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5.6 Swarm Effects, Mass Transfer, and Gas Hydrates
In oil and gas plumes, the rise velocity of single droplets and bubbles through stagnant water is not the only parameter that governs the distribution and fate of the
hydrocarbons. Layers of different salinity and density and the interactions of ocean
currents with the seafloor topography, upwelling and subduction currents, and turbulent eddy diffusion have to be considered to gain reliable plume modeling results
(see Sect. 5.3). But also more fundamental physical phenomena that alter the rise
velocity locally like swarm effects, mass transfer of gas components, partitioning of
hydrocarbons, and the formation of gas hydrates should be taken into account.
Some of those are discussed in separate chapters of this book. In the following, a
brief overview of the aforementioned effects is given.
The rise or sink velocity of a particle swarm in stagnant media is usually smaller
than that of single particles due to displacement and a hereby induced opposing flow
of the continuous phase. In principle, this process also holds for fluid particles like
droplets and bubbles (Richardson and Zaki 1954; Brauer 1971). However, in dense
droplet and bubble swarms, clustering of the fluid particles, i.e., the formation of a
group of particles that is rising collectively, causes the reverse effect. These clusters
can cause an upwelling flow of the surrounding water that allows the droplets and
bubbles to rise significantly faster. This effect has been observed for rising methane
bubble swarms with and without oil coating (MacDonald et al. 2002). The rise
velocity of a dense particle swarm is therefore supposed to be enhanced as compared to single rising particles, especially when it contains large amounts of gas
bubbles. In deeper water and in close vicinity to the blowout site, the rise velocity is
hence supposed to be higher than one would calculate from the correlations for
single particles presented above. In this region, breakup and coalescence of bubbles
and droplets can occur, too. In shallower regions and farther away from the spill site,
most of the pure gas bubbles are dissolved, and the distance between the droplets is
much larger. Therefore, swarm effects are negligible here and the aforementioned
correlations apply.
Due to the decreasing pressure along their rise path, gas bubbles tend to expand,
since their compressibility and specific volume are increasing. On the other hand,
mass transfer of gas molecules across the phase boundary and subsequent dissolution into the seawater counteract this effect. For pure bubbles rising in pure water,
dissolution dominates over expansion, so the bubbles are shrinking (Rehder et al.
2009; Zhao et al. 2016). However, mass transfer is governed by the respective driving gradient and thus by the concentration of the gas component in the surrounding
water. Particularly close to the blowout site and inside the intrusion layers caused by
an oil spill, where hydrocarbons are accumulated, high background concentrations
of methane and other gas components diminish the dissolution. Moreover, for gas
bubbles that are oil-coated, the mass transfer resistance of the interface is significantly higher, leading to much longer bubble lifetimes and larger rise distances –
also resulting in faster rising of the oil entrained, as known from natural oil and gas
seeps (MacDonald et al. 2002).
5 Behavior of Rising Droplets and Bubbles: Impact on the Physics of Deep-Sea…
5.6 Swarm Effects, Mass Transfer, and Gas Hydrates
In oil and gas plumes, the rise velocity of single droplets and bubbles through stagnant water is not the only parameter that governs the distribution and fate of the
hydrocarbons. Layers of different salinity and density and the interactions of ocean
currents with the seafloor topography, upwelling and subduction currents, and turbulent eddy diffusion have to be considered to gain reliable plume modeling results
(see Sect. 5.3). But also more fundamental physical phenomena that alter the rise
velocity locally like swarm effects, mass transfer of gas components, partitioning of
hydrocarbons, and the formation of gas hydrates should be taken into account.
Some of those are discussed in separate chapters of this book. In the following, a
brief overview of the aforementioned effects is given.
The rise or sink velocity of a particle swarm in stagnant media is usually smaller
than that of single particles due to displacement and a hereby induced opposing flow
of the continuous phase. In principle, this process also holds for fluid particles like
droplets and bubbles (Richardson and Zaki 1954; Brauer 1971). However, in dense
droplet and bubble swarms, clustering of the fluid particles, i.e., the formation of a
group of particles that is rising collectively, causes the reverse effect. These clusters
can cause an upwelling flow of the surrounding water that allows the droplets and
bubbles to rise significantly faster. This effect has been observed for rising methane
bubble swarms with and without oil coating (MacDonald et al. 2002). The rise
velocity of a dense particle swarm is therefore supposed to be enhanced as compared to single rising particles, especially when it contains large amounts of gas
bubbles. In deeper water and in close vicinity to the blowout site, the rise velocity is
hence supposed to be higher than one would calculate from the correlations for
single particles presented above. In this region, breakup and coalescence of bubbles
and droplets can occur, too. In shallower regions and farther away from the spill site,
most of the pure gas bubbles are dissolved, and the distance between the droplets is
much larger. Therefore, swarm effects are negligible here and the aforementioned
correlations apply.
Due to the decreasing pressure along their rise path, gas bubbles tend to expand,
since their compressibility and specific volume are increasing. On the other hand,
mass transfer of gas molecules across the phase boundary and subsequent dissolution into the seawater counteract this effect. For pure bubbles rising in pure water,
dissolution dominates over expansion, so the bubbles are shrinking (Rehder et al.
2009; Zhao et al. 2016). However, mass transfer is governed by the respective driving gradient and thus by the concentration of the gas component in the surrounding
water. Particularly close to the blowout site and inside the intrusion layers caused by
an oil spill, where hydrocarbons are accumulated, high background concentrations
of methane and other gas components diminish the dissolution. Moreover, for gas
bubbles that are oil-coated, the mass transfer resistance of the interface is significantly higher, leading to much longer bubble lifetimes and larger rise distances –
also resulting in faster rising of the oil entrained, as known from natural oil and gas
seeps (MacDonald et al. 2002).
5 Behavior of Rising Droplets and Bubbles: Impact on the Physics of Deep-Sea…