Computational modelling of coalescence filtration process

The low water-diesel interfacial tensions arising in biodiesels pose a problem for fuel filters designed to separate water contamination from diesel fuel. Such filters operate by passing the fuel through a fibrous non-woven material with the aim of capturing small water droplets on the fibres and holding them while further droplets coalesce with the captured droplets until the droplets are large enough to be carried away from the fibres and subsequently settle out of the fuel by gravity. The coalescence process is however less effective at lower interfacial tensions. The main purpose of this research is to explore the mechanisms at work in a coalescence filter by developing and applying computer simulations of the process, and to understand the effects of fibre properties such as wettability, size and separation on the filtration performance. Following a detailed review of the relevant literature, a macroscopic simulation of the flow within a filter housing is first presented, using finite element analysis via COMSOL Multiphysics to establish the main flow patterns through the filter system. The filter medium itself in this model is treated as a continuous porous medium. The flow at the pore/fibre scale is then analysed by means of a multiphase lattice Boltzmann method based on the multicomponent Shan-Chen model. The wettability of the fibres is incorporated through specification of a fluid density at the solid surfaces, allowing easy control of the local contact angle. The code developed is tested against previously published and validated finite volume/volume-of-fluid simulations of free droplet coalescence, with good agreement seen in the predicted dynamics. The interactions between individual water droplets and fibres is explored, in particular to establish critical conditions (flow speed, fibre contact angle, droplet/fibre size, droplet-fibre separation, fuel viscosity etc.) under which droplets carried by the flow can be captured by fibres, and the conditions under which droplets are released from fibres. The results confirm the difficulties in achieving rapid and effective coalescence when the interfacial tension is low, and reveal the sensitivity of the droplet-fibre dynamics to the contact angle on the fibres and the relative size of the droplets and fibres. In particular larger fibres are not effective for small droplets, so small fibres are essential in the filtration process. Also investigated are the dynamics of multiple droplets with arrays of fibres representing the filtration media. It is found that higher contact angles (>120°) lead to lower capture efficiency compared to lower contact angle, while contact angles less than 106° tend to produce a small variation in capture efficiency and capture most of the droplets at a filter porosity of 0.87. It is concluded that the inlet layer of the filter should have fibres with 78° contact angle and the exit layer fibres with contact angle 106°.