M.Sc Thesis | |
M.Sc Student | Fux Gali |
---|---|
Subject | Deformation, Deposition and Release of Oil Droplets from a Membrane Surface |
Department | Department of Civil and Environmental Engineering | Supervisor | ASSOCIATE PROF. Guy Ramon |
Full Thesis text | ![]() |
Oil is a
common pollutant in wastewaters from a wide range of industries. Some of
the
compounds found in oily wastewater are toxic and hard to degrade, and thus
pose
a significant environmental risk. Treatment of such streams is mostly
performed
by physical and chemical methods; however, those methods are limited
in
their abilities to remove small and well-dispersed oil droplets. Membrane
separation,
which has become widely common in recent years, operates as a
complementary
treatment for these processes. The membranes’ versatility, high
efficiency
and low demand for chemical additives serves as incentive for their
implementation
as treatment processes. However, despite their excellent
capabilities,
wider application of membranes for oil/water emulsion separation is
limited
due to severe fouling. The tendency of oil droplets to deform and coalesce
seems
likely to play an important role in fouling; however, such micro-scale
dynamics
of the oil-water-membrane system are poorly understood. The present
study
uses confocal microscopy at unprecedented resolution for direct observation
of
oil droplet deposition, deformation and detachment during separation and
cleaning,
respectively. The 3D shape of the droplets was imaged as a function of the
permeation
rate, J, droplet
radius, R, membrane
permeance, k, water
viscosity, μ,
and
the water/oil interfacial tension coefficient, σ. Using an
interfacial balance of
viscous
and surface tension forces, these parameters are combined to yield a
modified
capillary number, , which accounts for the extra
viscous
‘suction’ at close proximity to the membrane surface. A clear correlation was
observed
between the degree of droplet deformation and an increasing .
Furthermore,
the reversibility of droplet deposition and membrane performance
were
assessed through microscopic surface coverage and flux recovery analysis,
establishing
the existence of a critical flux. In general, operation at a low flux (3.9
μm/s)
yields spherical droplets that are easily removed by crossflow cleaning,
whereas
a high flux (85 μm/s) leads to significant deformation and mostly
irreversible
deposition. It is surmised that, for oil droplets, the critical flux defines a
transition
from a non-wetting state, where a thin water film separates the oil from
the
membrane surface, to wetting of the membrane by the oil, induced by
hydrodynamic
drainage of the film and eventual destabilization and rupture of the
thin
film. This transition is expected to be delayed if the membrane material is
highly
oleophobic
and hydrophilic. These results shed important new insight on the
influence
of hydrodynamic conditions on fouling reversibility during membrane based
emulsion separation, and may be used to guide better design of surface modified
membranes.