(47c) CFD-DEM Study of Flow Field and Pressure Drop inside a Degassing Silo

Plastic pellets release monomers, comonomers and other volatile hydrocarbons during and after their production. For further processing the pellets are usually stored in silos. The evolution and accumulation of hydrocarbons can lead to combustible mixtures with the air around the pellets and can cause silo fires or even explosions. Since such situations are unacceptable, companies which design and build silos are spending time and effort to prevent the uncontrolled degassing of the stored particles. Typically, such storage-silos are purged with air in order to remove the hydrocarbons and to keep the local concentrations in the silo low. The homogeneous distribution of the purge air is required to remove the volatile gas as reliable as possible and to therefore enable a safer operation.

Flow simulations with Computational Fluid Dynamics (CFD) were performed to describe the air distribution inside an industrial scale degassing silo with a complex geometry. The parameters of the drag model were determined through experiments with real product (plastic pellets) and the measured pressure drop depending on the flow velocity was described by an empirical power law function.

It is possible to address the emerging pressure drop and simulate the corresponding overall flow in the silo by modelling the particles as a simplified zone with the help of the experimental measurements. For this, the internal silo volume was discretized with the finite volume method (FVM) and the bulk material was modelled as a porous zone with a geometry which corresponds to the shape of the bulk product in the silo. The Superficial Velocity Porous Formulation was used to calculate the pressure drop through the porous media. Parameters from the experiment were used for the simulation. An isotropic pressure drop inside the bulk was assumed. Inside the areas of the silo without the bulk, the turbulent flow was calculated using SST k-w model. The flow velocity distribution and pressure drop in the silo were described and the areas of the slow flow were located.

However, the exact flow properties and possible dead zones inside the bulk material itself cannot be addressed with this approach. Additionally, the assumed isotropic pressure drop inside the particle zone is unlikely to represent the reality. The possible influence of the present silo walls on the particles and the interactions with the fluid are also neglected inside the porous zone.

The coupling of the CFD with the Discrete Element Method (DEM) could therefore be used to get a better understanding about the flow inside the bulk, since the pressure drop would be affected by single particle-fluid interactions instead of a simplified porous zone. Additionally, the wall influence could be addressed. This however is not easily doable, since widely used correlations for drag coefficients are failing to achieve the measured pressure drops through the bulk material. Even empirical equations, which inherit form factors and other system specific parameters are unable to calculate the measured pressure drops accurately.

The aim of this study is therefore to find a correlation for the drag coefficient that allows the proper calculation of the measured pressure drop inside the particle zone with CFD-DEM coupling. For this, a scaled down simulation setup will be used. The calculated pressure changes from this simulation will be compared to the empirical power law function. Possible reasons for the failure of other widely used empirical models for pressure drop and drag coefficients will be discussed.