Detailed compositional analysis of
nine crude oils and their saturate,
aromatic, resin, and asphaltene fractions were carried out by elemental
analysis and Fourier transform infrared spectroscopy. The crude oils
had a broad range of physicochemical properties. The largest difference
in composition was found in the asphaltene fractions of the oils,
followed by the resin fractions. These fractions had significantly
higher abundance of oxygen and nitrogen heteroatoms than the saturate
and aromatic fractions. Furthermore, the lightest crude oils had the
highest oxygen content in the asphaltene fraction, while it was the
highest in the resin fraction for the heaviest oils. Partial least-squares
regression models suggested that the total acid number was primarily
associated with carboxylic acids in aliphatic structures and that
high abundance of nitrogen and oxygen particularly in the resin fraction
enhanced the total base number. It is believed that the detailed structural
information provided here will help to improve the understanding of
the interfacial properties of the crude oils.
The
significance of interfacial chemistry for the oil removal efficiency
during flotation was demonstrated in a series of laboratory flotation
and pilot-scale compact flotation unit (CFU) tests. Three crude oils
with different physicochemical properties were used in the investigations.
The differences in drop size distributions and densities of the oils
could not fully account for the observed oil removal. However, taking
the time for drainage and rupture (i.e., induction time) of the thin
aqueous film separating the drops and bubbles into consideration resulted
in good agreement with the oil removal. Moreover, it was demonstrated
in a modified CFU setup that water-soluble hydrocarbons adsorbed onto
the bubbles and reduced the oil removal. This was most likely as a
result of increased induction times caused by the adsorbed components.
The interactions between crude oil droplets and air bubbles were studied by the droplet-bubble micromanipulator technique. Eight crude oils were investigated, and some aspects of the involved mechanisms were discussed. The induction time was measured for air bubbles approaching crude oil droplets in different aqueous phases. Distinct differences were observed in the presence and absence of salts, which showed the importance of long-ranged electrostatic repulsive forces on thin-film stability. The results also suggested that adsorption of dissolved hydrocarbons at air bubble surfaces may increase the potential energy barrier in the thin liquid film. Furthermore, the time needed for crude oil droplets to spread over the air bubble surfaces (referred to as coverage time) was determined for the crude oils. The results showed that the spreading velocity decreased with increasing viscosity of the crude oil. The detailed understanding of this type of interaction is considered to be a precursor for improving the oil removal efficiency during the flotation process.
A matrix
of 18 synthetic produced water samples was analyzed by
partial least-squares (PLS) regression modeling. This has been done
to investigate the correlation between crude oil properties and water
solubility and affinity of the dissolved hydrocarbons for air/water
interface which is referred to as produced water quality. The synthetic
produced water samples were prepared with nine crude oils and two
aqueous phases in the presence and absence of divalent cations (Ca2+, Mg2+). The total organic carbon content and
dynamic surface tension was determined for the samples. The findings
of the PLS modeling are summarized as follows: produced water quality
was not correlated to the amount (wt %) of aromatics, resins, and
asphaltenes in the crude oils but with the amount (wt %) of heteroatoms
present in the different fractions. The water solubility and affinity
for air/water interfaces of the crude oil hydrocarbons was promoted
by oxygen-containing compounds and reduced by nitrogen- and sulfur-containing
compounds. Heteroatom-containing molecules of similar class originating
from aromatic, resin, and asphaltene fractions of crude oils contributed
to different degrees of water solubility and affinity for adsorption
at air–water interfaces. This can be related to the differences
in the molecular structures of the compounds. Furthermore, the presence
of divalent cations showed more influence on the quality of produced
water samples of crude oils with high total acid number and nitrogen
content.
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