The experimental measurement and modelling of sedimentation and creaming for glycerol/biodiesel droplet dispersions

This paper presents both experimental measurements and numerical modelling on the separation of glycerol from biodiesel. Glycerol is a natural by-product of biodiesel production and removal of glycerol droplets from biodiesel is required to attain fuel quality standards. Experimental observations on the separation kinetics for sedimentation and creaming of droplet dispersions as a function of time and dispersion volume fraction are presented using both optical techniques and magnetic resonance imaging (MRI). Optical contrast between the biodiesel and glycerol enabled droplet behaviour to be followed optically and it was noticed that because of a droplet size distribution, smaller drops often flowed counter-current to the main direction of droplet flow. Droplet size distributions were measured using droplet size apparatus, but instability in the droplet sizes due to break up and coalescence resulted in uncertainties in the data. A numerical model was developed extending earlier work of, Davis and Russel. Multiple population droplet sizes were found to be necessary in order to describe the experimentally observed concentration profile evolution. The model also included an axial dispersion coefficient which accounted for the experimentally observed counter-current flow of some droplets. In general the model predictions were able to match experimental data with reasonable accuracy. ► Glycerol/biodiesel gravity separation was measured by optical and MRI techniques. ► Smaller droplets flowed counter to the predominant droplet flow. ► Model of polydisperse droplets captured main features of sedimentation. ► Counter flow of small droplets was modelled using a dispersion coefficient.