CFD modeling and simulation of turbulent fluid flow and dust dispersion in the 20-L explosion vessel equipped with the perforated annular nozzle

A three-dimensional CFD model was developed to simulate the turbulent flow field induced by dust feeding and the associated dust dispersion within the 20-L explosion vessel equipped with the perforated annular nozzle. The model was validated against experimental data for pressure and root mean square velocity.Simulation results have shown that the turbulent kinetic energy is rather uniformly distributed and its values are significantly lower than those attained with the rebound nozzle. Furthermore, the perforated annular nozzle is able to generate a uniform dust/air cloud. However, a consistent fraction of the dust remains trapped inside the nozzle and, thus, it does not contribute to the explosion process.     

Dust particle sedimentation in the 20 L standard vessel for dust explosion tests

According to the current international standards, to perform the correct evaluation of the explosion and flammability parameters, a uniform distribution of the dust particles should be achieved inside the 20 L and/or 1 m³ standard vessels.CFD simulations have shown that in both standard test vessels (20 L and 1 m³), the dust particles are not uniformly dispersed, being mostly concentrated at the edge of the macro-vortices generated by the injection of the fluid and particle through the nozzle. In addition, only a partial fed of the particles is obtained, and dust particles sedimentation phenomena can occur.As a result, the dust participating to the reactive process may be much lower than the expected nominal concentration in the vessel due to sedimentation and incomplete feeding. Consequently, misleading values of the flammability/explosion parameters could be measured.Particle sedimentation and incomplete feeding depends both on the Stokes number and on the Reynolds number, whereas the concentration distribution depends on the turbulence level, the fluid flow maps, and the number of particles which enter into the vessel through the nozzle.The aim of this work is to evaluate the key parameters (particle size, particle density, and fluid velocity) affecting sedimentation and incomplete feeding in 20 L vessel. To this end, CFD simulations of dust dispersion are performed at varying the particle density and size. Operating maps, in terms of the key parameters and/or their dimensionless combinations, are developed and a correlation for correction of the data is proposed.     

CFD simulations of dust dispersion in the 1 m3 explosion vessel

According to standard procedures, flammability and explosion parameters for dusts and dust mixtures are evaluated in 20 L and/or 1 m³ vessels, with equivalent results provided a correct ignition delay time (60 ms in the 20 L vessel; 600 ms in the 1 m³ vessel). In this work, CFD simulations of flow field and dust concentration distribution in the 1 m³ spherical vessel are performed, and the results compared to the data previously obtained for the 20 L. It has been found that in the 1 m³ vessel, the spatial distribution of the turbulent kinetic energy is lower and much more uniform. Concerning the dust distribution, as in the case of the 20 L, dust is mainly concentrated at the outer zones of the vortices generated inside the vessel. Furthermore, an incomplete feeding is attained, with most of the dust trapped in the perforated annular nozzle. Starting from the maps of dust concentration and turbulent kinetic energy, the deflagration index K_St is calculated in both vessels. In the conditions of the present work, the K_St is found to be 2.4 times higher in the 20 L than in the 1 m³ vessel.     

Measuring the violence of dust explosions with the “20 l sphere” and with the standard “ISO 1 m vessel”: Systematic comparison and analysis of the discrepancies

On the basis of a systematic testwork with a number of different dusts, the explosion indices as determined within the 20 l sphere and with the ISO-VDI 1 m³ vessel have been compared. The repeatability has been assessed and since some systematic deviations appear a refined physical analysis of the explosion processes is developed. It appears in particular that the cube root law supposed to link both vessels is not verified. A striking illustration of this appears when a dust with a significant explosion severity inside the 20 l sphere is not even explosible in the larger vessel. It is strongly suggested that the ignition energy is forcing very significantly the explosion in the smaller vessel inducing several tens of Celsius degrees of preheating. It is shown also that the inner level of turbulence is decreasing very fast in the 20 l sphere during the flame development so that difficult-to-ignite mixtures would tend to burn at a lower combustion rate. It is further demonstrated that the major bias between the chambers can be explained and quantified with these elements. A correlation with the standard 1 m³ vessel and a grid of interpretation of the data is proposed.     

CFD Simulation of the Dispersion of Binary Dust Mixtures in the 20 L Vessel

There are at least two main requirements for repeatable and reliable measurements of flammability and explosibility parameters of dusts: a uniform dispersion of solid particles inside the test vessel, and a homogeneous degree of turbulence. In several literature works, it has been shown that, in the standard 20 L sphere, the dust injection system generates a non-uniform dust cloud, while high gradients characterize the turbulent flow field. In this work, the dust dispersion inside the 20 L sphere was simulated for nicotinic acid/anthraquinone mixtures (with different pure dust ratios, while keeping the total dust concentration constant) with a validated three-dimensional CFD model. Numerical results show that the fields of dust concentration, flow velocity and turbulence are strongly affected by both diameter and density of the pure dusts. These different dust properties lead to segregation phenomena with the formation of zones richer in one component and leaner in the other one and vice versa, and also result in preferential paths for the solid particles inside the sphere. Overall, the obtained results highlight the need for developing a dust injection system able to overcome the shortcomings of the actual one even when testing dust mixtures.     

CFD simulations of dust dispersion in the 20 L vessel: Effect of nominal dust concentration

Measurements of flammability and explosion parameters for dust/air mixtures require uniform dispersion of the dust cloud inside the test vessel. In a previous work, we showed that, in the standard 20 L sphere, the dust injection system does not allow generation of a uniform cloud, but rather high gradients of dust concentration are established. In this work, we used a previously validated three-dimensional CFD model to simulate the dust dispersion inside the 20 L sphere at different dust nominal concentrations (and fixed dust diameter). Results of numerical simulations have shown that, as the dust nominal concentration is increased, sedimentation prevails and, thus, when ignition is provided, the dust is mainly concentrated at the vessel walls.     

Process loss prevention of petrochemical process: A case study of the flashing accident of the storage tank on acrylonitrile-butadiene-styrene powder in Taiwan

Qualitative analysis, process hazard analysis, thermal evaluation, and fault tree analysis were applied to a flashing accident involving a storage tank that contained acrylonitrile-butadiene-styrene (ABS) powder in Taiwan. The accident was caused by combustible powder attached to the inner wall of the tank reaching a high temperature and then melting. Thereafter, the molten powder became glue-like and dropped onto the ABS powder, burning at the tank bottom, causing decomposition of the styrene and butadiene derivatives as well as other combustible gases. The high concentration of combustible powder and low ignition temperature triggered the powder, initiating a dust explosion. Finally, we analyzed the findings of each method and examined the properties of ABS powder, realizing that the root cause of the accident included an insufficient understanding of the characteristics of ABS and the failure to comply with the management procedures of hot work. Recommendations and countermeasures were proposed that could proactively ameliorate process safety.     

Quantifying the effect of strong ignition sources on particle preconditioning and distribution in the 20-L chamber

Computational fluid dynamics is used to investigate the preconditioning aspect of overdriving in dust explosion testing. The results show that preconditioning alters both the particle temperature and distribution prior to flame propagation in the 20-L chamber. A parametric study gives the fluid pressure and temperature, and particle temperature and concentration at an assumed flame kernel development time (10 ms) for varying ignitor size and particle diameter. For the 10 kJ ignitor with 50% efficiency, polyethylene particles under 50 μm reach 400 K and may melt prior to flame propagation. Gases from the ignitor detonation displace the dust from the center of the chamber and may increase local particle concentration up to two times the nominal value being tested. These effects have important implications for explosive testing of dusts in the 20-L chamber and comparing to larger 1-m³ testing, where these effects may be negligible.