Modeling of liquid and air transport processes within a filtering mist separator

摘要

Description of transport processes of liquid and air within a filtering mist separator is an essential basis for investigations of filtration specific properties (pressure drop, separation efficiency ...). For mist separators there have already been detailed studies at microscopic level to understand the droplet deposition and movement on a single fiber. The application of those equations to macroscopic level, for example an entire filter, however, often fails due to the complex arrangement of the fibers (random orientation, unknown numbers of contact points …). On a first basis the complex processes of air and liquid transport within a filtering mist separator were modeled by a simple dynamic system. It was assumed that air and liquid masses (=holdup) within a system depend directly on each other and cause flows in and out. These transport flows were described by formulas which are simple equations including parameters for different transport mechanism (air transport, gravity, adhesive force). The air and liquid holdups in the system were furthermore used to estimate different resulting filtration specific properties (e.g. pressure drop). The developed formulas were applied on a 2-dimensional cell grid, which simplifies a complex real filter medium. For each cell the air and liquid holdup was calculated by a stepwise calculation of air and liquid flow values until a steady state was reached. Simulation results of the evolution of the liquid holdup and pressure drop were compared to experimental results of a real wire/glass-fiber filter. Thereby the tests revealed that the pressure drop of the filter still increased for some time although a certain constant liquid holdup already had been achieved. An explanation for that increase of the pressure drop could be made by the examination of the simulated liquid holdup profile of the filter. It was found out that the liquid is moving through the filter and is piling up at the filters clean gas side which causes an increase of the air resistance, respectively an increase of the pressure drop.