10-08-2012, 02:12 PM
Oil Recovery with Novel Skimmer Surfaces under Cold
Climate Conditions
Cold_Climate_Oil_Recovery_with_Grooved_Skimmers_Final_Report.pdf (Size: 1.33 MB / Downloads: 296)
Abstract
Increasing oil exploration, production and transport in Arctic waters will increase the
risk of an oil spill occurring in cold and ice-infested waters. The mechanical oil spill
recovery equipment currently used in warmer waters was not designed to collect much
more viscous oils, or oil-ice mixtures. The presence of ice crystals in oil emulsions
affects the adhesion processes between an oil slick and the surface of an oleophilic
skimmer and prevents oil from being efficiently recovered. Novel drum skimmer surface
geometry and materials, tailored to the conditions present under cold climates, are
expected to significantly increase the rate of oil recovery, reducing cost and risk.
The objective of this project was to perform a comprehensive analysis of the adhesion
between oil or ice-in-oil mixtures and various surface patterns and materials, under cold
climate conditions. This knowledge was then applied to improve existing mechanical
response equipment so that it can be applied efficiently under these conditions. The novel
recovery surfaces that proved to increase the recovery efficiency of a drum skimmer up to
two times in warm waters were also successful in cold climate conditions.
In the first phase of the project, laboratory bench-scale tests of different surface
materials were conducted, to determine contact angle and amount of oil adhered at subfreezing
conditions, with and without ice. It became clear that the physicochemical
property that would be most significantly influence by cold climate conditions would be
viscosity, and that the presence of ice would also have an important effect on viscosity,
although to a varying degree depending on the initial oil viscosity. Neoprene was the best
material surface, of those tested here, for adhering oil even under oil/ice conditions.
Based on the results of the laboratory tests at subfreezing conditions, we selected
materials and surface patterns with the highest oil recovery potential under cold climate
conditions, and performed field scale oil spill recovery tests with three different oils at
the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory
(CRREL), located in Hanover, NH. This provided valuable information about the
correlation between the laboratory tests and full scale experiments. It also demonstrated
the potential of the skimmer modifications under conditions similar to response
operations. The field tests were very successful, with high rates of oil recovery under cold
climates, with and without ice present. However, the presence of ice does decrease the
overall rate of oil recovery to some extent.
These studies served to advance the goals of the Coastal Response Research Center,
the Prince William Sound Oil Spill Recovery Institute, and the Minerals Management
Service by providing important information for the improvement of cleanup of oil spills
in cold climates. The outcome of this project advanced our understanding of the adhesion
of oil and oil emulsions (water containing and ice-containing) to recovery surface
material under cold climate conditions. This research will facilitate selection of materials
and surface configurations that result in significantly higher recovery rates of oil spills in
cold and ice-infested waters. This will ultimately lead to a faster oil spill cleanup and
greater protection of natural resources.
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1. Problem Statement
According to the U.S. Environmental Protection Agency (USEPA), almost 14,000 oil
spills are reported each year in the United States alone. The considerable increase of oil
exploration and transport in Arctic waters will increase the risk of an oil spill occurring in
cold and ice-infested waters. Currently, mechanical oil spill recovery in cold climates is
inefficient largely due to the fact that the equipment available to oil spill responders was
not designed to collect very viscous oils and oil-ice mixtures. The presence of ice crystals
in oil emulsions affects the adhesion processes between an oil slick and the surface of an
oleophilic skimmer and prevents oil from being efficiently recovered. Oil spill responders
have used weir type skimmers and large vacuum hoses to suck in oil-ice mixture,
resulting in a significant amount of free water in the recovered product, reducing oil spill
recovery efficiency and creating a discharge problem.
Oleophilic skimmers are based on the adhesion of oil to the rotating skimmer surface.
The rotating surface lifts the oil out of the water to an oil removal device (e.g. cleaning
blade, roller, etc.). The materials used to manufacture the surface of adhesion skimmers
have not been adapted to the special conditions in cold climates. Steel, aluminum, and
general-use plastics had been in use for more than 25 years. Material selection has not
been based on the adhesive properties, but rather on historical practice, price and
availability. Very little effort has been made to study the affinity of new materials for oil
and the recovery efficiency under cold climate conditions. Research conducted in our
laboratory indicates that the recovery material on the skimmer surface can change the
recovery efficiency up to 20%, and that tailoring the geometry of the skimmer surface
can have much higher recovery efficiencies, even up to 200%. To date we have only
studied oils and water-in-oil-emulsions at temperatures above 0±C. All the oils tested
were above their Pour Point. No ice-in-oil emulsions were tested. To our knowledge, no
scientific research has been done to study the effect of changes in oil properties at cold
temperatures and/or in the presence of ice in oil emulsion on oil adhesion to the material
of the recovery surface. Our research aims at studying this process in detail.
Various shapes of the recovery unit, such as a mop, belt, brush, disc, and drum, have
been developed to increase skimmer efficiency. Our research has shown that the
relatively low recovery rate of smooth drum, belt and disk skimmers can be explained by
their relatively small surface area. Only a limited amount of oil adheres to the recovery
surface in every rotation, requiring more time or more skimmers to increase the overall
recovery. Brush and mop skimmers attempted to address this issue by increasing the
surface area in contact with oil. Although these skimmers allow more oil to adhere to the
recovery surface, not all the adhered oil can be removed from the bristles. Thus, a
significant fraction of the oil remains on the bristles, reducing the overall recovery
efficiency.
The oil spill recovery process is composed of two equally important goals. The first
one is to remove oil from the water surface and the second one is to remove oil adhered
to the recovery surface and transfer it into to a collector. The recovery efficiency depends
on the achievement of both of these goals. In case of a smooth surface (e.g. smooth drum,
disk or belt), the amount of oil recovered from the water surface is relatively low, but
close to 100% of it can be removed by a cleaning blade. In the case of a brush surface,
the recovery of oil from the water surface is high on the first pass, but a significant
amount of oil remains on the surface, reducing the overall recovery rate.
The characteristics of an adhesion skimmer surface pattern and materials that can
significantly increase oil recovery efficiency can be summarized as follows:
It should have the maximum surface area possible for a given width of the recovery
surface;
• The formation of oil menisci is highly desirable, since this allows a thicker
layer of oil to be recovered from the water, and it slows oil drainage back into
the oil spill;
• The cleaning blade should be able to remove close to 100% of the oil adhered
to the recovery surface;
• The surface pattern and materials should be tailorable to the oil properties of a
particular region (e.g. Alaskan crudes);
• The recovery surface pattern and materials should take into consideration the
changes in oil properties that occur as the oil weathers, and in colder climates.
With these goals in mind, a surface pattern that satisfies all these criteria has been
developed in our laboratory. The materials used as the contact surface have been selected
based on their ability to adhere to oil, their durability and relatively low swelling, and
feasibility of implementation in existing skimmers. The basic configuration of the
recovery surface is shown in Figure 1.
Figure 1. V-patterned recovery surface. The arrow indicates the direction of oil recovery.
A V-patterned surface maximizes the surface area of a drum, belt or disc skimmer
(Broje and Keller, 2006). Depending on the angle and the depth of the channels, the
surface area can be increased 2-4-fold for the same width of recovery surface. It also
allows menisci to be formed in the depth of the channel, increasing the amount of
recovered oil and slowing down oil drainage. The variation in channel width with depth
allows efficient use of this surface pattern on oils with a wide range of viscosities. The
lighter oils will be collected in the depth of the channels, while viscous oils can be
collected in a wider part of the channel allowing water drainage in the deeper part of the
groove. The cleaning blade can be machined to almost perfectly match the recovery
surface. Thus, close to 100% of the recovered oil can be removed and transferred into the
oil collector in every rotation. Figure 2 shows two grooved drums installed into a
standard drum skimmer (Elastec/American Marine Mini Max®)
Figure 2. Mini Max® drum skimmer. Standard drums were replaced with grooved drums
and a matching cleaning blade.
Recent tests conducted at Ohmsett – The National Oil Spill Response Test Facility,
located in Leonardo, NJ, have shown that V-patterned drums yield to 2 to 3 times higher
recovery efficiency compare to the conventional smooth drums (Broje and Keller,
2007a). This is illustrated in Figure 3. Different materials on the drum surface may have
higher oil recoveries (Broje and Keller, 2007b).
Figure 3. Comparison of recovery rate between flat (smooth) and grooved drums.
Objectives
The data presented in Figure 3 indicate that the use of grooved drums instead of
conventional drums can more than double the oil spill recovery efficiency in warm waters
(10-30 oC). This includes the recovery of very light hydrocarbon mixtures such as diesel.
We believed that this surface pattern could be successfully used in the cold climate
conditions. There were several aspects that need to be studied in this respect, including
the effect of:
• Cold temperatures on the recovery of viscous oils by smooth and grooved
drums;
• Slush ice mixed with oil on the adhesion process between oil/ice and the
surface of the recovery unit;
• Material and geometry of the recovery unit on oil withdrawal and slip
condition;
• Drum rotation speed on the adhesion process, amount of recovered oil and
recovered free water.
The objective of this project was to perform a comprehensive analysis of the adhesion
processes between oil or ice-in-oil mixtures and various surface patterns and materials
that are being used or proposed for use in oil skimmers, under cold climate conditions.
This knowledge can be used to develop mechanical response equipment that can be
efficiently used under these conditions.
We studied the properties of oils (in particular, viscosity, pour point and density) with
increasing ice content. We evaluated how the formation of oil-and-brash-ice mixtures,
with various amounts of ice, affected the adhesion and recovery efficiency of the mixture.
We tested various materials (polymers and metals) and surface configurations (smooth
and patterned surfaces) in order to identify materials and configurations with the highest
recovery efficiency under variable conditions. The surface pattern presented in Figure 1
was modified to examine the effect of channel angle and depth, surface material, and
roughness on the recovery efficiency of various oils. Crude oil and oil-ice mixtures, as
well as refined products such as diesel and HydroCal, were used for these studies.
Following the laboratory tests, we selected the materials and surface patterns that
performed best under cold climate conditions, and performed full scale oil spill recovery
tests at CRREL. This will provide us with valuable information about the correlation
between the laboratory tests and full scale experiments, as well as demonstrate the
potential of the proposed skimmer modifications under conditions similar to response
operations.
Laboratory work
Physicochemical Properties of oils
The four most relevant physicochemical properties for understanding oil recovery
from surface water spills are density, viscosity, surface tension and dynamic advancing
contact angle.
Density
A Pyrex specific gravity bottle for viscous fluids was used to determine the density of
the oil according to ASTM D70 and D1429. The mass of the oil or oil and ice mixture
was divided by the volume of the specific gravity bottle to determine the density. All
weights were measured on a Mettler Toledo analytical balance to four decimal places.
The volume of the specific gravity bottle was calibrated with water at a known
temperature and density, which ranged from 29 to 33 mL depending on the temperature
at which the samples were measured.
Viscosity
A Brookfield DV-II+ Pro Programmable Viscometer (Figure 4) was used to analyze
the viscosities and percent torques of the oil samples. For each run approximately 250
mL of sample were analyzed in a container 120 mm high and with a minimum diameter
of 82.6 mm. The speed and spindle used were also recorded for consistent measurements
of samples. Diesel required a small sample adapter due to the low viscosity of the non-
Newtonian liquid. An average viscosity was measured from five separate locations in the
container.