133
miscible with each other. The change in dissolved concentration of gas (methane) in
oil with pressure was therefore determined to be the primary control on the partition
behavior of the studied oil constituents.
In addition to the pressure and temperature, the extent of aromatic hydrocarbon
alkylation also influences the partition ratios. The partition ratios of BTEX compounds were found to increase by approximately four times with the addition of
each methyl group to the aromatic ring, from benzene to toluene and toluene to
xylene (Table 8.1). The observed increase in organic solvent/water partition ratios
with the addition of alkyl groups to the benzene core, in homologous series, is a
well-recognized trend (Leo et al. 1971). With the addition of each methyl group, the
strength of nonpolar intermolecular forces (London dispersion) increases, offsetting
the polar interactions of the aromatic benzene ring with water (Leo et al. 1971). This
leads to decreased interactions with water, lowering the concentration of the molecules in water and increasing the partition ratio values (Table 8.1).
8.4.2 Equilibrium Partition Ratio Along the Water Column
Figure 8.4 shows the equilibrium partition ratio variation of BTEX compounds in
gas-saturated oils, corresponding to the pressure and temperature profile (Table 8.1)
along a 1500 m depth water column in the Gulf of Mexico, with and without dispersant Corexit added (at oil/dispersant of 1000:1, Table 8.1). During the plume migration in an actual oil release, upward through the water column, the decreasing
pressure contributes to the progressive loss of methane from the oil, which in turn
enhances the dissolution of BTEX compounds in the water phase. Methane exsolution may also mechanically aid partition between the phases. The cumulative effect
of decreasing pressure with increasing temperature, as the plume migrates to shallower waters, has a strong impact on the P ow behavior of the BTEX compounds,
especially for the more highly alkylated homologues (Fig. 8.4). At these conditions,
as the oil constituents move upward toward the surface (decreasing pressure,
increasing temperature), the P ow values of BTEX compounds keep decreasing, i.e.,
there is an increase in their partitioning to the water phase.
With the addition of dispersant in the partitioning experiments, at an oil/dispersant concentration of 1000:1, the partition ratio values were observed to shift to
lower values (Fig. 8.4) relative to the partition ratios measured without any dispersant in the system. This shift was within the error bars at higher pressure conditions (9 and 11 MPa) while being slightly higher at lower pressures (3 MPa). This
effect indicates that the extent of the BTEX compounds partitioning into the water
column increases with the addition of dispersant to the system, even if it is at low
concentrations (1000:1). The addition of dispersants to an oil-water system promotes the formation of smaller oil droplets, contributing to an increased surface
area of contact between oil and water and in turn resulting in an increased rate of
dissolution of organic species. Since the partition ratios were measured at equilibrium conditions both with and without dispersant, the smaller droplets are not
8 Partitioning of Organics Between Oil and Water Phases with and Without…
miscible with each other. The change in dissolved concentration of gas (methane) in
oil with pressure was therefore determined to be the primary control on the partition
behavior of the studied oil constituents.
In addition to the pressure and temperature, the extent of aromatic hydrocarbon
alkylation also influences the partition ratios. The partition ratios of BTEX compounds were found to increase by approximately four times with the addition of
each methyl group to the aromatic ring, from benzene to toluene and toluene to
xylene (Table 8.1). The observed increase in organic solvent/water partition ratios
with the addition of alkyl groups to the benzene core, in homologous series, is a
well-recognized trend (Leo et al. 1971). With the addition of each methyl group, the
strength of nonpolar intermolecular forces (London dispersion) increases, offsetting
the polar interactions of the aromatic benzene ring with water (Leo et al. 1971). This
leads to decreased interactions with water, lowering the concentration of the molecules in water and increasing the partition ratio values (Table 8.1).
8.4.2 Equilibrium Partition Ratio Along the Water Column
Figure 8.4 shows the equilibrium partition ratio variation of BTEX compounds in
gas-saturated oils, corresponding to the pressure and temperature profile (Table 8.1)
along a 1500 m depth water column in the Gulf of Mexico, with and without dispersant Corexit added (at oil/dispersant of 1000:1, Table 8.1). During the plume migration in an actual oil release, upward through the water column, the decreasing
pressure contributes to the progressive loss of methane from the oil, which in turn
enhances the dissolution of BTEX compounds in the water phase. Methane exsolution may also mechanically aid partition between the phases. The cumulative effect
of decreasing pressure with increasing temperature, as the plume migrates to shallower waters, has a strong impact on the P ow behavior of the BTEX compounds,
especially for the more highly alkylated homologues (Fig. 8.4). At these conditions,
as the oil constituents move upward toward the surface (decreasing pressure,
increasing temperature), the P ow values of BTEX compounds keep decreasing, i.e.,
there is an increase in their partitioning to the water phase.
With the addition of dispersant in the partitioning experiments, at an oil/dispersant concentration of 1000:1, the partition ratio values were observed to shift to
lower values (Fig. 8.4) relative to the partition ratios measured without any dispersant in the system. This shift was within the error bars at higher pressure conditions (9 and 11 MPa) while being slightly higher at lower pressures (3 MPa). This
effect indicates that the extent of the BTEX compounds partitioning into the water
column increases with the addition of dispersant to the system, even if it is at low
concentrations (1000:1). The addition of dispersants to an oil-water system promotes the formation of smaller oil droplets, contributing to an increased surface
area of contact between oil and water and in turn resulting in an increased rate of
dissolution of organic species. Since the partition ratios were measured at equilibrium conditions both with and without dispersant, the smaller droplets are not
8 Partitioning of Organics Between Oil and Water Phases with and Without…