3. Results and discussion
3.1. Crude glycerol
A sample of crude glycerol from a biodiesel plant was used for
these tests. An approximate analysis was obtained by thermogravimetry
(Q600 TA Instruments thermobalance with Pt crucibles),
using a thermal program previously developed with
synthetic samples using two isothermal steps at 50 and 105 C
for 15 min each. The mass losses at 50 and 105 C were correlated
with methanol and water contents, respectively. After drying, glycerol
was completely evaporated at 300 C to determine the solid
residue content. The following contents (by weight) were measured
for non-hydrocarbon fractions: 2.1% methanol, 8.3% water
and 3.4% non-combustible matter. Anion analysis by chromatography
yielded 3.7% sulfates, 0.08% chlorides and 0.01% fluorides.
These results were obtained with a Methohm ion chromatograph
equipped with a Metrosep A5 Supp anion column. A sodium
carbonate-hydrogen buffer was used as eluent and suppression
previous to conductimetric detection was applied.
Fig. 2 shows the evolution along the axial distance of the diameter
of crude glycerol droplets, for two initial sizes (d0 150 and
180 lm, respectively). The same results are plotted in Fig. 3 in
terms of the diameter squared, normalized with the value at injection.
Analogue results for droplets of fuel-oil No. 2 (distillate oil
normally used in domestic and industrial boilers) are included as
a reference of the behavior of a common fuel in the same experimental
conditions.
Glycerol droplets display qualitatively similar evolutions for
both diameters. The size of the droplets remains constant for some
time after injection due to the heating of the liquid with little evaporation.
This is followed by a decay with about constant slope in
Fig. 3, exhibiting a behavior very close to the theoretical ‘d2 law’.
At some distance, the droplets are suddenly shattered into smaller
droplets. This phenomenon can be clearly seen in Fig. 4, showing
sequences of several images of the same droplet over a distance