Cutting bubbles using direct numerical simulation

Due to an increase in the oil price, Fischer-Tropsch synthesis, methanol synthesis and other gas-to-liquid processes become increasingly attractive. These gas-liquid-solid processes are mostly performed in bubble slurry columns [Wang et al., 2007, Yang et al., 2007]. However, the efficiency of these columns is restricted due to limited heat removal rates or limited interfacial mass-transfer rates. To improve the efficiency of these reactors, a new reactor concept is developed: a micro-structured bubble column [Jain et al. 2013, Segers et al., 2013].In a micro-structured bubble column, a static wire mesh is introduced. This wire mesh can be used as a catalyst carrier, eliminating a filtration unit to remove the catalyst particles from the product stream. Furthermore, the wire mesh also ensures cutting of the bubbles. This will reduce the bubble size and enable a higher surface per volume ratio [Jain et al. 2013, Segers et al., 2013].To determine the efficiency gain due to the introduction of the wire mesh, a multi-scale modelling approach is used. In this approach there are three types of models. The largest scale models, the Euler-Euler and the Euler-Lagrangian models, need closures to accurately model the interactions between the bubbles, the liquid and the mesh. These interactions can be determined using the smallest scale model: the Direct Numerical Simulations (DNS). While these DNS models are able to simulate these interactions without any a priori assumptions, they are only capable of simulating a small part of the micro-structured bubble column [Segers et al., 2013, Roghair et al., 2011].In this work, a DNS model was developed to study the effect of a wire mesh in a micro-structured bubble column. The DNS model is a combination of the Volume Of Fluid (VOF) model of Baltussen et al. (2014) and the second order implicit Immersed Boundary (IB) method of Deen et al. (2012). The advantage of the use of the VOF model is the relatively easy treatment of break-up of bubbles and the inherent mass conservation. The used IB method enables an implicit fluid-solid coupling,Using this VOF-IB method, the effect of the simplest wire mesh, a single wire, on a single bubble is determined. Several simulations have been performed to study the effect of the alignment of the bubble with the wire and the relative size of the bubble upon break-up. An example of such a simulation is shown in figure 1