1. ABSTRACT
This research investigated WCO transesterification using Computational Fluid Dynamics
(CFD) based on laboratory experimentation in a 2 litre stirred tank reactor (STR) with an
axial and mixed flow impeller. Biodiesel predominantly produced in STR is limited by
hydrodynamic and kinetic factors, where the former is often unaccounted for in reactor
design for biodiesel production. In order to model the reaction, the hydrodynamics effects
in STR were experimentally obtained by particle image velocimetry (PIV) technique.
Numerical modeling was done using three numerical turbulent models (kappa-epsilon (к-
ε), shear stress transport (SST) kappa-omega (к-ω) and Reynold’s stress model (RSM)) in
3-D. Experimental transesterification of WCO (free fatty acid (FFA) < 2-5%) was guided
by using the Taguchi L9 orthogonal array (OA) design. The effect of radial and mixed
flow impellers on biodiesel yield was studied at impeller speeds of 600, 650 and 700 rpm;
temperatures of 60, 65 and 70°C and impeller bottom clearance (IBC) of 20, 25, 30 mm.
It was found that IBC and speed had the most significant effect using radial and mixed
flow impellers respectively. Maximum yield of 94.5% was obtained with the mixed flow
impeller in baffled STR compared to 91.3% with radial in unbaffled STR. The highest
correlation of 0.98 was found to be between FAME yield, peak time and temperature for
the mixed impeller in baffled vessel, with maximum FAME yield occurring at 5-30
minutes of reaction time. Fuel characteristic in terms of derived cetane number, glyceride
level, pour point and flash point were well within the ASTM standard. Monitoring of the
reaction was achieved by a novel photodiode analogue-signal circuitry based on spectral
difference of FAME and WCO. A correlation of 0.987 was found between this technique
and ASTM method in measuring reaction time. Experimental fluid dynamic study in a 2
litre vessel revealed that the mean velocity using the radial impeller was maximum at D /
T = 0.38, while for the mixed flow impeller it extended over D / T = 0.15 - 0.46.
Increasing the IBC did not significantly affect normalized mean radial velocity for both
impellers in baffled tank and coverage area for the mixed flow impeller was larger,
consequently increasing mass transfer rate. In the unbaffled tank, normalized radial
velocity increased with increase in IBC and speed for the radial impeller. The mean
velocity was maximum (0.1 to 0.2 Utip) at radial distance between (0.17-0.5) D/T and
(0.1- 0.8) D/T for radial and mixed flow impeller respectively. Impeller power, power
number and flow number for the mixed flow impeller were comparable with axial
impeller from literature, with lower energy consumption. The numerical solution using
the RSM agreed with observed trends of PIV results in estimating mean, radial and axial
velocity as against the к-ε and SST (к-ω) models at a position below the impeller for the
mixed flow impeller. Although this was at a higher computational cost compared to the
other two models. Combined axial and radial flow was confirmed by the RSM model for
the mixed flow impeller. Eulerian-eulerian coupling of mass flow and reaction kinetics
based on conservation of mass and species transport was used to achieved a scale up of
ratio 1:2 using user defined function (UDF) derived for the reaction in ANSYS Fluent.
The relationship between the tank and liquid volume was based upon a volumetric
gradient, assuming non-homogenous, statistically isotropic turbulence. The successful
application of the model achieved biodiesel production was attained for the 1 and 2 liter
in a 2 liter STR with an error margin of 18 and 38 % respectively. Scale up approach
could be further improved using numerical models that resolve the smaller scale flow
using mixed flow impeller.