of distribution profiles of solid volume fraction indicates the concentration and dispersion of solid particles. It can be seen from Figure 10 that the distribution pro files of solid volume fraction at the whole width of the bed are smoother and higher for case 1 and case 2, as compared to case 3 and case 4.

Figure 11 gives the distribution profile of solid volume fraction along the bed height for different distributors. It can be seen from Figures 10 and 11 that the profiles of case 1 and case 2 are relatively less fluctuated, and the pro files of case 4 and case 3 are rough. It means that the distribution of solid volume fraction in the bed by using the distributor with smaller ori fice size and ori fice pitch is more homogeneous than by using the distributor with larger ori fice size and ori fice pitch.

Figure 7 Bubble evolutions in the fluidized bed with different distributor

Figure 8 Experimental pro files of bed pressure drop

Figure 10 Time-averaged radial direction distribution of solid volume fraction(U=0.6 m/s)

Figure 9 Simulated transient solid volume fraction for four types of gas distributors

Figure 11 Time-averaged distribution of solid volume fraction along bed height

The profiles of solid volume fraction fluctuate roughly in case 3 and case 4, indicating that the concentration of solid particle in the bed fluctuates seriously with larger ori fice size and ori fice pitch of the distributor.

5 Conclusions

In this study, it has been shown that with the same opening area, the orifice size and orifice pitch of gas distributor have a significant effect on the fluidization characteristics of a gas-solid fluidized bed. The distributor pressure drop increased with an increasing ori fice size in the gas distributor. The ori fice resistance to the gas flow and the obstruction of solid surrounding the gas jets were increased with the small ori fice size of the distributor. The distributor pressure drop by using ori fice sizes of 3 mm and 5 mm was higher than that by using ori fice sizes of 1 mm and 2 mm. The CFD simulations indicated obviously that the orifice size and orifice pitch affected the flow behavior of gas and solid just above the distributor, while the formation of particle dead zone and the initial bubble size could be well regulated by using smaller orifice size and ori fice pitch of distributor in this study. The bed pressure drop with the distributor having smaller ori fice size (1 mm and 2 mm) were higher than that of larger orifice size (3 mm and 5 mm). The homogeneous flow structure of gas and solid could be achieved by using distributor of smaller ori fice size and ori fice pitch.

Nomenclature

CD—drag coefficient

D—bed width, m

Db—average bubble diameter, m

Db,0—initial bubble diameter, m

Db,∞—maximum bubble diameter, m

dm—diameter of the particle moving zone above distributor, m

do—ori fice diameter, mm

ds—solid particle diameter, mm

H0—height of initial bed material, mm

H—bed height, mm

Ld—height of particle dead zone, mm

Po—ori fice pitch, mm

Re—Reynolds number

U—super ficial gas velocity, m/s

Umf—minimum fluidization velocity, m/s

Uo—gas velocity through ori fice, m/s

Greek letters

α—volume fraction

β—drag coefficient

ε—solid volume fraction

φ—angle of particle dead zone, rad

ρ—density, kg/m3

Acknowledgements:This work was supported by the China Ocean Mineral Resources Research & Development Program(DY125-15-T-08), and the National Natural Science Foundation of China (21176026, 21176242).