Effect of solid density

Solid density is also important parameter which can influence on the hydrodynamic of airlift reactor. By choosing three different density, ρ_s as 1000, 2000 and 3000 kg/m³, simulations have been performed with 15% of initial solid loading in the whole reactor.

Figure 44 and 45 display the contours of solid distribution whenUg = 0.05 m/s in the system withρs= 2000 kg/m³ and 3000 kg/m³ respectively .

Initially, solid particles are uniformly distributed in the whole reactor. The solids motion is governed by the liquid, gravity (resulting in slip velocity of solid between the liquid) and the turbulent dispersion mechanism. When time goes solid starts to sediment in the system due to the effect of gravity. The system gets to the stable state after 25 sec.

Figure 45 displays that the solids fraction is relatively constant, but it is higher in the lower part of the reactor.

Figure 44: Instantaneous snap shot of solids concentration in the reactor with ρs = 2000kg/m³.

Figure 45: Instantaneous snap shot of solids concentration in the reactor with ρs = 3000kg/m³.

To ensure the effect of solids density on overall solids holdup of in the riser section, aver-aged solid holdup have been calculated. Three superficial gas velocities have been used for each case. The influence of solids density on solid holdup in the riser is represented in Figure 46 with the range of Ug.

Figure 46 reveals that averaged solids holdup increases with the increase of solids density.

Initially, solids are dispersed in the system. Due to higher density, solids start to settle down and it results in high solids holdup. Moreover, the results show that for the high density case, superficial gas velocity have no affect in solids holdup.

Figure 46: Average solids holdup in the riser with different superficial gas velocity.

The influence of the solids density on the averaged gas holdup for varying superficial gas velocities is shown in Figure 47. The gas holdup significantly increases with the increasing solid particle densities and it decreases with lower solid density. For lower density case, bubble rise velocity is higher. Due to this, the small bubbles signifies larger bubble diameters caused by increased bubble coalescence. A change in the density of the slurry phase helps the bubbles to rise faster, hence gas holdup decreases.

Figure 47: Average gas holdup in the riser section with different superficial gas velocity.

7 Conclusions

In the present work, the CFD simulations of gas-liquid two-phase flow in internal-loop airlift reactors have been performed. Transient Eulerian-Eulerian approach was used to predict phase distribution and phases velocities. Moreover, simulation have been conducted to investigate the comparison between 2D and 3D results, scale effect on the hydrodynamics and influence of grid and time-step size. The results of the present work of two-phase flow yields the following major conclusions:

• The Eulerian model is able to simulate two-phase flow in internal-loop airlift re-actors. In order to check, with increment in superficial gas velocities, gas phase loses their plug flow character and the velocity profile assume a parabolic shape.

The parabolic velocity profile becomes more prominent with increasing superficial gas velocities in both 2D and 3D cases. The results from 2D and 3D simulations reveal that the assumption of 3D leads to radial profiles of phases velocities that have more parabolic character than that for 2D simulations. The predicted local gas holdup is found to be very good agreement with 2D and 3D.

• The important point in designing airlift reactors is to determine the distance be-tween the riser tube and the reactor walls. Three configurations of internal-loop airlift reactors were used to determine the scale influence on the hydrodynamics.

The results show that as the downcomer area increases, the mixing of the gas and liquid increases and the gas holdup decreases. While downcomer area decreases, the mixing decrease and the gas holdup increase.

• In the present work, grid sensitivity have been studied with 2D and 3D cases. The refinement of the grid has a strong influence on the hydrodynamics of internal-loop airlift reactor for both 2D and 3D cases. It can be observed that gas holdup profiles become more parabolic when go toward coarse grid to fine grid for both 2D and 3D results. The results demonstrated that the grid refinement in the 3D cases seems to be more significant than 2D.

• Time-step size is an important parameter for transient simulation. To determine the influence of time-step on the hydrodynamics of internal-loop airlift reactor, three various time-step have been used. Influence of time-step study reveals that gas holdup has very less sensitivity with changes in the ∆tthan liquid velocity.

In the present work, CFD simulations for internal-loop airlift reactor with gas-liquid-solid three-phase flow system have been carried out by using Eulerian model where the continuous and dispersed phase(s) are mathematically treated as interpenetrating fluids.

Numerical calculations were performed to investigate the effect of solid loading, solid

particle size and solid density on the hydrodynamic characteristics of internal loop airlift reactor. Moreover, study of the solid distribution in the system with different operating conditions have been discussed. The major conclusions emerging from three-phase flow simulation work are listed below:

• For each solid loading cases from 5% to 20%, results shows that the superficial gas velocity looks to be the most effective parameter on the gas holdup, where the averaged gas holdup in the riser section increases with the increase in the superficial gas velocity. The averaged gas holdup is significantly decreased with increasing slurry concentration. Overall solid concentration in the riser section is increased with increment in the initial solid loading. For each solid loading cases, higher concentration of the solid particles is observed on the riser walls.

• By using different solid particle sizes varies from ds = 0.1 mm to 1 mm, simula-tions have been performed. The averaged gas holdup in the riser section increases significantly with the increase in the superficial gas velocity for each cases. It was observed that the riser gas holdup decreases with increase in solid particle diam-eter. The averaged solid holdup in the riser section is significantly increased with increasing solid particle size and the averaged solid holdup is reduced with incre-ment in superficial gas velocity. The solid concentration increases at the riser walls with the increment in solid particle size.

• By choosing three different solid densities, ρ_s, as 1000, 2000 and 3000 kg/m³ simulations have been carried out to investigate the influence on the hydrodynamic of internal-loop airlift reactor. It had found that the averaged solid holdup increases in the riser section with the increase of solid density. For lower solid density, the averaged solid holdup decreases with increment in superficial gas velocity but superficial gas velocity have not affect to solid holdup when density of solid is higher. In addition, it have been observed that the averaged gas holdup in the riser significantly increases with the increasing solid particle densities and it decreases with lower solid density.

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